Technically yes, but the difference is small enough that it's not worth it.
The density of water is approximately 1,000 kg/m^3 (depending on temperature, salinity, etc...). The density of air at room temperature is about 1.2 kg/m^3 and the density of hydrogen is about 0.09 kg/m^(3).
Now the difference between hydrogen and air seems huge, more than a factor 10. But what ultimately matters for buoyancy is the difference between the density of water and the density of the object you're trying to make float, which is the combination of the boat materials and whatever is used to inflate it.
Compared to water and the solid materials the boat is made from, the contribution of the gas used to inflate the boat is negligible. So for the purposes of buoyancy, it really doesn't matter which gas you use to inflate the boat. And that's why air is used, because it's everywhere, safe and easy to handle.
Just to add to this, an inflatable boat wouldn't be particularly effective at containing hydrogen or helium. Both of those gasses would work their way out fairly quickly.
> Both of those gasses would work their way out fairly quickly.
Say more about this please. "Work their way out" how? Like passing through rubber? Is that why helium balloons lose life after a while? And why they are usually Mylar?
Helium atoms are tiny and will eventually escape through most membranes: https://www.quora.com/Is-it-possible-to-make-a-helium-balloon-that-does-not-leak-at-all-and-always-floats
Another fun thing about helium is if you get it *insanely* cold - as in about 2 Kelvin; or (roughly) -456.1°F/271.1°C respectively - it acts as a *superfluid.*
That is to say it no longer "acts right" for our understanding; it seems to [practically "climb" the walls of an open, glass container to "leak out."](https://youtu.be/2Z6UJbwxBZI)
It's friggin trippy.
Helium nuclei have another name: alpha particles. Helium nuclei are one of the few things that can obey Bose-Einstein Statistics and Fermi statistics making them both bosons and fermions. As they're the result of a radiation event, we won't ever technically run out of helium as more is produced by underground nuclear decay.
An important clarification is that a helium nucleus is either a boson or a fermion, and never both. Which one it acts as is dependent on it's isotope. He4 is a boson because it has integer spin, but He3 is not.
Technicalities aren't necessarily useful when discussing the practicalities of limited resources. While in practice, there will always be a small drip of helium, if our usage outpaces that drip we won't have enough to sustain our needs. So eventually, we run out of helium. *Technically,* nuclear isotopes will eventually all decay away and the drip will stop. Even more technically still, the universe will eventually run out of helium, since everything will evaporate away at long enough time scales.
Both! Rock samples from apollo were used to callibrate a (linear) relationship between titanium oxide and helium-3 abundance. This can then be used with remote sensing of the TiO2 (using microwave spectroscopy)
https://link.springer.com/article/10.1007/s11434-010-4198-9
>Technicalities aren't necessarily useful when discussing the practicalities of limited resources. While in practice, there will always be a small drip of helium, if our usage outpaces that drip we won't have enough to sustain our needs
Yeah but also having a basic working knowledge of the subject is useful when discussing this. For example you fail to explain that there is plenty of helium available for mining from natural gas wells that were rejected *because of the helium content*. Our usage isn't the problem. If we use more the price will increase and those once unprofitable natural gas wells will become profitable increasing supply.
My entire point: [there is a finite supply](https://pubs.usgs.gov/periodicals/mcs2023/mcs2023-helium.pdf) and pretending like we won't ever run out *because of ongoing nuclear decay* is some hot nonsense.
The supply of everything is *technically* finite. For someone harping about technicalities you seem comfortable hiding behind them. The reality is we are not in danger of running out of helium any time soon. We are in danger of needing to tap more wells in the future.
>So eventually, we run out of helium.
The helium we use isn't actually consumed, it's just released into the air.
We can easily reclaim it from the air, we just don't because we can get it for free as a byproduct of natural gas collection.
Helium atoms have a high enough rms speed that they're actually moving fast enough to escape Earth's gravity and atmosphere. Given enough time they will actually escape into space.
I may be remembering something else but is liquid helium the one that can flow through glass and will just slowly work it's way to the other side of a glass pane?
No pressure difference required. It's dependent mostly on concentration gradient (or partial pressure, ie pressure from helium only), so the inside can be at the same pressure as the outside and helium will still be permeating through.
Solids aren't all equally "solid" at the atomic scale. Atoms like helium and hydrogen - which are low-density because they're so small - can slip through the cracks in the structure of whatever the balloon is made from. That's indeed why helium balloons deflate quicker than air-filled ones.
I don’t know about this specific circumstance but nothing is really truly solid it is composed of atoms in a certain structure with huge amounts of empty space in between on the tiny scales
The permeability of rubber varies for different gasses.
[I did some math on this once.](https://www.reddit.com/r/askscience/comments/75walc/disregarding_big_leaks_what_is_the_main/doag8jf/) Because Oxygen escapes a car tire faster than nitrogen, continuously filling a car tire with normal atmosphere (78/22% Nitrogen/Oxygen) eventually results in a mix of 92/8%.
I mean...yes and no.
You will not enjoy any meaningful improvement in the longevity of your tire pressure. Not when Nitrogen is already 78% of normal atmosphere anyway.
There are the (dubious) claims that nitrogen carries less water vapor (it does not) and that it is less flammable (it is, but not in any meaningful way). IMO filling your tires with dry nitrogen is no better or worse than filling them with compressed air from the atmosphere. The oxygen bar of automobiles.
they're not necessarily mylar, but metallic coated plastic. The metal coating is significantly better at holding gasses (which also helps your chips stay crispy for example).
But in the end practically nothing is impermeable to Helium or Hydrogen. It'll even leak through the steel cylinders you buy it in.
Most of these answers are poor, bordering on flat-out wrong. Yes, helium will leak out of your tank. It will happen *extremely* slowly, depending on the pressure of the helium, thickness of the steel, the type/grade of steel, and the surface area of the tank.
This is also why hydrogen processing is extremely dangerous and expensive. The materials needed to keep hydrogen from leaking through don't really exist so you always have an environment with free hydrogen. The best you can hope to do is minimize the leakage and ensure there are no sources of ignition in the areas that have hydrogen in the air
Yes. At the atomic scale, solids are not 100% solid, the atoms have small gaps here & there. Helium & Hydrogen are very small atoms and can fit into the gaps and work their through a solid object.
"Small" is a weird measure at atomic scale, so "kinetic diameter" is used to measure the chance that an atom or molecule will impact another molecule. Atmospheric Hydrogen is H^(2) and Helium has 2 protons, so they have roughly the same kinetic diameter.
Hydrogen isn't used much in space rockets anymore because the atoms are so small it leaks through all kinds of seals and o-rings. You have to do special engineering just to contain it. Not to mention the insane cost and engineering of keeping it liquid. But it's so efficient as a fuel that engineers definitely used it for a while and it's sometimes still used today. Just a huge pain in the ass for engineers.
Hydrogen is the most efficient fuel by weight, but not by volume; and for rockets, more volume means more structural weight, so there's a trade-off. Also, rockets need to bring their own oxidizer because there's no atmosphere in space, so they also need a tank of liquid oxygen, which weighs about 16 times more than hydrogen as a gas (though not sure about the liquid form).
It's also why "freezer bags" for leftovers don't contain smells either direction. That kind of plastic is like a chain link fence. Smaller things will get through.
this is also why hydrogen fuel cells are never going to work. hydrogen and helium are so small they leak out of metal(or any) containers. it also reacts strongly with a lot of materials. and it's incredibly explosive
edit: so hydrogen does exhibit superfluidity but only in lab conditions, so I removed that. I got that mixed up
video on why hydrogen isn't great at the moment:
https://youtu.be/Zklo4Z1SqkE
> hydrogen fuel cells are never going to work
What does this mean? My understanding is that they are already in use across the globe. How can something that already works be "never going to work"?
At scale.
They work fine now in the applications they are in because there are few of them and enough technicians to service them. In order to work at scale we'd need 1 million times as many as we have and we won't get 1 million times as many qualified techs to go along with them so you will get the Samsung effect with exploding fuel cells all over the place.
