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IconQuick Hull-buildingIcon

From: Dave Culp <dave@dcss.org>

I have been asked about a quick easy way to produce one-off foam sandwich hulls I used to build one of my boats. It's just a simple foam strip plank set-up, using CAD-generated mould sections. The trick is that a) the hull half-sections are small enough that the moulds can each be got out of a half-sheet (or less) of plywood (so you're just cutting holes in square pieces of 1/8" doorskin ply), and b) with CAD, it's no big deal to build a mould every 6-8", so you don't need stringers.

It is then put together on a large jig. In my case, 2 x 2's were screwed to my workbench, and to saw-horses set beyond each end. The foam strips (3/8" x 1.5", for a 20' hull), are laid in, and just hot-glued to the moulds. Very little tapering is done; what's necessary is done by edge-bending the foam, on the bench, and cutting with a straightedge and knife. (Very elegant tapers result, and it takes only seconds to do).

Once the foam is in, the inside is glassed (and left very rough--no finish resin yet, and no sanding). The half-hull is braced (half a dozen sticks, again hot-glued across the hull), and popped/cut out of the moulds. The whole mould/jig is disassembled and re-assembled, front-for-back, resulting in a "mirror" image in three dimensions, and the whole thing is repeated to get the other half-hull.

Both halves are carefully trimmed to centreline marks, and the hull glued together (not hot glue this time, but resin). We used scrap foam "tabs," glued alternately on the inside of the hull-halves to assure alignment. When the resin set, the tabs were removed and the inside joint taped with 2" glass tape. The stern got a foam transom later, and the bow a solid block of foam, carved to get the shape.

I now had a "solid" hull, glassed/taped together inside, and raw foam on the outside. I cut a bunch of moulds down to make a cradle, and began fairing the foam until I liked it - Surform and a long sanding block. (I'd eliminate the Surform next time, and just use a long block with 36-grit paper). I filled the gaps in the foam with microballoons (less than two quarts, on a 20' hull, and 3/4 of that ended up on the floor as sawdust), faired again, and glassed it.

Lastly, I finished off the cockpit rim, put in the seat-back/bulkhead, and a hatch for the beer. The hull weighed 40lbs, before we added mounting pads and hardware.

I did all the above, completely alone in a single weekend - about 18 hours total, from gluing the plotted paper stations to doorskin ply with rubber cement (much faster than transferring the lines) to final sanding. The only power tools I used were a table saw to rip the foam strips, and a sabre saw to cut the doorskins. I did all the sanding by hand. Newly cured glass is as soft as pencil eraser, if you get it at just the right time. As I had laid up the boat with small batches of resin, I could follow this cure state around behind myself. This trick I learned from Jim Brown on Searunner tris.

Mind you, I have built a hull or two, but never before with either the CAD stations, nor of foam strips. I have hand-carved hulls from solid foam, so am not afraid of shaping to eye, but I used no tricks beyond mixing the resin pretty hot, and moving fast. I expected a pretty rough finish, and would have allowed broad tolerances (speed boats are throw-aways, after all, and this hull was not meant to touch water at anything over 12-15 kts), but was astonished at the quality it produced.

I give complete credit to this technique to Greg and Dan Ketterman. -- They used it to prototype all the TriFoilers, Avocets, and for LongShot herself.

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