Plywood-foam-plywood composite: why not?...

Ply being an inherently stiff material, and cores being mainly to compensate for materials deficient in stiffness, it is hard to warm to cored ply skins. And I think you do have a hard spot issue there.

 

I identify with your reasons, and I think the methodology should work fine.

The problem of hard-points from the internal frames only matters IF the adjoining parts of the hull are less stiff, and if you are going to apply a Core to ALL the insides, this would hardly be a problem.

Also, as you say, 4-5mm ply is pretty damn stiff by itself, and would only show hard spots with the help of badly spaced trailer rollers or say a jetty column impact.

 

Far from ideal, I'd say. Even a sandwich with two skins meeting a bulkhead abruptly, without longitudinals, or transitioning, is not ideal.

 

So what do you recommend for a standard ply hull? Won't it have the same problem?
If not, given the inherent stiffness of the ply, wouldn't it be better to go through to the outer skin?

 

A "standard" ply hull would have longitudinal battens laid over bulkheads/frames, then the ply. You could do a little experiment with small samples, to see how the various ways stack up, by testing how much force is required before each method fails. Commonsense tells you the thin outer skin in direct contact with the frame, will give way first.

 

@Coling

No, the hardspots are a very real problem that continue to develop over time. To control a 4mm panel over 30 feet during construction, you would need about 24 molds just to distribute the curvature (Or a substantial batten system). Nether allow you to install inner panels and retain the shape. The method is used extensively for hatch covers and other places where a perimeter frame controls panel shape and provides strength, and the inner foam and ply are splotched in last.

There are some specialty uses for this though. In multichine construction, where the panels shape can be controlled by closely spaced chines (the skin thickness is reasonable considering the width of the panels), you can run a foam and ply inner panel post assembly, but permanent frames land on the inner skin with a hat strip. The inner skin stops with a bevel a few inches from the chine and tabbing provides the additional strength there. This gets you additional positive floatation, and better sound and thermal insulation. And it lets you organize and accommodate the fitout of electrical, plumbing and hardware mounting.

As PAR notes, this is not a particularly light way to get the hull done, and it is relatively expensive and very time consuming. But it can work out in the end if you needed additional floatation anyway, and you wanted a clean inner hull on a small section such as a catamaran hull. And at least you have enough hull thickness to screw stuff to the hull without going all the way through. My Catalina had about 50 curtain hardware screws poking out though the cabin sides when I bought it.

But it always pays to compare the scantlings and build requirement of different methods to some standard like an equivilent single-ply 18 oz woven roving layup. Yes, the latter would seem like a rubbermaid trashcan rigidity-wise on paper, but with careful attention to compound curves where you need rigidity, it works just fine at 30 feet.