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)

[Thunderhead19]

I see what you're getting at. Something like the reverse of a submarine. Pressurizing a balloon, for example, (or a condom) does make it stiffer, but it makes it a lot weaker. Take a condom and fill it with water (not so much that it stretches), and drop it. It probably won't break, and it has only become marginally stiffer (and thats with water in it, not air). Now fill one with water until it streches a fair bit. It will only be a tiny bit more rigid, but if you drop it, it will burst. The impact loads that a boat sees are many times greater than those seen by a boat hull. With aircraft design Aircraft fuselages are not pressurized to make them stronger of more rigid. When Federal Express buys old passenger planes to use to ship cargo they de-pressurize the passenger cabin to make them stronger ( compensating for the degree of fatigue that the airframes have undergone in their lifetimes from constant pressurizing and depressurizing.

What you're suggesting is definitely achievable.

[MikeJohns]

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

Not in this forum, and the results of the investigative FEA showed that the pressure needed to fairly high to be worthwhile and that other methods worked out lighter and cheaper such as adding more fibre.

From an engineering viewpoint it's a non-starter for rigid materials.

[MikeJohns]

www.fourhulls.com[/url]

Gareth

Small hulls rarely fail from anything if built to sensible scantlings ( exempting racing hulls). What kills them is the unplanned loads following which you will often get tensile failure of parts of the structure (as well as buckling).

Ships for example often fail in tension when the deck/topsides structure prove inadequate .

Also be aware that a lot of buckling in ship structures is often not a catastrophic failure in itself but leads in itself to related tensile failure.

Thats why we model everything in those expensive computer packages these days. Its all horribly complex.



Cheers

[DanishBagger]

Its all horribly complex.



Nah, you're kidding me ;)

[SeaSpark]

Finally, some numbers.

Thanks MikeJohns, still curious how this FEA analysis has been done.

By the way i posted a picture of the lattice mast from your gallery in the " Mast aft rig" thread http://www.boatdesign.net/forums/showthread.php?t=11164 and followed the 2 other lattice threads in this forum. Do you have more information to share on this subject?

There also is a remark about the A-mast alternative. Do you have experience with this?

[grob]

"Hulls rarely fail in tension" OK I worded that pretty badly, the point I am trying to make is that while pressuring a rigid hull is not going to be the answer to all hull problems there are situations where it has some merit. i.e where you are trying to protect from faliure due to local buckling. Local buckling (bezier buckling) is found in structures with relatively low stiffness and thin walls.

The eqn for bezier buckling of a thin walled tube is:- stress to cause buckling=.25*E*t/r

where E=matl stiffness, r=radius of a tube, t=thickness of tube.

If you plug a typical carbon mast dimensions into this you will see it is too stiff and thick walled to suffer from local buckling. A rotomolded catamaran hull is a different matter it uses a cheap low stiffness matl and is a relatively large diameter with a small wall thickness. e.g 0.25*1000MPa*6mm/150mm=10MPa. 10MPa is well below the normal faliure stress of the matl. Therefore unlike a carbon mast it could benifit from being Pretensioned.

If you want to see how this works in practice, get a large plastic thin walled bottle of fizzy drink and take the lid off to releive the pressure, then put it back on and give the bottle a squeeze, its not vey stiff!. Next give the bottle a shake and try and squeeze it again, it is now much stiffer. This is a rigid structure made stiffer by pressure.

The downside may be that while local buckling would not be a catastrophic faliure as you say (they will often spring back into shape when the load is removed), pressurising a hull may indeed put you into the realms of catasrophic faliure if you have not done your sums right. Think of the fizzy pop bottle experiment taken to the extremes. I think this is why people have stayed away from this method of hull stiffening.

All the best
Gareth
www.fourhulls.com

[granite]

The catapult is a catamaran with inflatable hulls, there is beam to stiffen them and a fiberglass bow section fairing

http://www.catapultcats.com

[SeaSpark]

Thanks for the link Granite.
After some googling i found this, a Catapult Cat with foils!

http://www.dcss.org/speedweek/toastrack.html

[MikeJohns]

...................

Thanks MikeJohns, still curious how this FEA analysis has been done.
By the way i posted a picture of the lattice mast from your gallery in the " Mast aft rig" thread. Do you have more information to share on this subject?

There also is a remark about the A-mast alternative. Do you have experience with this?

SS

If you want FEA tips you need to access tutorials on pressure vessels.

As for lattice masts, yes lots of information, we design them; mostly for larger sailing vessels we charge $400 Australian per design . They work out very cheap to build and no heavier than the equivalent Al extrusion. You have the steel maintenance work ongoing but with modern epoxies that is minimal.

A-frame spars suit modern lightweight materials but you have to allow for the compression on the mainstay trying to spread the frame so you need cross members and then the whole structure starts to get a bit too much windage.

I'm not keen on aft mast designs either, too much windage aft in the bare rig so running off is dangerous in the situation that it shoulkd be safest (in a severe storm).

cheers

[SeaSpark]

Mike Johns, your reputation in the trial we are having has just gone up.

[tspeer]

It's been done. (http://www.catapultcats.com/)
http://www.catapultcats.com/images/image46.jpg