PVC pipe idea I haven't seen yet

Hmm, how about taking an inflatable dinghy and fiberglassing it over? Would it have enough strength to withstand both rough beating and punctures? If yes, then that's what I've been looking for.

Edit: Or just make some fiberglass pipes and go from there?

 

Large HDPE pipe might be useful for hulls on a catamaran dinghy.

Using an old inflatable dinghy as a removable form for fiberglass sounds interesting.

 

Glassing over a non rigid surface can be problematic. As the resin cures, it tends to distort if the supporting surface isnt very rigid.

 

that means : ".... if the supporting surface isn´t very rigid."

 

I would guess PVC pipe that large would be costly, it would be far less expensive to build a better shape from stitch-and-glue plywood and glass that over. It would weigh a lot less too.

Or if you have some way of making fiberglass pipe over a low cost mandrel than you can do away with the PVC pipe all together.

 

I made a couple of small (75 mm dia, 200 mm long) test cylinders made of inflatable boat's pvc fabric and fiberglassed them over with something like a 400 g fabric at right angles, and epoxy resin with a 1:2 ratio, 20 minute hardener. I used a jar/bottle as a 'mold' (think 'inflated') for rigidity which I later removed.

1 layer is soft to the touch
2 layers is already quite rigid
3 layers does not bend by hand

Inflatable boat fabric is non-removable and has effectively fused with fiberglass.

The boat design I want to use is inflatable dinghy with a plywood and stern, 3000 mm length / 400 mm dia pontoons, plus < 750 mm cones on the back, plus about 1000 mm of curved bow part, for a total of about 5 meters of length, with a slightly pronounced keel (the keel is actually inflatable on the dinghy).

Now I cannot decide:

(1) Replace plywood floor on my old inflatable dinghy, inflate it and fiberglass over with 3-4 layers, making sure it stays inflated;

(2) A hybrid: Use PE/PVC pipes as pontoons for additional rigidity over that of an inflatable boat, and fiberglass over as in version 1;

(3) Mix plywood parts with a metal fence mesh, approximating the shape of a dinghy, put a cut-to-shape inflatable pvc fabric on, and fiberglass it over as in version 1.

(4) Scrap the inflatable dinghy and make an all-plywood boat and fiberglass it over. But then again I don't need pontoons or anything from a dinghy design so it would be an entirely different boat.

Also recommended by a friend of mine:

(5) Make styrofoam male mold using multiple layers cut with a machine, mix it with plywood floor, and fiberglass it over.

 

I'd lean towards (5). But regarding fiberglass over inflatable fabric: 1) would mold release allow the inflatable fabric to be removed and 2) is a tightly inflated bladder inside a thin fiberglass shell useful (ie, stiffer, stronger, worth the extra weight)?

 

What about PE (polyethylene) pipe used in floating docks and such; how well does it bond to epoxy+fiberglass?

 

PE doesn't glue.

From a common sense perspective one has to look around and see how long boats last. Of all boats it's the fiberglass ones that last and last and last. In fact, they are the longest living boats there are with the least care.

I won't use PVC or any plastic material in pipe form I very much doubt it can be made into something ok, but what I have done before was to use tubing as part of a former to obtain a certain shape. When you glass around this shape you pretty much get something that is better. So if you glass around three pipes wrapped in plastic then pull the pipes out you get a triangular shape where one V is down and the top is a bit of a deck.

The biggest problem building a boat, because of it's size is forming the shape and to get both sides the same. The easiest method I came across (for me) is to make a shape with the features I want around a pipe, wrap it in plastic and then fiberglass it.

 

I used thickened epoxy to glue pvc pluming pipe to wood. I sanded the pipe first with 80 grit sandpaper so the epoxy would have something to grab. After it set up I was unable to pull the pipe off the wood by hand.