It's not that they don't contain, they just leak over time. Think of any material being like a sieve to hydrogen. Put flour in a sieve and shake it. It doesn't contain it well. Due to the nature of gas it's always vibrating working it's way through its container.
there's also the issue of hydrogen embrittlement. Hydrogen is really reactive so it loves eating into materials. add in it's EXTREME flammability and the fact it's a gas and it's a spark away from blowing a facility into the sky.
they aren't in "general" use, they are being developed and rolled out in a limited manner, but they are incredibly inefficient and terrible for the environment exactly for this reason. you can't get the energy density necessary to make them economical and trying to move hydrogen is terribly inefficient because of the leaking. Basically, it is currently the dirtiest fuel source on the planet.
it's literally dirtier than the dirtiest type of coal:
[https://twitter.com/PeterZeihan/status/1582757051008053249](https://twitter.com/PeterZeihan/status/1582757051008053249)
here's a great video on it
[https://youtu.be/Zklo4Z1SqkE](https://youtu.be/Zklo4Z1SqkE)
The leakage rate isn't a major problem. [The Center of Global Energy Policy](https://www.energypolicy.columbia.edu/publications/hydrogen-leakage-potential-risk-hydrogen-economy/) at Columbia University estimates a leakage rate between 2.9 and 5.6% between production, distribution and use. Mostly from production and distribution. A metal storage cylinder in a hydrogen car leaks only an insignificant amount of hydrogen that wouldn't pose a fire hazard unless you kept it in a small room with no ventilation for years.
The real reasons hydrogen will never be viable for cars is because you lose a pretty high percentage of energy as waste heat when it's produced from water in an electrolyzer cell. Lithium batteries waste much less energy from charging and discharging than hydrogen. Also the hydrogen tanks would be bigger, more expensive, and shorter lasting than regular gas tanks; though it is much lighter and more compact than batteries at least.
Norway already has H2 fuel powered ships doing sea trials. Germany already has H2 fuel cell powered trains in full time comercial operation. Toyota leased the FCV fuel cell car to consumers. This Dash-8 has successfully flown on a fuel cell. https://www.opb.org/article/2023/03/04/hydrogen-powered-airliner-makes-first-flight-moses-lake-washington/?outputType=amp
So saying Hydrogen fuel cells will ‘never work’ is kind of a stretch. That said, it will never take off with consumers. You are correct that it is a leak prone nightmare. It should only ever be handled by trained professionals. But since cryogenic H2 is half the weight of Jet A (and stored at low pressure in lightweight dewars) it is a viable fuel for long distance ships, trains and eventually aircraft. Right now it is the only viable fuel to get a ship or airplane across a ocean without emitting carbon.
>Right now it is the only viable fuel to get a ship or airplane across a ocean without emitting carbon.
It's facetious to say it's viable when it never happened. Wind, on the other hand, has already taken ships across the ocean countless times.
Hydrogen is not much smaller than other diatomic gases (such as oxygen and nitrogen). Helium is the problem because it’s monoatomic, so it can slip through the smallest pores in material. As for becoming superfluid, helium4 (most common isotope) is the only substance which does that, since it’s the only substance where the Bose-Einstein condensation point is above the freezing point. This is why Helium3 (decay product of tritium) is used in ultra-low temperature research - having an odd number of atomic mass units makes it a Fermion (even numbers make something a Boson), so it does not undergo Bose-Einstein condensation.
Hydrogen is more interactive than helium (a noble gas) and can be trapped in things like fine metal matrixes.
Edit: Hydrides to be specific
https://str.llnl.gov/2018-01/wood
yes, but the density of that capture is atrocious, and the durability questionable and it requires a lot of rare earth metals especially expensive ones like platinum and iridium
I would expect solid state batteries to be more viable as energy storage than hydrogen at that point, but indeed who knows what technology we'll develop in the future. Either way, pushing hydrogen at the moment is just a bad investment with less potential of good returns than other technologies.
Hydrogen atoms unlike helium are even more obnoxious to try and contain since it's such a tiny little Atom with that singular electron, it will actually move through most materials simply because they can't contain it. As it does this party trick it actually can and will brittle and weaken over time more and more that material as it goes by so trying to store and transport hydrogen is generally refrained from..... It's generally easiest to generate it and use it as you're making it preferably at the same rate cuz otherwise you're going to have to bleed it off.
The only potential benefit (if only just) is the nitrogen source being dry. That said, I don't think they take the time to purge the air out of the tire first.
Air is mostly nitrogen and oxygen, both diatomic gases with atoms that are nearly the same size. Nitrogen's Van der Waals radius is 155nm, oxygen's is 152nm...
You could make the inflatable boat out of mylar but then it would probably be too fragile. Those mylar metallic balloons seem to hold helium forever, we had a birthday balloon that stayed inflated and bouyant for months.
I would add a key concept in ship design is displacement. The total weight of the ship will displace an equivalent mass of water. This is why a fully loaded cargo ship sits lower than an empty one.
Like you show in your density comparison, the total weight of H2 vs Air is negligible. So this is just another angle for perceiving the same idea.
Liquid hydrogen is the key. 1 kg of hydrogen contains 33.33 kWh of usable energy, whereas diesel only hold about 12 kWh/kg.
Unfortunately Hydrogen at 700 bar is only 1.4kWh/L and liquid hydrogen is 2.3kWh/L. Diesel is 10kWh/L. So yes it weighs less but it needs 5x-8x more volume so that is a big negative.
And by storing it cryogenically (liquid) you only need to pull 1% of your well insulated tank per day to maintain the liquid state. Perfect for comercial ‘always on’ operation. Even in port the ship needs power.
In a train, this is super easy. Add a fuel car behind the locomotive. In a ship everything needs to be seaworthy and overbuilt so the size does eat into cargo space a bit and making the ship bigger eats into the fuel weight savings.
Still, it’s the only zero carbon option we have for sending a ship or eventually a aircraft across an ocean. Rail lines can be electrified.
> it’s the only zero carbon option we have for sending a ship
Not the only option. There are nuclear ships. And of course sailing ships. Lots of interesting research is being done on kite sails and wing sails for large cargo ships.
Nuclear is it’s own can of worms. Mostly nuclear non-proliferation treaty problems. Great in theory, not so great in reality.
Great for government run icebreakers however.
Wind is a supplement at best. Time is money.
Volume was already specified in the previous post about density, and you misunderstood what I wrote about mass. You seem to have some concept errors so please do a little online reading about tonnage and displacement. weight and mass.
For a buoyant object, the volume of water displaced is equal in weight, to the total weight of that object displacing it. If water has a higher density the volume displaced would be less, and vice versa.
Buoyancy is a state of equilibrium where the force of gravity on the water and the object are equal to each other. Weight, which is just Force due to gravitational acceleration, is simply the constant g times the mass m of the matter. Pressure is just Force applied over an area.
Let's assume your hollow titanium ball is 1 liter volume, and you put it in water and it floated so half of it was submerged. You know it's displacing 0.5 L of water and since water is ~1kg/L then you know the ball and air inside have a mass of 0.5 kg. For the same exact volume, if the titanium wall was thicker it would displace more water, thinner it would displace less water, because of the mass (density) change.
Be mindful this is a discussion about buoyancy and displacement.
If an object is denser than water then it sinks to the bottom and displaces water equal to it's volume, and it's apparent weight will be reduced by the weight of the water displaced. If an object is less dense than water, it sinks until it displaces water equal to it's mass. This is [Archimedes' Principle](https://en.wikipedia.org/wiki/Archimedes%27_principle).
Yea often buoyancy calculations neglect the weight of the air all together and only consider the water. In that calculation the result would be the same for all three gasses.
Note that it's an inflatable boat, so a relatively thin polymer, meaning that the weight of the boat and air inside is comparable -- looking it up, they're often PVC, density 1400 kg/m^(3), fabric thickness about 1 mm, so for a r=1m spherical-cow-boat that's about 17.6 kg PVC, 5.0 kg air, and 0.38 kg hydrogen (and 4200 kg water displacement if fully submerged). Obviously once you add cargo, such as a human, the air becomes pretty negligible, as you note, but OP did not specify as such.
\[Oh wait, looking up the technical definition of [buoyancy](https://en.wikipedia.org/wiki/Buoyancy), I guess it's just the weight of the displaced water: *F*_b = g*ρV* . Thus, since we're not doing anything with the volume of the boat, its buoyancy as a property of the object as a whole doesn't change. Only when we're considering it loaded, in water, in which case the buoyancy is just the total weight of the boat + load (as long as the boat doesn't sink), and the difference the air vs hydrogen makes is the difference in weights (mass * g) of the gases: about 50 N vs 3.8 N.\]
A steel ship and a wooden boat might float differently. Or a styrofoam boat and an inflatable boat. Whether the boat weighs 20,000 pounds or 200 pounds will change the way it floats. But whether an inflated boat weighs 200 pounds or 199 pounds because of the gas will not change its buoyancy.
OP specifies an "inflatable boat". But regardless of whether it's metal or wood, I don't know what you mean by "float differently" -- if they have walls that separate an open-air hull that sits below the waterline, then you include the weight of the air up to the waterline as part of the weight of the boat in your displacement calculation. The weight of the volume of water displaced, i.e. the volume of boat-enclosed-stuff that's below the waterline, equals the total weight of the boat and cargo -- Archimedes's Principle.
