Hi there. After some advice regarding a project I'm starting. I run a small offshore race boat of a catamaran design. Between the two hulls is a flat deck of around 1m wide by 2m in length. It was made of 10mm marine ply wrapped in a glass fibre mat and finished with gelcoat. It comprises of three panels.  This broke last year due to the impacts of the two passengers landing on it and the ravages of time. The deck does take a beating as it also has the fuel tank strapped to it. I'd like to replace the deck with some boards that have the same or better strength as the plywood but be as light as possible. I'm looking at using 10mm foam board with outer layers of glass fibre matting and a resin topcoat. Will this be suitable and as strong? Thanks very much. Any advice would be greatly appreciated. Steve. 

In principle yes, this will work.
I'd rather use woven glass though, or bi or triaxial UD, this will result in a much better fiber to volume ratio and will be stronger. The outermost layers can be made from mat to prevent print through.
If you can make the deck a little thicker then do it, every millimeter counts and will make the boat much stiffer and stronger since stiffness of a sandwich is proportional to the (thickness)³.

Post pictures of the boat and the damage.

Thanks for the reply. No idea how to post pics on here, although if you google 'Zapcats' you'll get an idea of the boats and what the deck had to put up with! The old deck cracked right through the ply and soaked up water so is scrap. In addition, the boat only runs a 50bhp motor so weight saving is critical. The deck has to be 12mm thick as it sits in a rubber channel down each side and metal 'H' sections between. The deck also has D links glued to it to allow the fuel tank to be strapped down. Seeing the woven mat is fine as it would stop the deck being so slippy. 

This was my first attempt at a new deck. Made of a polypropylene material from Germany. Proved to be way too flexible and became soft in the sun. Shame. 

A few more pics to give you an idea. Boat on trailer and front view showing original deck..

I would think you'd be better off just sticking with a plywood core. You need a reasonable amount of impact/crush resistance and to get that with foam core you will need thicker glass either side. You'd probably end up having to go down to about 8mm core and maybe even have to build it up with a combination of carbon and kevlar to get the same strength. Ply may be a bit heavier, but at that thickness I think it would be much stronger. You could stick with 10mm ply but use a biaxial cloth (1x600gsm or 2x300gsm) instead of mat either side and do it with epoxy rather than polyester resin.

Thanks for the advice. And I know just what you mean, I keep coming back to that conclusion myself. 

Any other thoughts? 

Hi Stepto,

Just A thought on this one... as the others have said at that thickness plywoodis hard to beat for out and out resilience, How about using ply for the railsat the edge where it slots into the channel and then tapering this into athicker foam core, I would definitely look at using a woven glass or carbon with epoxy for the skins; this would vastly out-perform chopped strand matting. I wouldimagine for reasonable crush and puncture resistance the skin would need to bea minimum of 0.75mm thick - that would be around 3 plys of 200g glass.

Another alternative that avoids thickening the core would beto cut out areas of the 10mm plywood core that have little stress and loadingand replace those with 10mm foam before skinning them with the reinforcement, Iwould imagine that you may easily be able to replace 30% of the core, combinethis with thinner, more high performance carbon or glass skins and you couldstill save some real weight!

Keep us updated!

Paul Statham
Easy Composites / Carbon Mods - Technical

I would say...  if you do not want to spend too much on this, then plywood, glass and polyester ...

If you are willing to spend a bit on it, and want it lightweight, then I would go for foam core + 2 x 4 layers of woven glass  + Carbon/kevlar or Twaron/CF.

maybe build it up asymmetrically, with 3 layers 200gsm on the bottom side and 5 on the top (deck) .. It should be more than strong enough, especially with a few layers of Kevlar/CF in there.

That will not be cheap, however ... you would need 16 sq. meters of CF/Kevlar + E-glass + 2 sq. meters of foam core ... add resin and consumables to that (Peel-ply, etc. ) and you will hit + £200

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