Lifting gases used on naval vessels?

Something that's been floating (ha) around the old noggin:

I'm no expert by any means, but it seems to me an envelope or series of envelopes of lighter than air gas would do much to decrease the weight of a naval ship, increase buoyancy, possibly increase speed and maneuverability, and allow for heavier loads, more weapons, or more defenses.

I'm envisioning bladders of hydrogen or helium incorporated inside the backs of sails and/or enclosed along the edges of the ship's hull, perhaps?

I'll just let you deal with the mechanics of it...

At any rate, I think this would improve naval progress to some degree, but mostly it leads to wider use and understanding of airship technology far earlier.

Is this at all possible and if so, what is the earliest it can be done (I'm thinking realistically the 1790's), and some of the effects of such a development?

Fire away.
 
Not worth the investment by the time you could actually build it.

I have to disagree, it's not all that difficult or expensive to fill bladders with lifting gas. Ships would benefit greatly from the increases in speed and capacity and decreases in weight, right?
 
Something that's been floating (ha) around the old noggin:

I'm no expert by any means, but it seems to me an envelope or series of envelopes of lighter than air gas would do much to decrease the weight of a naval ship, increase buoyancy, possibly increase speed and maneuverability, and allow for heavier loads, more weapons, or more defenses.

I'm envisioning bladders of hydrogen or helium incorporated inside the backs of sails and/or enclosed along the edges of the ship's hull, perhaps?

I'll just let you deal with the mechanics of it...

At any rate, I think this would improve naval progress to some degree, but mostly it leads to wider use and understanding of airship technology far earlier.

Is this at all possible and if so, what is the earliest it can be done (I'm thinking realistically the 1790's), and some of the effects of such a development?

Fire away.
Oh yes extremely large bags of hydrogen on a warship would be progress alright(think Hindenberg), the size of the gasbag needed to even start to "lighten" the load would be insane, Helium wasn't readily available USA had a virtual monopoly pre-WW2 state secrets and all, here's some math
http://science.howstuffworks.com/helium2.htm
 
If the gas is contained inside the hull, you're spending valuable hull space on it. Your crew can only breathe where there's oxygen, and most of the things you want to put inside your warship need to be crew-accessible. Surface ships aren't as bad as submarines, but my understanding is that hull volume is still at a premium.

Then there's damage control aspects. Normally, a ship made of heavier-than-water materials will sink if the hull is punctured and too much water gets in. This is mitigated with pumps to bail out small-to-moderate leaks and internal bulkheads dividing the ship into water-tight compartments to limit how much of the ship can be flooded by a single hole. Make your warship dependent on lifting gas to stay afloat, and not only do you need to keep water out, you also need to keep air out. That's a whole new set of surfaces you need to seal and worry about getting punctured, and you can't use bailing pumps to mitigate damage because you're limited to the lifting gas you brought with you. Adding airtight bulkheads and other redundant containment systems helps, but isn't perfect, and costs you some of your displacement advantage from using the lifting gas.

Hydrogen as a lifting gas can be replenished through electrolyzing seawater, but it's a major fire hazard on a warship: engines, firing your guns, enemy shells or bombs exploding, someone lighting a cigarette, or even just any metal object striking another metal object could set off a hydrogen fire or explosion if a significant concentration gets where it's not supposed to be.

In addition, if your lifting gas tanks puncture, the lifting gas will flow first into inhabited areas of the ship and suffocate your crew by displacing the regular air. Water getting in will suffocate your crew, too, but water's obvious enough that you've got a chance to get out of the way. Inert gas asphyxiation is so subtle that unless you're actively on alert for the warning signs, you're unlikely to notice anything wrong before you lose consciousness.

If instead you store your lifting gas outside the hull (effectively a zeppelin or blimp hull tethered to the ship's superstructure), you've got other problems. Damage control is an even bigger problem, since your enemies now have a great big unarmored gasbag to aim at, knowing that if they put enough holes in it they'll sink your ship. It'll also have a huge surface area, slowing your ship down due to drag, and making your ship more vulnerable to bad weather because of how it catches wind.

A couple variations that mitigate (but don't completely avoid) these problems to a certain extent:

  1. Fill your hull with a low-pressure pure-oxygen atmosphere. You can breathe this just as well as regular air, it'll give you some extra lift from the lower density, and you can (at least theoretically) refresh your supply of pure oxygen in mid-ocean by processing it out of the air or electrolyzing it out of the water.
  2. Fill your hull with a normal-pressure gas mixture of 20% oxygen, 80% helium. Not as much lift as pure O2, and you still have the problem that you can't easily replace lost helium mid-ocean, but there's advantages to not having to try to contain a big pressure difference.
 
Do you realize how much hydrogen you'll need? Look at the size of the blimp in relation to the gondola beneath it. You'll need a bladder ten times the size of the ship just to reduce its weight by a fraction.
 
