around 7x atmospheric.

gauge pressure would be 620kPa / 90psi

Thanks a stack.

No we were discussing surfboard and board design. I bought some wood strip kayak plans from laughing loon and was discussing making a surfboard first as the kayak is a pretty big job and I do not want to cock it up. Thy look really great and can buy them off the shelf for around $8k the plans cost me about $300 bucks. The secret is in the finishing and fibreglassing and I want something to practice on before I bite the bullet.

Wooden surfboards built on the same lines look great but they give away weight. If you used the same principles but used aircraft ply you would achieve the same strength. Because the ply is glassed on both sides it forms a very rigid board. That was when we got around to talking about pressurizing it, which you could do quite easily. It would be as tight as a drum and in comparison to a filled board almost indestructible. It would need varnishing once a year to avoid bleaching the veneers because fiberglass is not uv resistant.

Then someone mentioned helium!

Japie-its a good theorie but but slightly flawed, depending on your purpose..

You are blurring "weight" and "mass".

Think of it like a brick that weighs 10 kg here at sea level. It takes a certain amount of energy to start and stop it moving (kinetic energy).

Take that brick into space, it weighs nothing but the mass is still the same. Still takes the same energy to stop/start it moving etc.

Hence if your 25 kg surfboard that is too heavy to turn easily and will crack your skull if it hits you, will still be exactly the same if you fill it with helium and it only weighs 5 kg.
However it will be more buoyant, if that is your sole purpose.

The idea of pressurising it was purely to do away with the necessity of having supporting ribs which all wooden surfboards of any length require to prevent the wood strips from bowing under weight. We figured that if it was going to be pressurised if we used helium it may give it that extra little bit of weight advantage.

Have a look at some of the photos on this site and you will see what I mean. The boards look great but you would not want to have to carry them too far.

mooncussersurfboards.blogspot.com/

This is the Laughing Loon site:http://www.laughingloon.com/canoe.kayak.html

The finished boats look really great and by all accounts have superior performance as well.

what happens if the gas heats up and expands causing a blow out?

less then 70g. would be almost half a kilo at 90psi thou.

however you will struggle to contain the gas at that pressure, your structure would have to be very rigid to contain the pressure and also your bottom shape would flex a lot unless rigid enough, i.e. it might go from convex to concave bottom shapes which is probably not what you want. I'd say it would be much heavier then a rib supported structure by the time you resolve the rigidity issues.

There is probably a sweet spot strength wise that you could find by balancing skin thickness, rib thickness and distance between the ribs. In the links, I'd say that you probably want ribs lighter and more closely spaced then those examples.

The Kayaks are built over a frame which is removed before the deck is glassed to the hull. The deck and the hull are glassed inside and out which results in a very rigid structure.

The board I want to build will be glassed inside and out. Given that the internal space is small in comparison to a kayak I am hoping that it will require little pressure to give it rigidity. Too much and it definitely poses a risk changing the shape and possibly splitting.

Heat is a problem with glassed timber. Dark timber left in the sun will heat up and give off gas which causes de-lamination . It the board were under pressure and if left in the sun the internal pressure would increase. The secret would be to pressurize to maximise rigidity but allow for further expansion through heat without the structure splitting.

I don't think that heat would bump up the pressure inside by much... it's proportional to absolute temperature, so going from 0 to 40 deg C is around 13% increase in pressure. The skin will expand as well, reducing the increase in pressure.

Helium is a small molecule though, it escapes through balloons in a couple of days so I'd think your main worry would be the pressure decreasing over time. If you made the bladder thick enough to contain the helium then it might have trouble conforming to the shape.

I'd love to see the final product though!

I think you are right Nebbian. Helium is difficult to contain. My idea was to make the board maintenance free as far as possible and to seal off the valve. Buoyancy was a pay off.

We had had more than a few mind alterers before the subject of helium came up and then visions of people air surfing followed!

Rigidity looks like it will be achieved by air pressure. Realistically all we want to achieve is a center core that does not yield to foot pressure on land. I cannot see that being much over 30psi bearing in mind that this is a double glassed .5mm veneer deck glued to a rail.

I am not a surfer myself but am close to it. The turn over of traditional boards is what makes them environmentally unfriendly and expensive. Every time my young bloke buys one he goes to bed with and within a short space of time it ends up in the recyclers. It is because they have a soft core. And he is a bit rough!

Rigidity in the kayaks is achieved through hull shape. Hopefully we can achieve this with a board that is pressurized. I like to think that you could strike it with a round hammer without risk of inflicting damage you would expect in a soft core.

As far as weight give away is concerned I have not worked it out yet. Over and above the glass and resin use in a traditional board it's weight give away will be an extra layer of glass and resin on the inside, tip and stern filled with resin say 100gms plus the whole surface area with a sandwich of .5mm aero ply. Add a couple of grams for a decent valve and two coats of varnish!

Watch this space!

rigid boards are a totally different feel to standard pu boards. some people like it. personally i like a little feed back into my feet.

Thanks Gorgo I watched the video. If you were a serious surfer you would have to consider one, they would last for ever.

It is possible to do a light weight board out of wood. The best way would be like you said jappie, to do a wood sandwich ad using balsa. they can also be done the traditional way built up over an internal frame. http://www.grainsurf.com/forum has some very good wood boards. For a good light weight board have a look at 5' 8" Balsa Fish , also there are some threads by woodyads on light weight wooden boards.