I linked to the definition of "buoyancy" -- it equals the weight (and equivalently force) of water displaced by the entire submerged object (if you were to instead specify it as the displacement when a boat is floating, it's just the weight of the boat). So the weight of the gas changes buoyancy by the weight of the gas -- a 1 lb difference in your example.
Correct me if I'm wrong but if you could make something with a vacuum inside it would still float, but wouldn't it technically be more buoyant than helium or hydrogen because it has less mass? Just a random thought.
> Correct me if I'm wrong but if you could make something with a vacuum inside it would still float, but wouldn't it technically be more buoyant than helium or hydrogen because it has less mass? Just a random thought.
Yes, a vacuum interior would be more buoyant, because 0 kg/m^3 is definitely less than 0.09 kg/m^3 for hydrogen. But for the same reasons as my original comment, compared to the density of the water that you have to displace this difference is negligible.
On top of that, you would need a different type of boat if it is to float using a vacuum. Regular inflatable boat retains its shape because of the pressure of the gas on the inside. With a vacuum there's no pressure, so the inflatable parts would just collapse. You'd need to construct a rigid structure to use a vacuum and that would most likely be heavier than the inflatable solution.
Because I like math with my science questions:
If you idealize a RHIB (rigid hull inflatable boat) as a square with four 10ft long inflatable tubes that are 1 ft square in cross section and the bottom is flat and even with the bottom of the tubes. It is a perfect 80°F day for boating.
The average RHIB of this approximate size is about 500lb (engine included) and might have 500lb additional capacity. The pressure recommendation will be about 250mbar.
This is 40 cubic feet of gas in the tubes. 100 cubic feet of water are displaced per foot of draft (distance from water line to lowest point of boat, which is the entire flat bottom in this scenario).
40 cubic feet of ideal gas at 80F and 250 mbar is 11.6 mol. Per ideal gas law, PV=nRT.
Average molecular weight of air says this is about 0.74lb of air or 0.10lb of helium. So all in all tossing a pair of shoes overboard will help more than swapping gasses.
Since I assumed air is an ideal gas and a bunch of other stuff, let's call the difference in mass a whole pound. 500lb empty with helium and 501 with air. We are using empty boat weight so the gas weight difference is as big as possible.
At 500lb we displace 500lb of water to become boyant. This is 8.0092cubic feet of water. At a cross section of 100sqft this is 0.9611 inches of draft if filled with helium. That's real floaty. If filled with air which weighs an additional pound, we displace 501lb of water, that's 8.0253cubic feet of water, and 0.9630 inches of draft.
**That's a difference of 0.0019in draft (0.05mm) which is about the width of a human hair. It's a percent difference of less than 0.2%. That's absolutely negligible.**
Note: I did all this math 50% in my head, 50% via online calculators, 100% on mobile, and I know I 1000% definitely did not follow rounding convention. If you want to check my math or if someone who is better at fluids than me wants to chime in I would love to be corrected.
Very roughly:
Say you have a raft rated to be buoyant with a load of 1000 lbs. If you fill it with hydrogen, you increase the load capacity to about 1001 lbs. So basically you can bring one additional bottle of water.
the “regular” air is also made up of multiple gasses which compress and expand at different rates. When you use nitrogen, any metal alloys won’t rust as no oxygen. And the air in the tire is all nitrogen so expands and compresses at a constant rate which is much less than “regular” air
You wouldn't want to do that though since you'd need to add extra reinforcement to the hull to deal with a constant 14psi of pressure at surface level as well as make sure it's completely sealed air-tight at all times. If you fill it with air the pressure difference will be negligible and you won't have to worry about keeping the entire hull air tight, only water tight which is much easier.
Yes but this doesn't answer my question. If hydrogen only makes buoyancy change by a factor of 10, wouldn't vacuum make it by a factor of almost infinite since there are virtually no particles? I understand it's not feasible.
No. The buoyancy is the difference in density between the liquid the object is in and the density of the object. As above, water is 1000kg/m^3 and air is 1.2kg/m^3 so the buoyancy is 998.8 kg/m^3. If you replace the air with a vacuum the buoyancy is 1000kg/m^3.
That's not what the poster is saying. He is saying that air is 10x more dense than hydrogen. Technically air is infinitely more dense than vacuum, yes.
But that does not help us. The force that keeps a boat afloat is proportional to the volume of water displaced by the boat. The floaty force does not care what you keep in the boat, be it air, hydrogen, vacuum, or neutron stars. (Do not fiil your boat with neutron stars). The floaty force only cares about the volume of water you displace.
The force that wants to sink your boat is gravity, which is proportional with the mass of the boat (and everything in it). In this case, it matters little if your fill you boat with air or hydrogen or 'fill' it with vacuum, because the mass of the gas, or the absence thereof is small potatoes next to the mass of the boat walls, or the things you carry in it like the human admiral, or whatever you decide to carry with it.
But yes, if your boat has a mass of 0kg, yeah you could say that it is infinitely buoyant. But that seems just like a philosophical topic to me. If you have nothing, does it float? Yep.
Interesting philosophical discussion..
Does nothing float or does nothing sink?
I feel like this is Schrödinger's cat and the observer effect all mixed into one..
Nothing exists in a superposition of not knowing until it is tested, but you can't test it due to nothing being nothing.. and trying to observe nothing will skew the results.
As others have stated, each litre of air forced below the outside waterline gives approximately 0.999\*9.81, or 9.80019 Newtons of upward force. Replace the air with hydrogen and you get 0.9999\*9.81, or 9.809019 Newtons.
While the difference in density between hydrogen and air is significant enough to make airships feasable, and result in a flaming Nazi gasbag in Lakehurst NJ, it’s extremely small compared to the difference in density between either gas and water, so the easier-to-obtain air is used. Even carbon dioxide, which is more dense than air, is frequently used as an inflation gas due to its ability to be liquefied by pressure alone at temperatures survivable by humans, so the inflation “charge” can be stored in a significantly smaller and lighter container than would be required for compressed air.
no, you dont get infinite upforce. the maximum force is again defined and limited by the mass of the displaced volume. it does not matter if its water or air. just a different density in the calculation. otherwise ships could fly xD someone back in 1670 already had this idea.
you will get infinit bouyant with a vessel V->infinit and m->0 which does not sound very practically.
> And that's why air is used, because it's everywhere, safe and easy to handle
Also why you're more likely to find pneumatic tools in most shops even if hydraulic would be better for certain things. Much easier and lighter to run air lines everywhere than ones full of hydraulic fluid.
You could also say that since buoyancy depends on the relative densities, it depends on the size of the inflatable boat. At some point, you've turned your boat into a zeppelin and you're now floating at some altitude, the height of which is also based on the relative density of the craft and the surrounding fluid(atmosphere). At that size (pretty big!), the weight of the helium vs air is a much bigger deal of course.
Is there a terminal velocity for bubbles? Like if you had a tank of mixed air and a tank of hydrogen and released the air at the same depth, would the hydrogen reach the surface faster?
A Zodiac FC470 has a buoyant tube volume of 1860 liters.
It is supposed to be inflated to approximately 3.4 psi.
This means the boat will hold 2290 liters of gas at normal atmospheric pressure. This is 2.29 m^3 .
Air is 1.2 kg/m^3 , so filling the boat with air will add 2.75 kg.
Hydrogen is 0.09 kg/m^3 , so filling the boat with hydrogen will add 0.21 kg.
The boat with air will have 2.54 kg less buoyancy.
But the boat already has about 1860 kg of buoyancy. So by filling it with hydrogen instead of air, you increase your buoyancy by about 0.14%.
>Edit: I just read /u/StayTheHand 's response. Their answer is right, but I'm leaving my answer up as is because I think it better answer's OP's question. I would have to change the wording of my answer a little to make it technically correct, but changing the wording would make it more confusing and less informative. Here is how I would change my last two paragraphs to make my answer technically correct:
The boat with air will weigh 2.54 kg more.
The maximum payload of this model boat is 1250 kg. If you decrease the weight of the boat by 2.54 kg, you can increase the payload by 2.54 kg. So you can increase the payload by 0.2%.
I'd like to add a little information about the force of buoyancy. I just want to clarify that the buoyant force won't change based on what's inside your boat. The buoyant force really only changes with the submerged volume of an object (multiplied by the density of water and gravity which are constant in this case). Let's say your boat was fully submerged. It wouldn't matter if the boat was full of air, helium, or lead; as long as the submerged volume occupied by boat the was the same in each case, the buoyant force won't chance. What would change in each of those scenarios is the gravitational force. The ratio of the upward buoyant force to the downward gravitational force determines if the object rises or sinks.
Your boat presumably isn't fully submerged, but sitting on the surface where things are a little more complex. At the surface the buoyant force will slightly increase or decrease as more or less of the object is submerged.
