Maximizing Buoyancy in a Closed Helium Container: Tips and Techniques

[ucf-fisher21]

question: I have a closed container filled with helium and I can't change the shape or size of the container. How can I achieve maximum buoyancy in order to make the container seem as light as possible? Would I want to jam pack the container with helium so that the pressure inside container is high, or have the helium in the container be at the same pressure as the outside air, etc?

This isn't a HW question, its for something I want to build.

I would really appreciate any help.

 

[russ_watters]

Net buoyancy is about volume displaced vs the mass of the object doing the displacing. So to have maximum buoyancy, you want the object to be as light as possible.

 

[ucf-fisher21]

exactly, I want the container to be as light as possible, but how should I use the helium in order to make the container lighter (like the way a hotair balloon works)?

 

[tiny-tim]

exactly, I want the container to be as light as possible, but how should I use the helium in order to make the container lighter (like the way a hotair balloon works)?

erm … fill it with helium … then remove all the helium!

 

[ucf-fisher21]

tiny-tim,

Are you saying that the container will have maximum buoyancy if it's a vacuum?

 

[tiny-tim]

Are you saying that the container will have maximum buoyancy if it's a vacuum?

Yup!

Of course … it'll probably implode!

 

[ucf-fisher21]

tiny-tim,

now that I know a vacuum provides maximum buoyancy, I have another question: The container will be rectangular (say 18x5x2 ft) and it will be made of a fiberglass/kevlar composite (which would have really good mechanical properties); do you think this would be rigid enough to keep from imploding at sea level pressure?

 

[russ_watters]

It is possible, it we won't be able to tell you from that information. You'll just have to find out!

The thing is, though, helium is pretty light, so the extra buoyancy to be gained by evacuating it vs filling it with helium isn't very much.

 

[ucf-fisher21]

so I am guessing that keeping helium in it won't effect buoyancy that much, but it should help the container from imploding.

I could actually weigh the container with and without helium, and see how much the buoyancy is effected.

The important thing is that with the project I want to do, implosion is a matter of life and death, so it's obviously a top priority.

 

[russ_watters]

The density of helium is 0.011lb/cu ft and the density of water is 62.4 lb/cu ft. So the difference in buoyancy between a helium filled and evacuated vessel is .02%.

For that matter, air's density is only .075 lb/cu ft...

 

[isly ilwott]

With a rectangular vessel, pressurizing with helium would likely increase the volume slightly, yielding more bouyancy by displacing slightly more water. If you are planning to submerge the vessel to great depths, you might consider filling it with oil to preclude implosion. Oil being only marginally lighter than water will not offer as much bouyancy as a light gas, but it will not compress.

Since you are evidently planning to submerge whatever is attached to this bouyancy device, you will in fact have to add weight or pull it down with a cable attached to the sea floor. Which is it going to be?

 

[ucf-fisher21]

isly ilwott,

I said rectangular just to give everyone a rough idea of the dimensions. It's actually going to be a boat. I need the boat to draft in as little water as possible. So I plan on giving the bottom of boat as much surface area as possible, but not so much that it will affect the performance of the boat.

Also, the boat will be very rigid (fiberglass and kevlar) so i don't think the helium will affect the shape at all. Since it's going to be very rigid and it will be at sea level pressure, I think I can make the hull of boat a vacuum to achieve as much bouyancy as possible.

 

[isly ilwott]

Incidently, a friend of mine used to build party barges with two or three rectangular floats made of sheet steel. Each float was divided into air-tight chambers, each of which was fitted with an air check valve (as on most all car wheels) to pressurize each cavity to about 4 psi. The air made the flat sides "pooch out" a bit, making the raft float a bit higher in the water. This system also aided in the detection of leaks below the waterline...you see bubbles...you've got a leak.

 

[isly ilwott]

isly ilwott,

I said rectangular just to give everyone a rough idea of the dimensions. It's actually going to be a boat. I need the boat to draft in as little water as possible. So I plan on giving the bottom of boat as much surface area as possible, but not so much that it will affect the performance of the boat.

Also, the boat will be very rigid (fiberglass and kevlar) so i don't think the helium will affect the shape at all. Since it's going to be very rigid and it will be at sea level pressure, I think I can make the hull of boat a vacuum to achieve as much bouyancy as possible.

The weight of air removed will be negligible. Consider filling with foam as is done with the Boston Whalers. That way, when the integrity of the hull in compromised by a sharp rock, you will still float. I've seen a Boston Whaler sawn in half...each half remained afloat...even with an outboard attached and a heavy man in each half.

Even with many structural trusses between the two surfaces of a relatively flat fiberglass structure, the flat surfaces will become slightly concave when a vacuum is introduced between them. I think you are wasting your time trying to gain a smidgeon of bouyancy by vacuum.

