Pressurizing a hull to make it more stiff

[SeaSpark]

This old idea of mine has been boggling me for many years now, time to post it:

Would it be possible to increase the stiffness of hull by pressurizing it?

The most obvious example are the tubes of a RIB, they have no stiffness at all until you inflate them. Another is the use of pre tensioned concrete in bridges and other structures. Here the concrete is delibaretly put under a high pressure load to make it more resistant to bending forces.

On many boats this could never be a practical idea, the need for pressure doors would make it totaly unworkable.

The hulls of some catamarans have almost circular sections over most of their length, like a pressure vessel. The idea might work on a hull like this.

The question is:

Could the increase in weight needed to make the hull pressure resistant be compensated by the higher stiffnes making the hull more efficient?

[solrac]

Spark, may I correct some of your thoughts:
../pre tensioned concrete../ The concrete itself can't be practically tensioned, what is pre or post-tensioned is the steel reinforcement... once the cement mix is hard enough, that makes the entire piece resist as much as a 25/30% more due to internal compression forces in the hole section. think it can't be of use in boat hulls, because it's not practical the necesary weight of the sections needed to post-compress without breaking risk...
on another fact, the concrete & steel are "compatible" materials, as they have similar temperature coeficients, not the same between resin/fiberglass & any metalic reinforcement, that probably will cause severe delamination on temp variations along time.... sorry

[SeaSpark]

In my idea the pressurized gas plays the role of the steel rods in pre-tensioned concrete structures.

[MikeJohns]

Yes the concept is not daft.... but ... you still have to account for the forces. Plus the additional force of the pressurized vessel even at rest. So it would end up much heavier and considerably more complex. Much much lighter to use compression and tension elements to carry the forces which you may as well do in a monocoque structure ......ie ....a rigid hull.

Works well with flexible materials

Has also been a suggestion to pressurize carbon fibre masts to increase the buckling strength........it too has some big drawbacks.

[Ike]

Just how would you pressurize the hull? Would you seal the hull completely and pressurize it that way, or are you talking about making frames & longitudinals, or deck beams hollow or what? Frankly it sounds a bit impractical. In fact some boats are built to deliberately give a little. If you make a boat hull that spans the crest of several waves too rigid it can break from it's own weight. That's why supertankers are never taken to sea unloaded. The cargo adds to the strength of the hull. I know we aren't talking ships here but elaborate on your idea so we can get a little better idea of how you plan to do this.

[SeaSpark]

MikeJohns, can you point me in the direction of the pressurize carbon fibre masts discussion?

[grob]

This may be a good idea, I have thought about it in the past.

Any thin walled structure (Hulls) tends to fail by buckling which occurs in compression, so by pressuring the hulls you are pretensioning them making them less likey to fail by buckling. Hulls rarely fail in tension.

Mother nature uses this method in plants alot, long thin stalks and stems are often hydrostatically stiffened by the osmotic pressure of the sap.

One advantage of using stiffer materials in the first place, is that you are only having to add enough air to raise the pressure and not to inflate the boat.

Small catamarans are often made cheaply from rotomoulded polyethylene which is not stiff at all and so they tend to be built heavy to get the stiffness back. You may be able to make a lighter rotomoulded catamaran by pneumatically stiffening it.

Gareth
www.fourhulls.com

[DanishBagger]

Until a hole develops ..

[SeaSpark]

Gareth, thank you for your clear explanation.

DanishBagger, i am aware of the fact that holes and hulls don't mix, pressurized or not.

I would realy like to put this idea into practice to test the effectiveness, but i lack funds.
Using a rotomoulded hull seems to fit to this idea perfecty did not think of this before.

[DanishBagger]

Hehe, sorry, I wasn't trying to be funny, I'm serious. I think that gaining extra strength by pressurising is okay, but if something as complex as a "real" boat is designed to only be strong enough when pressurised, to me that would be a recipe for disaster. Even the slow depressurising that would inevitable occur would be really bad. You would have to have a means to re-pressurising it, under any circumstances, and at any time. What would happen if in really bad weather, because of flexing a small crack would develop, then you would have to keep up the pressure somehow, and that at the worst possible time.

Of course, as an engineering feat it could be interesting, but if the boat didn't inherently had the strength to do without, I wouldn't be able to trust it.

I didn't post this to put the idea down, I just think that you woud need complex, overbuild systems backing up the complex system to begin with - on top of that you would need to make it even bigger, just so that it can stay pressurised for a long time, even with a hole or crack somewhere.

Not only is it hard to make something completely waterproof, but it is so much more difficult to make something completely airproof.

Oh, another question - I just imagined, what if the boat was dark? Then during the night it would become much less strong, whereas during the day, in the sun, it would become very stiff, perhaps even so much so that cracks would develop.

I know, I'm a party pooper. Sorry about that.

[solrac]

The only idea I think may be worth the pain, can be pre-tensioning some plastic wires (kind of fishing lines?)at the time you glue the roving & resin, then, when it has been catalyzed & hardened, let's say next day, you can cut the tensioning device to see what effect have you achieved. the final effect will be (in theory) simmilar to a concrete pre-tensioned slab, a 30% increase in lateral flexing strenght. still there are some issues:
- On concrete slabs, the steel reinforcements are never more than 5% of the section
- the section height/total lenght ratio of the slab is on the 7 -10% order
- the tensioning device can only work on a straight plane, not posibility of curves. the curvatured cable tensioning on bridges is always done by inserting the cables on a plastic pipe (200% the section of cables), post-tensing them before concrete hardened & finally injecting under pressure a cement mix on the piping space.
Think this limitations are not easily aplicable on reinforced plastics

[Thunderhead19]

I don't get it. Pre-tensioned steel rods are intended to provide increased tensile strength to concrete members. By pressurizing a concrete hull you'd be doing two bad things. 1. steel and concrete have similar coeficcients of thermal expansion, concrete doesn't share this with any gas that I know of. 2. you'd be pre-loading the concrete in every direction. This is not good. You'd get a more rigid boat, but the strength at the hulls weakest point would be proportuionally reduced. With concrete slabs, for example pre-loading is in the strongest direction in order to increase rigidity and decrease the percent loding in the weakest direction. Gases are not anisotropic.

[SailDesign]

I think you will find that in order to pressurise a hull enough to stiffen it, you would have to add a lot more hoop strength (which means weight) to resist the pressure, thus ending up with a heavier hull......

[SeaSpark]

A structure that resembles my idea very well is the pressurized aircraft fuselage. Here the cabin is put under pressure to enable people to breath at 10km height.

Ill try to find to what i does to fuselage stiffnes.

Suggestions on where to find information on this matter are very welcome.

Solrac, thunderhead i hope my idea is more clear to you now, no i don't want to build a concrete boat.

SailDesign: The skinn of a small boat is stronger then necessary in many cases to be able to resist point loads, this extra stenght could also be used to withstand the pressure force.

Sorry if my english is not always that clear.

[DanishBagger]

Well, the difference being that in an aircraft, an engine is running constantly, having "eternal" power, so to speak. I guess in a motor-hull, that'd make somewhat sense (to me)