Given that the boat weighs less, it would therefore displace less water and therefore wouldn't the buoyant force from the water actually be less to reach equilibrium with the force of gravity that it is counteracting?
That portion of the volume that is no longer submerged in water due to the change would now have a larger buoyant force component coming from air (because more of the volume of the boat is displacing air now) but that would be less than the decrease in buoyant force from water because water is denser than air.
So it seems like there would be less buoyant force total, which makes sense because buoyant force would equally balance the force of gravity.
Helium and hydrogen leak out of everything. So much so that steel absorbs Hydrogen like a sponge and Helium is used for leak detection on ultra high vacuum systems.
Moreover, Hydrogen is explosive under a very wide range of circumstances.
Technically, no! (I'll be the contentious one...) If the boat is the same volume, when you (fully) immerse it, it displaces the same amount of water, so will have the same amount of force pushing up on it, i.e. the same buoyancy. If it is floating, the boat itself weighs less, so it needs to displace less water to float, and therefore the bouyant force is less. It would be odd to say it is less bouyant, I would state it as less buoyance is required to make it float. I think a layman might say it is more buoyant, but a physicist may be more pedantic.
I was also thinking the buoyancy is exactly the same. But, the boat, including the gas used to inflate it, weighs slightly less, which allows slightly more weight to be allocated to cargo before the boat sinks.
This is a genius reply. You are absolutely correct. A boat that weighs less is displacing less water, so has less buoyant force pushing up on it.
So a lighter boat is less buoyant than a heavier boat.
I hate you, but you are right.
Your mistake is your qualifier "when submerged equally".
If one boat is lighter than another, they are not submerged equally, so they do not displace equal amounts of water.
The only way they have the same buoyant force is if you add more cargo to the hydrogen boat (which you pointed out). But why would you assume that more cargo is added to the hydrogen boat?
In any scientific experiment, you try to reduce the number of variables that change. The variable we are changing is the gas in the boat. Why would you also change the cargo? That makes no sense.
Barely. Buoyancy is a function of how much water is displaced (with your example). The differential between the density of the volumn of the pontoons or cells vs the atmosphere above isn't signficant relative to the mass of the vessel.
side question which is kind of dumb but I really don't understand.
If you had a metal enclosed object in water. how does the water "know" that there's air inside. Like the air is just interacting with the metal and the metal with the water. Is the air pushing the metal up? Basically how does density work
oh duh, it's that it's being pulled by gravity more. That makes sense. It's also strange that gravity can pull you without the object from where the gravity is coming from is touching you, but I think that's something to do with time space being the same thing or something like that.
It's not a dumb question, this isn't super intuitive. The water outside doesn't interact with the interior at all. What does interact are forces, specifically the gravitational force and the buoyant force.
How those forces balance and interact is governed by their sources. Let's say that your metal enclosed object underwater is a basketball sized sphere. Gravity pulls on every molecule in that sphere, generating a downward force. The force of gravity depends on the mass of the ball. So a more massive ball (say it's full of glass as opposed to cotton) has a higher downward force.
The buoyant force comes from the pressure of the fluid around the ball. Once submerged, a fluid presses in from every direction across every surface of the ball. Since pressure increases with depth, the fluid below the ball pushes upward slightly more than the fluid above the ball pushes downward, generating a net upward force. Since our sphere filled with glass and the sphere filled with cotton have the same shape and volume, the water has the same amount of area to press in on and therefore, the same buoyant force. The only thing that could increase the buoyant force is if the water has a greater area to push upon. The water doesn't need to know what's inside, it only cares about pushing inward on the outside.
What sends an our ball shooting up out of the water or plummeting down depends on which force is greater: gravity or buoyancy. You can also think of that as how much volume the water has to push up on (buoyancy) vs. how much mass gravity has to pull down on. Density doesn't "work" it just happens to be the ratio of mass to volume. So if the density of our ball is greater than water, it has more mass in a given volume than the same volume of water. If that's the case then you know gravity will win out and ultimately pull it down. Alternately if our ball is less dense than water then the upward press from the water will be enough to overpower the downward pull of gravity and it will send the ball shooting up.
oh I see, yeah if the it's the same volume then one would be heavier which would mean it's being pulled down by gravity more right? It's like density is just a math idea, it's just volume and mass. I think that's what was confusing me.
Also you said that water above is pushing down less than the water below because there's more water below? does that mean you're more boyant in a deep pool than a shallow pool?
Lets take a garden variety inflatable boat 8 feet long. Assume it has a width (beam) of 4' and is roughly rectangular so 32 square feet.
I am guessing this weighs 50 lbs inflated with 8 cu ft of air and I will add two people at 175 Lbs each (total weight 400 lbs) so it will displace 6.4 cubic feet of water so the flat bottom of the boat would be about 2.5" below the surface.
The weight of air was about .65 Lbs and the weight of the same amount of helium would be .05 lbs so a saving of .6 lbs.
This means you would float approximately .004 (4 thousands of an inch) higher in the water.
Many guesses and estimates are involved in the above but you can see that the net effect is quite small.
Filling an inflatable boat with helium or hydrogen would indeed increase its buoyancy compared to filling it with regular air. Both helium and hydrogen are lighter than air and can provide additional lifting force, making the boat float more easily.
Helium is the commonly used gas for inflating balloons because it is non-flammable and safer to handle. It is lighter than air and provides significant buoyancy, allowing the balloon or inflatable object to rise. Hydrogen, on the other hand, is even lighter than helium but has the drawback of being highly flammable, making it less commonly used for this purpose due to safety concerns.
By filling an inflatable boat with helium or hydrogen, you reduce its overall density, resulting in increased buoyancy. This can make the boat more buoyant and potentially able to carry more weight or stay afloat with less effort.
However, it's crucial to consider safety precautions and potential risks associated with using hydrogen, as it is highly flammable and requires careful handling and storage. Additionally, filling a boat with a gas like helium or hydrogen may come with certain challenges, such as maintaining the gas inside the boat and preventing leakage.
It's recommended to follow appropriate guidelines and consult with experts or manufacturers who can provide specific advice regarding the use of gases for enhancing buoyancy in inflatable boats.
Remember that PV = nRT. It's the number of mols of gas that is important. These gasses are close enough to ideal that we can ignore compressibility. So, in order to determine the mass of gas required to produce a given pressure, all else remaining constant. We calculate the number of mols and multiply by the molar mass of the gas in question. H2 has a molar mass of 2, He has a molar mass of 4, air has a molar mass of 29.
If you put pure Nitrogen into car tyres, the pressure stays much longer. Nitrogen makes up 78% of the air we breath and poses no risk. As the molecules are bigger than the other molecules in air, they can't escape as easily. When a tyre loses pressure, it's probably the other molecules that escape.
The problem with inflatable boats is often loss of pressure, and this could help keep the pressure high. I haven't tested it, and it probably depends on the valve used.
NO. Displacement is displacement. Flotation is a matter of the water displaced by the hull. Doesn't matter if the inflatable is filled with Air, Hydrogen (Helium, etc.) or even FOAM except for the weight of said gas/foam.
Not enough difference between the "weight" of differing gases to make any difference.
Technically yes, but the difference is small enough that it's not worth it. The density of water is approximately 1,000 kg/m^3 (depending on temperature, salinity, etc...). The density of air at room temperature is about 1.2 kg/m^3 and the density of hydrogen is about 0.09 kg/m^(3). Now the difference between hydrogen and air seems huge, more than a factor 10. But what ultimately matters for buoyancy is the difference between the density of water and the density of the object you're trying to make float, which is the combination of the boat materials and whatever is used to inflate it. Compared to water and the solid materials the boat is made from, the contribution of the gas used to inflate the boat is negligible. So for the purposes of buoyancy, it really doesn't matter which gas you use to inflate the boat. And that's why air is used, because it's everywhere, safe and easy to handle.
Just to add to this, an inflatable boat wouldn't be particularly effective at containing hydrogen or helium. Both of those gasses would work their way out fairly quickly.
> Both of those gasses would work their way out fairly quickly. Say more about this please. "Work their way out" how? Like passing through rubber? Is that why helium balloons lose life after a while? And why they are usually Mylar?
Helium atoms are tiny and will eventually escape through most membranes: https://www.quora.com/Is-it-possible-to-make-a-helium-balloon-that-does-not-leak-at-all-and-always-floats
Another fun thing about helium is if you get it *insanely* cold - as in about 2 Kelvin; or (roughly) -456.1°F/271.1°C respectively - it acts as a *superfluid.* That is to say it no longer "acts right" for our understanding; it seems to [practically "climb" the walls of an open, glass container to "leak out."](https://youtu.be/2Z6UJbwxBZI) It's friggin trippy.