...and a floating boat will displace its weight, regardless of the vacuum between the two hulls. Not only that, but with a vacuum, a pinhole leak will causes water to be forced into the cavity due to atmospheric pressure on the outside of the cavity.

Are you making a "bonefisher" boat to run in the shallows?

If you really want to save weight, leave the beer cooler at home.

 

[ucf-fisher21]

yeah, I want to make an ultralight flats boat ( I live in florida).

From what you said, making a vacuum is pointless so I'm scratching that out.

Compared to having regular air in the hull, do you think having helium will make a noticable difference in the weight of the boat?

 

[Topher925]

Building a boat hull from those materials and then having a large delta P is madness. Just fill it with a medium density foam like airplane wings and be done with it.

 

[tiny-tim]

… balloons …

yeah, I want to make an ultralight flats boat ( I live in florida).

Hi ucf-fisher21!

If you have access to plenty of helium, why not lighten the boat by attaching helium balloons to it?

 

[isly ilwott]

yeah, I want to make an ultralight flats boat ( I live in florida).

From what you said, making a vacuum is pointless so I'm scratching that out.

Compared to having regular air in the hull, do you think having helium will make a noticable difference in the weight of the boat?

I am confident that you could not tell the difference without an extremely sensitive scale. The amount of helium I visualize between the two hull shapes is of the order of ounces, not pounds. Carrying an extra rod and reel would probably add more weight than the helium saves over the weight of air.

Again, even pressurizing the hull cavity will do no good when the hull is cracked open. You will lose pressure, fill with water and sink as if the protection was never there.

Use foam. There are many products that expand like crazy when injected into such cavities. Any one of them that is not affected by fuel will last as long as you are expected to live and will keep water out of the cavity even when the cavity is cracked open.

What horsepower will you use to turn the screw(s)? [Engineers love redundancy...I'd carry two or more outboard motors if I were doing it...especially in the ocean, where currents can carry you miles from where you want to be. They don't have to be the same size, just each be long enough to reach below the surface...and if money is not a problem, I'd use inboard engines with jet drives. You can adapt something from a jet-ski to do that...then there's no prop to submerge below the bottom of the boat, allowing passage through very shallow waters...and you can slide over manatees without slicing them up.]

A hand-held marine radio is recommended also...in case you need assistance.

 

[ucf-fisher21]

tiny tim,

that would work, but I don't think i have the self courage to be out on the water with a bunch of balloons attached to my boat! plus it would make running the boat more difficult.

 

[ucf-fisher21]

If the helium thing won't work, ill just maximize the surface area of the hull to displace as much water as possible, and use the best weight reducing construction (lightest materials and vacuum infusion).

 

[Sakha]

You could design you boat as a trimaran, but with almost all the weight in the center hull, and the two outside hulls filled with foam or polyurethane which has a density of about 1lb/cu ft.
This would be very helpful to float, but it would have a big impact on maneuverability.

 

[russ_watters]

From what you said, making a vacuum is pointless so I'm scratching that out.

Compared to having regular air in the hull, do you think having helium will make a noticable difference in the weight of the boat?

See my numbers: it's less than .1% difference. Not enough to notice.

 

[ucf-fisher21]

the hulls on each side would provide extra flotation and also improve stability a lot, but I definitely can't sacrafice manuverability.

i'll be using the boat mostly in Mosquito/Indian River lagoon (cape canaveral area. You can actually see the NASA assembly building and the lauch towers. it's really cool.) Manuverability is key because the channels are only 2-4ft deep and outside channel is only inches-2ft deep. sandbars are everywhere as well.

 

[Sakha]

Good luck on whichever design you decide to build.

 

[ucf-fisher21]

thanks sakha. its going to be fun

 

[gmax137]

"ill just maximize the surface area of the hull to displace as much water as possible"

You're being sloppy here, either in thinking or in writing. The amount of water displaced depends only on the weight of your craft (about 0.016 ft3 water displaced per pound). Increasing the area will allow you to displace that volume with a shallower draft, but you *will* displace that volume or water.

 

[ucf-fisher21]

gmax,

sloppy in writting (i was never good in english class). I was trying to say that I wanted to spread the weight of boat over as much surface area as possible (bottom of boat) to maximize the buoyancy force.

 

[Astronuc]

bouyancy depends on density and volume. When a boat displaces water, the mass of the boat divided by the volume of water displaced = the density of water.

The other matter is one of drag. The drag coefficient increases with cross-sectional area of water displaced and surface area of a mass/vessel in contact with water. The structural material is much more dense than air, and aluminum (or alloy) is perhaps the most practical structural material for building a boat/water craft, although one might consider carbon-fiber composites if cost is not too high.