Plywood vs GRP

I am considering building a small racing class dinghy and would like to hear some opinions on the merits of building the boat in Plywood as oppossed to foam core fibreglass.
From experience weight is not much of an issue as most of the boats need to use corrector weights to get up to the min specified by the class rules.
the main characteristics I will be look at are

• stiffeniss
• durability
• longevity

I would also consider using a light glass cloth to sheath the ply hull.

 

If it's a hard chine design with more or less developable surfaces, then I think plywood is a faster method than grp sandwich.
Stiffness can be very high (locally) for a sandwich panel, but in a small dingy the outer layer of glass will be very, very thin.
So durability may be a problem.
If a plywood dingy is stored in a dry place it will last "forever"

 

The dinghy is hard chined.

Timber also looks better in my opinion.

 

And plywood is cheaper than most foam cores

 

And if you can't get the hull weight down around 8lbs per foot of length for a plywood dinghy up to around 18ft in length, you just aren't trying hard enough.

And that's not using any complicated stringer and frame setups. Just a couple of bulkheads, couple of buoyancy tank tops, the centrecase structure and maybe a total of two bottom stringers if not multichine.

Those sort of weights are measured weights.

15ft 6" dinghy - measured weight 126lbs with epoxy coating but no fittings. Built of 6mm (1/4") gaboon.

7ft 9" dinghy with a lot of surface area - side decks etc 57lbs with epoxy coating and no fittings
Built of 2 sheets 4mm exterior hardwood ply and 1 sheet of 6mm gaboon.
(the hardwood ply was chosen for cheapness - not light weight!)

15ft 6" canoe
6mm gaboon ply 44lbs
3mm gaboon ply with light glass over bottom - 34lbs

All with durability and no need for a strongback to build the boat over.

In general I wouldn't suggest glassing a wooden raceboat. It generally adds more stiffness for the same weight to simply go for thicker plywood - glass is heavy and not particularly stiff.

There is a use for glass with really thin ply to stop concentrated loads like crew's feet going through 3-ply (3 layer) plywood but often just going to a thicker 5-ply plywood will resolve the problem for the same weight but add more stiffness to the structure - which is why the 6mm bottom on the small dinghy above - it was a replacement for a 4mm bottom that the crew put their foot through.

MIK

 

Does the class have any restrictions on materials?

 

No carbon or kevlar.
GRP sandwich or plywood construction only