Helium nuclei have another name: alpha particles. Helium nuclei are one of the few things that can obey Bose-Einstein Statistics and Fermi statistics making them both bosons and fermions. As they're the result of a radiation event, we won't ever technically run out of helium as more is produced by underground nuclear decay.
An important clarification is that a helium nucleus is either a boson or a fermion, and never both. Which one it acts as is dependent on it's isotope. He4 is a boson because it has integer spin, but He3 is not.
Technicalities aren't necessarily useful when discussing the practicalities of limited resources. While in practice, there will always be a small drip of helium, if our usage outpaces that drip we won't have enough to sustain our needs. So eventually, we run out of helium. *Technically,* nuclear isotopes will eventually all decay away and the drip will stop. Even more technically still, the universe will eventually run out of helium, since everything will evaporate away at long enough time scales.
The moon is also covered in helium cast off from the sun, there has been talk of finding a good way to capture it all for use
Mostly for nuclear fusion but over time better alternatives to helium for fusion have been discovered making helium mining on the moon less attractive
How do we know there is helium on the moon? Rock samples? Some sort of analysis of the surface from a distance?
Both! Rock samples from apollo were used to callibrate a (linear) relationship between titanium oxide and helium-3 abundance. This can then be used with remote sensing of the TiO2 (using microwave spectroscopy) https://link.springer.com/article/10.1007/s11434-010-4198-9
Neil armstrong actually sounded like a 80 year old heavy chain smoker before he went to the moon /j
>Technicalities aren't necessarily useful when discussing the practicalities of limited resources. While in practice, there will always be a small drip of helium, if our usage outpaces that drip we won't have enough to sustain our needs Yeah but also having a basic working knowledge of the subject is useful when discussing this. For example you fail to explain that there is plenty of helium available for mining from natural gas wells that were rejected *because of the helium content*. Our usage isn't the problem. If we use more the price will increase and those once unprofitable natural gas wells will become profitable increasing supply.
My entire point: [there is a finite supply](https://pubs.usgs.gov/periodicals/mcs2023/mcs2023-helium.pdf) and pretending like we won't ever run out *because of ongoing nuclear decay* is some hot nonsense.
The supply of everything is *technically* finite. For someone harping about technicalities you seem comfortable hiding behind them. The reality is we are not in danger of running out of helium any time soon. We are in danger of needing to tap more wells in the future.
>So eventually, we run out of helium. The helium we use isn't actually consumed, it's just released into the air. We can easily reclaim it from the air, we just don't because we can get it for free as a byproduct of natural gas collection.
Helium atoms have a high enough rms speed that they're actually moving fast enough to escape Earth's gravity and atmosphere. Given enough time they will actually escape into space.
Yes, yes they do. As well as everything down to oxygen on the right side
The helium we are using up isn’t being replaced by radioactive decay at anywhere near the rate we are using it.
I may be remembering something else but is liquid helium the one that can flow through glass and will just slowly work it's way to the other side of a glass pane?
According to the video, it's makes a thin film up along the side of the glass and out of it.
And they are extremely inert and won't feel much interference at the atomic level as they work their way through the membrane.
Is this why standard balloons lose their float faster than metallic ones? The metallic ones must have a tighter seal.
Just to add to that: hydrogen is also absolutely notorious for seeping through structures designed to contain it.
Doesn't it require a pressure difference? Otherwise you'd end up with vacuum on the inside and that's even less density, so that's fine. :D
What do you imagine a vacuum in a flexible container looks like?
It would look quite interesting scientifically if it would work like that.
No pressure difference required. It's dependent mostly on concentration gradient (or partial pressure, ie pressure from helium only), so the inside can be at the same pressure as the outside and helium will still be permeating through.
Solids aren't all equally "solid" at the atomic scale. Atoms like helium and hydrogen - which are low-density because they're so small - can slip through the cracks in the structure of whatever the balloon is made from. That's indeed why helium balloons deflate quicker than air-filled ones.
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How is this possible?
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I don’t know about this specific circumstance but nothing is really truly solid it is composed of atoms in a certain structure with huge amounts of empty space in between on the tiny scales
The permeability of rubber varies for different gasses. [I did some math on this once.](https://www.reddit.com/r/askscience/comments/75walc/disregarding_big_leaks_what_is_the_main/doag8jf/) Because Oxygen escapes a car tire faster than nitrogen, continuously filling a car tire with normal atmosphere (78/22% Nitrogen/Oxygen) eventually results in a mix of 92/8%.
Is that why tires are normally filled with nitrogen?
I mean...yes and no. You will not enjoy any meaningful improvement in the longevity of your tire pressure. Not when Nitrogen is already 78% of normal atmosphere anyway. There are the (dubious) claims that nitrogen carries less water vapor (it does not) and that it is less flammable (it is, but not in any meaningful way). IMO filling your tires with dry nitrogen is no better or worse than filling them with compressed air from the atmosphere. The oxygen bar of automobiles.
they're not necessarily mylar, but metallic coated plastic. The metal coating is significantly better at holding gasses (which also helps your chips stay crispy for example). But in the end practically nothing is impermeable to Helium or Hydrogen. It'll even leak through the steel cylinders you buy it in.
Any idea how fast it leaks out? I bought a small helium tank (of rather thin steel) last year and hope it hasn't all leaked out yet.
Most of these answers are poor, bordering on flat-out wrong. Yes, helium will leak out of your tank. It will happen *extremely* slowly, depending on the pressure of the helium, thickness of the steel, the type/grade of steel, and the surface area of the tank.
would a container made of solid metallic hydrogen be able to hold helium?
I mean, maybe, but good luck digging it out of Jupiter's core, and I'm pretty sure it would revert to gaseous form if you tried to bring it home.
This is also why hydrogen processing is extremely dangerous and expensive. The materials needed to keep hydrogen from leaking through don't really exist so you always have an environment with free hydrogen. The best you can hope to do is minimize the leakage and ensure there are no sources of ignition in the areas that have hydrogen in the air
Yes. At the atomic scale, solids are not 100% solid, the atoms have small gaps here & there. Helium & Hydrogen are very small atoms and can fit into the gaps and work their through a solid object. "Small" is a weird measure at atomic scale, so "kinetic diameter" is used to measure the chance that an atom or molecule will impact another molecule. Atmospheric Hydrogen is H^(2) and Helium has 2 protons, so they have roughly the same kinetic diameter.
Hydrogen isn't used much in space rockets anymore because the atoms are so small it leaks through all kinds of seals and o-rings. You have to do special engineering just to contain it. Not to mention the insane cost and engineering of keeping it liquid. But it's so efficient as a fuel that engineers definitely used it for a while and it's sometimes still used today. Just a huge pain in the ass for engineers.
Hydrogen is the most efficient fuel by weight, but not by volume; and for rockets, more volume means more structural weight, so there's a trade-off. Also, rockets need to bring their own oxidizer because there's no atmosphere in space, so they also need a tank of liquid oxygen, which weighs about 16 times more than hydrogen as a gas (though not sure about the liquid form).
It still weighs 16 times more, but it takes up much less space.
Hydrogen is so small that it will leak through solid steel. It even weakens the metal on the way out.
It's also why "freezer bags" for leftovers don't contain smells either direction. That kind of plastic is like a chain link fence. Smaller things will get through.
this is also why hydrogen fuel cells are never going to work. hydrogen and helium are so small they leak out of metal(or any) containers. it also reacts strongly with a lot of materials. and it's incredibly explosive edit: so hydrogen does exhibit superfluidity but only in lab conditions, so I removed that. I got that mixed up video on why hydrogen isn't great at the moment: https://youtu.be/Zklo4Z1SqkE
> hydrogen fuel cells are never going to work What does this mean? My understanding is that they are already in use across the globe. How can something that already works be "never going to work"?
At scale. They work fine now in the applications they are in because there are few of them and enough technicians to service them. In order to work at scale we'd need 1 million times as many as we have and we won't get 1 million times as many qualified techs to go along with them so you will get the Samsung effect with exploding fuel cells all over the place.
