Copyright 2017 - Woods Designs, Foss Quay, Millbrook, Torpoint, Cornwall, PL10 1EN, UK
  • home built Flica 37

  • plywood Romany 34

  • lightweight 14ft Zeta mainhull

  • Strike 15 trimaran at speed

  • 28ft Skoota in British Columbia

  • 10ft 2 sheet ply Duo dinghy

  • 24ft Strider sailing fast

  • 36ft Mirage open deck catamaran

In principle any flat panel hulled catamaran, like a beach cat, the Gypsy or even a Meander, can be built using solid grp panels. Having said that I have found that you cannot "torture", ie compound curve, a grp panel like you can a plywood one, for, unlike plywood, grp doesn't stretch.

Boats are usually strong enough, it is getting enough stiffness that is usually the problem. Adequate stiffness is easiest to achieve by making a thick hull (and that comment applies whatever material is used). Solid GRP is thin, typically a 30ft catamaran hull need only be a few mm (say 3/16in) thick to have enough STRENGTH, after all GRP is as strong as steel for a given thickness. But it wouldn't be STIFF enough. So you need to use either a number of bulkheads and stringers, or a cored hull. The alternative is to use a very thick grp hull, as in the early "chopper gun" grp boats from the 1960-70s.

I have built large solid grp paneled boats, but I found that adding the stringers, necessary for the stiffness, was time consuming and also heavy. So I decided that even when building on a budget, a foam core was preferable, especially as it resulted in a cleaner interior.

For a good rule of thumb is that you save about 1/3rd of the weight by using a foam core. So, in imperial units (to keep the numbers easy) say your typical 8ft dinghy/yacht tender used a solid laminate of 3oz/sqft. If you built the same boat in foam sandwich you could use 1oz/sqft on each side of a core for the same strength. You'd be lighter and also stiffer. However the 1oz skin would be very prone to impact damage (which is OK on a racing dinghy that is looked after and handled gently ashore, but not for a hard-used yacht tender) So a thicker skin is often needed for practical reasons and thus there is a sensible minimum skin one can use, typically 11/2oz.

If you built a 24ft cat hull in solid glass the laminate would be about 6oz/sqft (again in imperial to keep the numbers easy). If you use a foam core (say 1/4in-3/8in foam) then you'd use 2oz each side (so pretty close to the recommended minimum thickness).

You would save about 5oz/sqft or roughly 30lb per hull. But, as you'd also need to add stringers on a solid hull (adding say 10lbs) you'd maybe actually save 40lbs per hull by using foam. So not a lot, but maybe a worthwhile saving, especially when you add in extra factors like extra buoyancy, reduced condensation, more space because no stringers or frames are needed.

Clearly as boats get larger (say over 30ft) the advantage of using a foam core becomes more obvious as the skins become thicker and the weight savings are much greater.

A compromise to solid grp on a smaller boat, like the Strider for example, is to use Coremat as the core. (This is a product that looks something like thick blotting paper - it isn't to be confused with Corecell). However it uses a lot of resin and, surprisingly, I haven't found that it bonded as reliably to the skins as when using foam. In fact early production Striders were built in Coremat, but when we changed to foam on later boats we found they were lighter, stiffer and also no more expensive, as less resin was used.

The only time I recommend using a solid laminate is when the panel size is narrow. Typically the Gypsy hull bottom, for example, or the Romany lower chines. These panels become extra stiff once the chine joints are completed while it is easier to make a joint if the panels are solid glass rather than cored. And of course abrasion is more of a potential problem on the hull bottom than it is on the topsides.

Solid grp is always heavier than plywood (it sinks, it's specific gravity is 1.2 at best). So plywood is the lightest/stiffest for small boats (it floats, specific gravity 0.6 at best). And because it is thicker than grp it is stiff (and as I said at the beginning, achieving enough strength is rarely an issue). 1/4in ply weighs about 20lbs a sheet, or 2/3 lb/sqft. You have to be a good laminator and use a lightweight foam panel to get anywhere near that weight using grp, while a 1/4in thick solid grp hull would only be used on boats over 35ft long. Typically a 35ft plywood hull owuld be built in 9mm (3/8in) plywood which weighs about 1lb/sqft. But you need stringers and frames as well of course.

It makes no sense to me to build a foam sandwich beach cat if a tortured ply one is lighter, cheaper and much quicker to build, yet still stiff and strong enough. Only the Olympic standard Tornado sailors can tell the difference between a high tech resin infused foam hull and a tortured ply one. To everyone else a Tornado is a fast boat! And, despite all that extra cost, time and skill required, the hulls are no lighter than the 4mm (3/16in) plywood Quattro hulls.