  1. Fill your hull with a low-pressure pure-oxygen atmosphere. You can breathe this just as well as regular air, it'll give you some extra lift from the lower density, and you can (at least theoretically) refresh your supply of pure oxygen in mid-ocean by processing it out of the air or electrolyzing it out of the water.

That is not practical either; most materials burn in the presence of pure oxygen, so it's a huge fire hazard, plus there is the problem of increased oxidation; your ship will rust away in no time.
 
That is not practical either; most materials burn in the presence of pure oxygen, so it's a huge fire hazard, plus there is the problem of increased oxidation; your ship will rust away in no time.

I've seen conflicting claims as to whether the relative concentration of oxygen that matters for flammability and oxidation or just the absolute concentration (partial pressure) of oxygen. I was looking at the wikipedia article on Apollo 1 when writing my post, according to which the problem was that the capsule was pressurized with over a full atmospheric pressure of pure oxygen, but the fire hazard would have been no more than with normal air if it had been at the planned in-flight atmosphere of 5 PSI of pure oxygen.

I did a bit more digging just now, and found this article that makes the same claims:
http://blogs.discovermagazine.com/badastronomy/2007/01/27/apollo-1-fire-40-years-ago-today/

But I also came across this, which claims that even at 5 PSI, a pure oxygen atmosphere poses extreme fire risks:
http://www.ocii.com/~dpwozney/apollo2.htm

I only have basic knowledge of chemistry, so I'm going to have to confess ignorance on this point.
 
Aside from the specific objections, the real problem (alluded to above often) is that the sheer volume of gas you'd need means that what you'd have would basically be an airship somewhat overburdened--frankly if you could make the gas cells on a scale sufficient to lift a significant percentage of the ship's weight, you might as well kick it up to 100 percent and have yourself a proper airship, flying around in the sky completely independent of the water!

Water is about 800 times denser than sea-level air. To be sure a surface ship has most of its volume above the water line, which is to say the volume contained in its structure is less dense than water. I typically figure a ship is about 1/5 the density of water, so the actual ratio of volumes would be something like 160. Say you only want to lift half the weight, halving the submerged volume for the benefits you envision. So you need 80 times the volume of the ship for that gas, presumably hydrogen (well, nowadays helium is still expensive but it can be made available for a price in very large quantities--but if all the shipping in the world tried this plan we'd hit the limits of even modern extraction fast). The cube root of 80 is about 4.31--if your gas bag has the same proportions as your ship hull, that's how much longer and broader in beam this volume overhanging the ship would be.

The gas volume would have a huge sail area too; crosswinds would be a major problem. The bag, even if built lightly, would be a significant mass itself being extended over such a large volume, and yet, given the stresses winds would put on it, dangerously flimsy. That's quite aside from the risks of people shooting at it!

If the purpose of the exercise is to lower drag below the waterline, consider that the huge gas bag would have not inconsiderable drag in air of its own. I think you are right that overall it would be reduced, but you have to consider that offset--getting half the drag area out of the water will not halve the drag since you now have the gas volume's drag to factor in.

Much of the drag on a surface ship is wave drag, something that has no analogy for a free-flying airship (or a fully submerged submarine)--aircraft only have to worry about that if they go supersonic!

Might as well finish the job and build a proper airship and get free of the water completely.

Though to be sure, I've proposed my own version of an airship designed to be bound to the surface of the sea, most recently here. The main economic niche I envision is to serve as a fast ferry. In NomadicSky's timeline, Cuba is ISOTed to our world from an ATL where it is a US state, so I was thinking Havana to Key West, but I've mentioned it in other contexts too--someone's "Nebraska Sea" Alt timeline for instance, for crossing the Sea from the Midwest to Texas.

I approached the idea from the opposite direction--take an airship, and anchor it firmly to the sea surface with submerged azipods that provide both directional control and propulsion. In my version the "ship" hull is entirely an airship, it has streamlined struts going down under the water and its engines are down there, but the bottom of the airship hull and any gondolas that might be suspended there are well clear of the wave tips, by meters or even tens of meters. Only a small amount of the ship's total mass is in water. Aside from the sheer HeliumHead wankery, the point is that it can go a lot faster than any conventional watercraft and is a technology that is older and less fuel-hungry than hovercraft or hydrofoils, the only vehicles I can think of in its speed class.

A society that could and would build these things would probably build more free-flying airships as well.

But I don't see either displacing most surface ships, and I don't think you can get any worthwhile benefit from lifting such ships partially out of the water with gas lift.
 
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For most of the history of fighting ships it was the amount of hull in the water that counted not the other way round. This is particularly true in rough seas such as the North Sea and Adlantic.

On occasion the ships did need to reduce their draft they would use lighters (basically tie two ladeden barges to a ship, then empty out the barges, the ship will lift as the barges go up).
 
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