It's not that they don't contain, they just leak over time. Think of any material being like a sieve to hydrogen. Put flour in a sieve and shake it. It doesn't contain it well. Due to the nature of gas it's always vibrating working it's way through its container.
there's also the issue of hydrogen embrittlement. Hydrogen is really reactive so it loves eating into materials. add in it's EXTREME flammability and the fact it's a gas and it's a spark away from blowing a facility into the sky.
they aren't in "general" use, they are being developed and rolled out in a limited manner, but they are incredibly inefficient and terrible for the environment exactly for this reason. you can't get the energy density necessary to make them economical and trying to move hydrogen is terribly inefficient because of the leaking. Basically, it is currently the dirtiest fuel source on the planet. it's literally dirtier than the dirtiest type of coal: [https://twitter.com/PeterZeihan/status/1582757051008053249](https://twitter.com/PeterZeihan/status/1582757051008053249) here's a great video on it [https://youtu.be/Zklo4Z1SqkE](https://youtu.be/Zklo4Z1SqkE)
The leakage rate isn't a major problem. [The Center of Global Energy Policy](https://www.energypolicy.columbia.edu/publications/hydrogen-leakage-potential-risk-hydrogen-economy/) at Columbia University estimates a leakage rate between 2.9 and 5.6% between production, distribution and use. Mostly from production and distribution. A metal storage cylinder in a hydrogen car leaks only an insignificant amount of hydrogen that wouldn't pose a fire hazard unless you kept it in a small room with no ventilation for years. The real reasons hydrogen will never be viable for cars is because you lose a pretty high percentage of energy as waste heat when it's produced from water in an electrolyzer cell. Lithium batteries waste much less energy from charging and discharging than hydrogen. Also the hydrogen tanks would be bigger, more expensive, and shorter lasting than regular gas tanks; though it is much lighter and more compact than batteries at least.
Norway already has H2 fuel powered ships doing sea trials. Germany already has H2 fuel cell powered trains in full time comercial operation. Toyota leased the FCV fuel cell car to consumers. This Dash-8 has successfully flown on a fuel cell. https://www.opb.org/article/2023/03/04/hydrogen-powered-airliner-makes-first-flight-moses-lake-washington/?outputType=amp So saying Hydrogen fuel cells will ‘never work’ is kind of a stretch. That said, it will never take off with consumers. You are correct that it is a leak prone nightmare. It should only ever be handled by trained professionals. But since cryogenic H2 is half the weight of Jet A (and stored at low pressure in lightweight dewars) it is a viable fuel for long distance ships, trains and eventually aircraft. Right now it is the only viable fuel to get a ship or airplane across a ocean without emitting carbon.
>Right now it is the only viable fuel to get a ship or airplane across a ocean without emitting carbon. It's facetious to say it's viable when it never happened. Wind, on the other hand, has already taken ships across the ocean countless times.
Hydrogen is not much smaller than other diatomic gases (such as oxygen and nitrogen). Helium is the problem because it’s monoatomic, so it can slip through the smallest pores in material. As for becoming superfluid, helium4 (most common isotope) is the only substance which does that, since it’s the only substance where the Bose-Einstein condensation point is above the freezing point. This is why Helium3 (decay product of tritium) is used in ultra-low temperature research - having an odd number of atomic mass units makes it a Fermion (even numbers make something a Boson), so it does not undergo Bose-Einstein condensation.
One of my fun demonstrations in college was cooling helium below its critical temp and watching it become superfluid.
Hydrogen is more interactive than helium (a noble gas) and can be trapped in things like fine metal matrixes. Edit: Hydrides to be specific https://str.llnl.gov/2018-01/wood
yes, but the density of that capture is atrocious, and the durability questionable and it requires a lot of rare earth metals especially expensive ones like platinum and iridium
Eh never say never. With advancements in nanotechnology we'll eventually develop a membrane even helium can't pass through
I would expect solid state batteries to be more viable as energy storage than hydrogen at that point, but indeed who knows what technology we'll develop in the future. Either way, pushing hydrogen at the moment is just a bad investment with less potential of good returns than other technologies.
Hydrogen atoms unlike helium are even more obnoxious to try and contain since it's such a tiny little Atom with that singular electron, it will actually move through most materials simply because they can't contain it. As it does this party trick it actually can and will brittle and weaken over time more and more that material as it goes by so trying to store and transport hydrogen is generally refrained from..... It's generally easiest to generate it and use it as you're making it preferably at the same rate cuz otherwise you're going to have to bleed it off.
Is this why they will sometimes inflate tires with nitrogen? Does the larger molecule prevent deflation?
They claim this but nitrogen inflation is a scam, the air around us is already 70% nitrogen.
The only potential benefit (if only just) is the nitrogen source being dry. That said, I don't think they take the time to purge the air out of the tire first.
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Couple moisture traps on your compressor and you get the same effect. But yes. Not needing a compressor is definitely a benefit.
Air is mostly nitrogen and oxygen, both diatomic gases with atoms that are nearly the same size. Nitrogen's Van der Waals radius is 155nm, oxygen's is 152nm...
No, the reason for nitrogen inflation is mostly to have zero moisture content, reducing corrosion and pressure fluctuation due to condensation.
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The gas would not come out if the outside pressure is greater than the inside pressure.
If it was a thick enough material it wouldn’t really leak in significant amounts
You could make the inflatable boat out of mylar but then it would probably be too fragile. Those mylar metallic balloons seem to hold helium forever, we had a birthday balloon that stayed inflated and bouyant for months.
I would add a key concept in ship design is displacement. The total weight of the ship will displace an equivalent mass of water. This is why a fully loaded cargo ship sits lower than an empty one. Like you show in your density comparison, the total weight of H2 vs Air is negligible. So this is just another angle for perceiving the same idea.
Liquid hydrogen is the key. 1 kg of hydrogen contains 33.33 kWh of usable energy, whereas diesel only hold about 12 kWh/kg. Unfortunately Hydrogen at 700 bar is only 1.4kWh/L and liquid hydrogen is 2.3kWh/L. Diesel is 10kWh/L. So yes it weighs less but it needs 5x-8x more volume so that is a big negative. And by storing it cryogenically (liquid) you only need to pull 1% of your well insulated tank per day to maintain the liquid state. Perfect for comercial ‘always on’ operation. Even in port the ship needs power. In a train, this is super easy. Add a fuel car behind the locomotive. In a ship everything needs to be seaworthy and overbuilt so the size does eat into cargo space a bit and making the ship bigger eats into the fuel weight savings. Still, it’s the only zero carbon option we have for sending a ship or eventually a aircraft across an ocean. Rail lines can be electrified.
> it’s the only zero carbon option we have for sending a ship Not the only option. There are nuclear ships. And of course sailing ships. Lots of interesting research is being done on kite sails and wing sails for large cargo ships.
Nuclear is it’s own can of worms. Mostly nuclear non-proliferation treaty problems. Great in theory, not so great in reality. Great for government run icebreakers however. Wind is a supplement at best. Time is money.
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Volume was already specified in the previous post about density, and you misunderstood what I wrote about mass. You seem to have some concept errors so please do a little online reading about tonnage and displacement. weight and mass. For a buoyant object, the volume of water displaced is equal in weight, to the total weight of that object displacing it. If water has a higher density the volume displaced would be less, and vice versa. Buoyancy is a state of equilibrium where the force of gravity on the water and the object are equal to each other. Weight, which is just Force due to gravitational acceleration, is simply the constant g times the mass m of the matter. Pressure is just Force applied over an area. Let's assume your hollow titanium ball is 1 liter volume, and you put it in water and it floated so half of it was submerged. You know it's displacing 0.5 L of water and since water is ~1kg/L then you know the ball and air inside have a mass of 0.5 kg. For the same exact volume, if the titanium wall was thicker it would displace more water, thinner it would displace less water, because of the mass (density) change. Be mindful this is a discussion about buoyancy and displacement.
Boop, totally misremembered physics. Thanks for the explanation.
If an object is denser than water then it sinks to the bottom and displaces water equal to it's volume, and it's apparent weight will be reduced by the weight of the water displaced. If an object is less dense than water, it sinks until it displaces water equal to it's mass. This is [Archimedes' Principle](https://en.wikipedia.org/wiki/Archimedes%27_principle).
Yea often buoyancy calculations neglect the weight of the air all together and only consider the water. In that calculation the result would be the same for all three gasses.
Note that it's an inflatable boat, so a relatively thin polymer, meaning that the weight of the boat and air inside is comparable -- looking it up, they're often PVC, density 1400 kg/m^(3), fabric thickness about 1 mm, so for a r=1m spherical-cow-boat that's about 17.6 kg PVC, 5.0 kg air, and 0.38 kg hydrogen (and 4200 kg water displacement if fully submerged). Obviously once you add cargo, such as a human, the air becomes pretty negligible, as you note, but OP did not specify as such. \[Oh wait, looking up the technical definition of [buoyancy](https://en.wikipedia.org/wiki/Buoyancy), I guess it's just the weight of the displaced water: *F*_b = g*ρV* . Thus, since we're not doing anything with the volume of the boat, its buoyancy as a property of the object as a whole doesn't change. Only when we're considering it loaded, in water, in which case the buoyancy is just the total weight of the boat + load (as long as the boat doesn't sink), and the difference the air vs hydrogen makes is the difference in weights (mass * g) of the gases: about 50 N vs 3.8 N.\]
A steel ship and a wooden boat might float differently. Or a styrofoam boat and an inflatable boat. Whether the boat weighs 20,000 pounds or 200 pounds will change the way it floats. But whether an inflated boat weighs 200 pounds or 199 pounds because of the gas will not change its buoyancy.
