Many boats like the Escape Playcat and Hobie wave are made using rotomoulded polyethylene (PE). PE is very tough but not very stiff. As a thin walled structure a PE hull will most likely fail by local buckling and buckling is a function of a materials stiffness not its strength and only occurs when a material is in compression.
There are four ways to prevent this and each is used by both nature and boat builders,
The first is simply to make the thin wall thicker.
The second is the most common method and it is to use ribs and stringers.
The third is to use a sandwich construction where a low density layer is sandwiched between two or more stiffer layers.
The final method is used by leaves and is known as hydrostatic stiffening, I propose to use a variation on this method (pneumatic stiffening) to get lighter hulls.
To show how effective this method can be buy yourself two bottles of pop (lemonade or other fizzy drink). Empty one and stand on it. It probably won’t break but is not very stiff and will crease. Now take the other bottle skake it vigourosly and stand on it. This bottle has been pneumatically stiffened.
What is happening is that the bottle or skin has been put into permenant tension by the compressed air in the bottle. The skin is carrying all the tension loads and the air inside all the compression loads. Since the skin can no longer go into compression it won’t fail by buckling. And this creates a very efficient structure.
I therefore propose that we make the hulls thinner walled say 3mm thus lowering the weight but restoring its stiffness by providing a valve by which it can be pressurised with a common pump. This will somewhat limit the types of shapes that we can use for a hull, but a long thin hull with no convex curvature like that used by a multihull design would be an ideal shape for making the most of this process.
We are effectively reducing weight without losing stiffness by using air which is free.
Does anyone think this idea has any merit, it is currently subject to a patent GB0223164.5.
"providing a valve by which it can be pressurised with a common pump"
Or perhaps just filling the hulls with soda pop!
Interesting idea. But don't the hulls normally need to breathe, so as to avoid expanding in the hot sun?
Sheldon
that can be remedied with a pressure sensitive safety valve like is common on most pneumatic storage tanks.
I would think that something like this would have a designed max pressure and therefore have a relief valve built in to control over pressurizing. It would solve the problem with my Prindle 18 hulls taking on a little bit of water every time out, but I'd probably have to have a 12 volt air pump to keep a steady pressure.
Jack
The same methode is used in intercontinental missiles and various other applications.
It is definately a smart thing to do, although I'm not sure about the patent though. As this idea has been used in various other applications is stands to reason that it is not new technology any more. Also its application in boat design is not new. How about the RIB's ? Rigid inflatables boats. In effect the rubber tube around the polyester plaining surface is stiffned in the same way.
But you're definately thinking outside of the box.
Wouter
I raced a Catapult for nearly ten years. Pump it up and away you go !. www.catapultcats.com
Ultimately the question would come down to how much air you would need to pump into the hulls to get the pressure required to for the desired stiffness. Air has weight - the material removed from the hull better weigh significantly more than the air pumped in to restore appropriate stiffness or the whole thing is pointless. One of our resident engineers can do the calculations, I'm sure.
*searches frantically for calculator and pocket protector*!!!!
Say you sail in an area with warm air temps and cold water temps. Will I have to carry a pump with me onboard to adjust the air pressure as the temp and thus volume of the air inside the hull changes? Perhaps an attached bottle of air (compressable by an ordinary tire pump) to automatically maintain pressure?
Use Helium, or another lighter than ambient air gas, it will reduce overall weight!
I've always wanted to put some sort of vessels like inner tubes, with helium in my hulls, noone would know! 
grob,
Cool idea with some interesting challenges to pull off. One challenge everyone with a cat faces is how to keep your hull sealed to keep water out never mind presurized to the point where you are requiring it for structural stability. It would not be too bad if traditional catamaran construction did not require so many attachments.
The second issue is to provide a hull shape consistant with good performance. Weight is not the sole factor in how efficient a hull form is hydrodynamically. To provide adequate stiffness increase to a thin skin material like polyethelene, you will end up creating a tube. This shape would be more efficient fully submerged, but not on the surface.
I would be really curious as to how this ends up working. Please keep us informed as you go.
Good Luck
Matt
Every once and awhile, somebody proposes the helium idea for reducing unsprung weight in car tires. Just use helium instead of air in your tires. Except that by the time you pump in enough to get the pressure you need, you've got about the same mass as other gasses and the big weight difference goes away.
Interesting idea but it's been done.
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What I've always wondered is why not roto-mold with stiffer material?
Actually your MASS will be about the same but your weight will be a little less resulting in a smaller displacement.
David Ho
TheMightyHobie18 1067
This is a very good idea and please pursue it. You should look at people who have tried similar things and the problems that they had.
The main problem is once the pressure goes above 1/2 atm the boat wants become spherical or at best a cylinder. This is not a good shape for a cat. You can fix this if you put in stringers, bulkheads or something similar to make it hold it's shape but this adds weight and cost.
The other major problem is that even at 2 atm an inflatable structure is still relatively flexible compared to rigid structure of the same weight. Again this is not good for a cat.
There have been a couple of tries at solving these problems. One solution involved inflating a double walled skin instead of the whole structure. The area between the skins contains something like a low stretch open cell foam. With this you end with a structure like foam core fiber glass. There was even a proposal to inflate kevlar open cell foam to 100 atm. I have heard a couple of stories about this and they all say you can do a lot of damage if it fails
One of the big inflatable companies Avon or Zodiac or ? had a report on some of things they had tried and their results
Most of the work on building better molded plastic structures have been by other industries (auto, aerospace, toy etc). The two major areas are a better technique and a much better plastic. .
An example of a better technique: First you mold a thin hard high melting point outer skin. Then you add a lower melting point (so you don't effect the skin) foam for the core. Last you add an even lower melting point inner skin. The big problem with this is making sure the layers bond together.
A much better plastic: Most these involve a post molding cure. After you finish molding the boat you subject the boat to a special temperature, pressure, gas, liquid, etc and the plastic turns into a light weight high strength material. An example of this is converting rayon into carbon-carbon. So far these are really expensive.
Please continue looking at this, but look at what other have done so you don't repeat their mistakes.
Uh, I think there is some interesting physics in the statement "your MASS will be about the same but your weight will be a little less". Mass and weight are different, we all agree, but two objects with same mass, in same gravitational field, have the same weight, don't they?
I'm gonna have to belly up and agree with with Jcasto.
That was exactly my point.