How about a thick(ish) 3D core?  That gives a nice stiff panel and the additional hexagon structure should help with impact resistance.

I've thought about cutting out sections of plywood but in my mind it just seems a compromise. And a lot of work! 

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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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

Any other thoughts? 

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

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.

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

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. 

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.