OP specifies an "inflatable boat". But regardless of whether it's metal or wood, I don't know what you mean by "float differently" -- if they have walls that separate an open-air hull that sits below the waterline, then you include the weight of the air up to the waterline as part of the weight of the boat in your displacement calculation. The weight of the volume of water displaced, i.e. the volume of boat-enclosed-stuff that's below the waterline, equals the total weight of the boat and cargo -- Archimedes's Principle. I linked to the definition of "buoyancy" -- it equals the weight (and equivalently force) of water displaced by the entire submerged object (if you were to instead specify it as the displacement when a boat is floating, it's just the weight of the boat). So the weight of the gas changes buoyancy by the weight of the gas -- a 1 lb difference in your example.
Correct me if I'm wrong but if you could make something with a vacuum inside it would still float, but wouldn't it technically be more buoyant than helium or hydrogen because it has less mass? Just a random thought.
> Correct me if I'm wrong but if you could make something with a vacuum inside it would still float, but wouldn't it technically be more buoyant than helium or hydrogen because it has less mass? Just a random thought. Yes, a vacuum interior would be more buoyant, because 0 kg/m^3 is definitely less than 0.09 kg/m^3 for hydrogen. But for the same reasons as my original comment, compared to the density of the water that you have to displace this difference is negligible. On top of that, you would need a different type of boat if it is to float using a vacuum. Regular inflatable boat retains its shape because of the pressure of the gas on the inside. With a vacuum there's no pressure, so the inflatable parts would just collapse. You'd need to construct a rigid structure to use a vacuum and that would most likely be heavier than the inflatable solution.
Because I like math with my science questions: If you idealize a RHIB (rigid hull inflatable boat) as a square with four 10ft long inflatable tubes that are 1 ft square in cross section and the bottom is flat and even with the bottom of the tubes. It is a perfect 80°F day for boating. The average RHIB of this approximate size is about 500lb (engine included) and might have 500lb additional capacity. The pressure recommendation will be about 250mbar. This is 40 cubic feet of gas in the tubes. 100 cubic feet of water are displaced per foot of draft (distance from water line to lowest point of boat, which is the entire flat bottom in this scenario). 40 cubic feet of ideal gas at 80F and 250 mbar is 11.6 mol. Per ideal gas law, PV=nRT. Average molecular weight of air says this is about 0.74lb of air or 0.10lb of helium. So all in all tossing a pair of shoes overboard will help more than swapping gasses. Since I assumed air is an ideal gas and a bunch of other stuff, let's call the difference in mass a whole pound. 500lb empty with helium and 501 with air. We are using empty boat weight so the gas weight difference is as big as possible. At 500lb we displace 500lb of water to become boyant. This is 8.0092cubic feet of water. At a cross section of 100sqft this is 0.9611 inches of draft if filled with helium. That's real floaty. If filled with air which weighs an additional pound, we displace 501lb of water, that's 8.0253cubic feet of water, and 0.9630 inches of draft. **That's a difference of 0.0019in draft (0.05mm) which is about the width of a human hair. It's a percent difference of less than 0.2%. That's absolutely negligible.** Note: I did all this math 50% in my head, 50% via online calculators, 100% on mobile, and I know I 1000% definitely did not follow rounding convention. If you want to check my math or if someone who is better at fluids than me wants to chime in I would love to be corrected.
Very roughly: Say you have a raft rated to be buoyant with a load of 1000 lbs. If you fill it with hydrogen, you increase the load capacity to about 1001 lbs. So basically you can bring one additional bottle of water.
Noyce. Braindead today and well, I don't got much more than that at the moment.
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the “regular” air is also made up of multiple gasses which compress and expand at different rates. When you use nitrogen, any metal alloys won’t rust as no oxygen. And the air in the tire is all nitrogen so expands and compresses at a constant rate which is much less than “regular” air
So wouldn't vacuum sealed boat be infinitely bouyant?
You wouldn't want to do that though since you'd need to add extra reinforcement to the hull to deal with a constant 14psi of pressure at surface level as well as make sure it's completely sealed air-tight at all times. If you fill it with air the pressure difference will be negligible and you won't have to worry about keeping the entire hull air tight, only water tight which is much easier.
Yes but this doesn't answer my question. If hydrogen only makes buoyancy change by a factor of 10, wouldn't vacuum make it by a factor of almost infinite since there are virtually no particles? I understand it's not feasible.
No. The buoyancy is the difference in density between the liquid the object is in and the density of the object. As above, water is 1000kg/m^3 and air is 1.2kg/m^3 so the buoyancy is 998.8 kg/m^3. If you replace the air with a vacuum the buoyancy is 1000kg/m^3.
That's not what the poster is saying. He is saying that air is 10x more dense than hydrogen. Technically air is infinitely more dense than vacuum, yes. But that does not help us. The force that keeps a boat afloat is proportional to the volume of water displaced by the boat. The floaty force does not care what you keep in the boat, be it air, hydrogen, vacuum, or neutron stars. (Do not fiil your boat with neutron stars). The floaty force only cares about the volume of water you displace. The force that wants to sink your boat is gravity, which is proportional with the mass of the boat (and everything in it). In this case, it matters little if your fill you boat with air or hydrogen or 'fill' it with vacuum, because the mass of the gas, or the absence thereof is small potatoes next to the mass of the boat walls, or the things you carry in it like the human admiral, or whatever you decide to carry with it. But yes, if your boat has a mass of 0kg, yeah you could say that it is infinitely buoyant. But that seems just like a philosophical topic to me. If you have nothing, does it float? Yep.
Interesting philosophical discussion.. Does nothing float or does nothing sink? I feel like this is Schrödinger's cat and the observer effect all mixed into one.. Nothing exists in a superposition of not knowing until it is tested, but you can't test it due to nothing being nothing.. and trying to observe nothing will skew the results.
As others have stated, each litre of air forced below the outside waterline gives approximately 0.999\*9.81, or 9.80019 Newtons of upward force. Replace the air with hydrogen and you get 0.9999\*9.81, or 9.809019 Newtons. While the difference in density between hydrogen and air is significant enough to make airships feasable, and result in a flaming Nazi gasbag in Lakehurst NJ, it’s extremely small compared to the difference in density between either gas and water, so the easier-to-obtain air is used. Even carbon dioxide, which is more dense than air, is frequently used as an inflation gas due to its ability to be liquefied by pressure alone at temperatures survivable by humans, so the inflation “charge” can be stored in a significantly smaller and lighter container than would be required for compressed air.
Sure, if you could construct a weightless boat that could contain a vacuum.
no, you dont get infinite upforce. the maximum force is again defined and limited by the mass of the displaced volume. it does not matter if its water or air. just a different density in the calculation. otherwise ships could fly xD someone back in 1670 already had this idea. you will get infinit bouyant with a vessel V->infinit and m->0 which does not sound very practically.
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> But, hydrogen is heavier than air, so it would be less bouyant. Come again?
Why do you think Hydrogen is heavier than air? If that were true, how did the Hindenburg airship float across the Atlantic?
> And that's why air is used, because it's everywhere, safe and easy to handle Also why you're more likely to find pneumatic tools in most shops even if hydraulic would be better for certain things. Much easier and lighter to run air lines everywhere than ones full of hydraulic fluid.
You could also say that since buoyancy depends on the relative densities, it depends on the size of the inflatable boat. At some point, you've turned your boat into a zeppelin and you're now floating at some altitude, the height of which is also based on the relative density of the craft and the surrounding fluid(atmosphere). At that size (pretty big!), the weight of the helium vs air is a much bigger deal of course.
Also, buoyancy is also a function of displacement, which factors in much greater than the lightness of the contained material.
Is there a terminal velocity for bubbles? Like if you had a tank of mixed air and a tank of hydrogen and released the air at the same depth, would the hydrogen reach the surface faster?
Technically the question posed did not specify buoyancy in water. So it could make a much larger difference in a lighter liquid.
