Composite panels in place of plywood for frames on stitch and glue boat?

On a boat such as this one:

http://www.selway-fisher.com/Mcup16.htm#HARRIER

Would it be possible/advisable/advantageous to replace the plywood frames, seats, and foredecks with composite panels constructed of foam board laminated with fiberglass or possibly door skin plywood, foam core, and fiberglass?

 

What do you hope to gain? It is just a 13 ft pram.

 

door skins do not use water proof glue. It would not be advisable since no matter how well you seal it in fiberglass eventually moisture seeps in and will weaken the primary structure. If your intent is to make it lighter, a fiberglass boat is heavier than a wood boat, so there would not be much advantage to the extra cost of and effort to make fiberglass foam sandwich panels. It can be done, but you will add cost and extra work without a lot of benefit. the stiffness to weight ratio of wood is about double that of fiberglass, so you usually a lot more fiberglass to keep it stiff enough, which ads weight. Some of the lightest boats are actually plywood with a layer of fiberglass to make it more durable.

 

Having a few small craft designs under my belt, I can tell you straight up, it's very difficult to get similar strength, stiffness and comparable price point, particularly for the home build, with any form of composite panel. Unless you toss out the price point, you'll just never get there and home builds are all about price point, which is a primary driving force behind most.

This said, I could build a competitive weight, stiffness, strength composite, much like that dory sort of thing, but it will not be home built friendly, nor cost competitive. Even using cheap foam, unrefined epoxies and crappy plywood formers, the labor is much higher, then cutting up some marine grade with a jig saw and taping the seams.

Simply put, yeah, it can be done, but not as easily, nor as cheaply, which is the whole point, when interested in developing, a successful home built design. A successful design like this, should be easy to build, with a minimum of parts, simple yet robust structure, with built in idiot proofing, using commonly available materials and techniques, that an average person can perform, with the tools they have in their garage. That's one hell of a set of goals, let alone the design's basic SOR.

 

Assuming you are trying to keep this cartoppable, the things I would do is use all okume ply. It is light and stiff. Some of the furniture looks like you could get away with 4mm. But you have to check your sheet layouts, it may cost you an extra sheet. You'll be trading an hour or two of labor for every pound you save during the build, and another hour or two in the redesign, and probably add about $20 of cost for every pound you save under the listed 145.

You can forego glass entirely and just paint the hull, also. Might only last 20 years, though. Personally, I'd skin it with 4oz deck weave and use 9oz tape on the chines and keel.

Workmanship is the key to keeping the weight out. How much bubbles can you mix into the fillet epoxy? What is the lightest wood that will work for those floor boards? (Is it scrap 6mm ply?, Can 4mm hardwood be substituted?) Can you avoid all metal hardware and just use ropework? Can you make some panels removeable? That's usually easier than making something not exist.

I'd add a piece of that floor board material across the inside of the front and rear transoms, as well. Little boats tend to get there noses banged.

Very nice looking little boat. I had a 14' Mac dingy for many years and had a blast with it.