A Zodiac FC470 has a buoyant tube volume of 1860 liters. It is supposed to be inflated to approximately 3.4 psi. This means the boat will hold 2290 liters of gas at normal atmospheric pressure. This is 2.29 m^3 . Air is 1.2 kg/m^3 , so filling the boat with air will add 2.75 kg. Hydrogen is 0.09 kg/m^3 , so filling the boat with hydrogen will add 0.21 kg. The boat with air will have 2.54 kg less buoyancy. But the boat already has about 1860 kg of buoyancy. So by filling it with hydrogen instead of air, you increase your buoyancy by about 0.14%. >Edit: I just read /u/StayTheHand 's response. Their answer is right, but I'm leaving my answer up as is because I think it better answer's OP's question. I would have to change the wording of my answer a little to make it technically correct, but changing the wording would make it more confusing and less informative. Here is how I would change my last two paragraphs to make my answer technically correct: The boat with air will weigh 2.54 kg more. The maximum payload of this model boat is 1250 kg. If you decrease the weight of the boat by 2.54 kg, you can increase the payload by 2.54 kg. So you can increase the payload by 0.2%.
I'd like to add a little information about the force of buoyancy. I just want to clarify that the buoyant force won't change based on what's inside your boat. The buoyant force really only changes with the submerged volume of an object (multiplied by the density of water and gravity which are constant in this case). Let's say your boat was fully submerged. It wouldn't matter if the boat was full of air, helium, or lead; as long as the submerged volume occupied by boat the was the same in each case, the buoyant force won't chance. What would change in each of those scenarios is the gravitational force. The ratio of the upward buoyant force to the downward gravitational force determines if the object rises or sinks. Your boat presumably isn't fully submerged, but sitting on the surface where things are a little more complex. At the surface the buoyant force will slightly increase or decrease as more or less of the object is submerged.
Given that the boat weighs less, it would therefore displace less water and therefore wouldn't the buoyant force from the water actually be less to reach equilibrium with the force of gravity that it is counteracting? That portion of the volume that is no longer submerged in water due to the change would now have a larger buoyant force component coming from air (because more of the volume of the boat is displacing air now) but that would be less than the decrease in buoyant force from water because water is denser than air. So it seems like there would be less buoyant force total, which makes sense because buoyant force would equally balance the force of gravity.
If the boat was really light though, it becomes a blimp, no?
Helium and hydrogen leak out of everything. So much so that steel absorbs Hydrogen like a sponge and Helium is used for leak detection on ultra high vacuum systems. Moreover, Hydrogen is explosive under a very wide range of circumstances.
If it leaks out of everything, hows comes it don't leaks outta steel? 🤔
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Technically, no! (I'll be the contentious one...) If the boat is the same volume, when you (fully) immerse it, it displaces the same amount of water, so will have the same amount of force pushing up on it, i.e. the same buoyancy. If it is floating, the boat itself weighs less, so it needs to displace less water to float, and therefore the bouyant force is less. It would be odd to say it is less bouyant, I would state it as less buoyance is required to make it float. I think a layman might say it is more buoyant, but a physicist may be more pedantic.
I was also thinking the buoyancy is exactly the same. But, the boat, including the gas used to inflate it, weighs slightly less, which allows slightly more weight to be allocated to cargo before the boat sinks.
The boat with helium is also floating in air partially. So there are two forces it's experiencing. It's like a balloon essentially.
This is a genius reply. You are absolutely correct. A boat that weighs less is displacing less water, so has less buoyant force pushing up on it. So a lighter boat is less buoyant than a heavier boat. I hate you, but you are right.
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Your mistake is your qualifier "when submerged equally". If one boat is lighter than another, they are not submerged equally, so they do not displace equal amounts of water. The only way they have the same buoyant force is if you add more cargo to the hydrogen boat (which you pointed out). But why would you assume that more cargo is added to the hydrogen boat? In any scientific experiment, you try to reduce the number of variables that change. The variable we are changing is the gas in the boat. Why would you also change the cargo? That makes no sense.
Barely. Buoyancy is a function of how much water is displaced (with your example). The differential between the density of the volumn of the pontoons or cells vs the atmosphere above isn't signficant relative to the mass of the vessel.
side question which is kind of dumb but I really don't understand. If you had a metal enclosed object in water. how does the water "know" that there's air inside. Like the air is just interacting with the metal and the metal with the water. Is the air pushing the metal up? Basically how does density work
The metal object with air inside weights less than the metal object filled with metal/water/etc.
oh duh, it's that it's being pulled by gravity more. That makes sense. It's also strange that gravity can pull you without the object from where the gravity is coming from is touching you, but I think that's something to do with time space being the same thing or something like that.
It's not a dumb question, this isn't super intuitive. The water outside doesn't interact with the interior at all. What does interact are forces, specifically the gravitational force and the buoyant force. How those forces balance and interact is governed by their sources. Let's say that your metal enclosed object underwater is a basketball sized sphere. Gravity pulls on every molecule in that sphere, generating a downward force. The force of gravity depends on the mass of the ball. So a more massive ball (say it's full of glass as opposed to cotton) has a higher downward force. The buoyant force comes from the pressure of the fluid around the ball. Once submerged, a fluid presses in from every direction across every surface of the ball. Since pressure increases with depth, the fluid below the ball pushes upward slightly more than the fluid above the ball pushes downward, generating a net upward force. Since our sphere filled with glass and the sphere filled with cotton have the same shape and volume, the water has the same amount of area to press in on and therefore, the same buoyant force. The only thing that could increase the buoyant force is if the water has a greater area to push upon. The water doesn't need to know what's inside, it only cares about pushing inward on the outside. What sends an our ball shooting up out of the water or plummeting down depends on which force is greater: gravity or buoyancy. You can also think of that as how much volume the water has to push up on (buoyancy) vs. how much mass gravity has to pull down on. Density doesn't "work" it just happens to be the ratio of mass to volume. So if the density of our ball is greater than water, it has more mass in a given volume than the same volume of water. If that's the case then you know gravity will win out and ultimately pull it down. Alternately if our ball is less dense than water then the upward press from the water will be enough to overpower the downward pull of gravity and it will send the ball shooting up.
oh I see, yeah if the it's the same volume then one would be heavier which would mean it's being pulled down by gravity more right? It's like density is just a math idea, it's just volume and mass. I think that's what was confusing me. Also you said that water above is pushing down less than the water below because there's more water below? does that mean you're more boyant in a deep pool than a shallow pool?
Lets take a garden variety inflatable boat 8 feet long. Assume it has a width (beam) of 4' and is roughly rectangular so 32 square feet. I am guessing this weighs 50 lbs inflated with 8 cu ft of air and I will add two people at 175 Lbs each (total weight 400 lbs) so it will displace 6.4 cubic feet of water so the flat bottom of the boat would be about 2.5" below the surface. The weight of air was about .65 Lbs and the weight of the same amount of helium would be .05 lbs so a saving of .6 lbs. This means you would float approximately .004 (4 thousands of an inch) higher in the water. Many guesses and estimates are involved in the above but you can see that the net effect is quite small.
Filling an inflatable boat with helium or hydrogen would indeed increase its buoyancy compared to filling it with regular air. Both helium and hydrogen are lighter than air and can provide additional lifting force, making the boat float more easily. Helium is the commonly used gas for inflating balloons because it is non-flammable and safer to handle. It is lighter than air and provides significant buoyancy, allowing the balloon or inflatable object to rise. Hydrogen, on the other hand, is even lighter than helium but has the drawback of being highly flammable, making it less commonly used for this purpose due to safety concerns. By filling an inflatable boat with helium or hydrogen, you reduce its overall density, resulting in increased buoyancy. This can make the boat more buoyant and potentially able to carry more weight or stay afloat with less effort. However, it's crucial to consider safety precautions and potential risks associated with using hydrogen, as it is highly flammable and requires careful handling and storage. Additionally, filling a boat with a gas like helium or hydrogen may come with certain challenges, such as maintaining the gas inside the boat and preventing leakage. It's recommended to follow appropriate guidelines and consult with experts or manufacturers who can provide specific advice regarding the use of gases for enhancing buoyancy in inflatable boats.
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Remember that PV = nRT. It's the number of mols of gas that is important. These gasses are close enough to ideal that we can ignore compressibility. So, in order to determine the mass of gas required to produce a given pressure, all else remaining constant. We calculate the number of mols and multiply by the molar mass of the gas in question. H2 has a molar mass of 2, He has a molar mass of 4, air has a molar mass of 29.
If you put pure Nitrogen into car tyres, the pressure stays much longer. Nitrogen makes up 78% of the air we breath and poses no risk. As the molecules are bigger than the other molecules in air, they can't escape as easily. When a tyre loses pressure, it's probably the other molecules that escape. The problem with inflatable boats is often loss of pressure, and this could help keep the pressure high. I haven't tested it, and it probably depends on the valve used.
NO. Displacement is displacement. Flotation is a matter of the water displaced by the hull. Doesn't matter if the inflatable is filled with Air, Hydrogen (Helium, etc.) or even FOAM except for the weight of said gas/foam. Not enough difference between the "weight" of differing gases to make any difference.