How to build a foam sandwich composite boat

Foam Sandwich Manual from bateau2.com tutorials

Deck:

The deck and superstructure can be built either on the hull or better, separately on a plug, the same way as the hull.

It is tempting to use the existing hull with its frames and bulkheads as a mold for the deck but you will do a much cleaner job building the deck on a separate mold. This may even be easier if the designer used a constant camber for all the parts. In that case, one slatted mold the size of your largest part will be sufficient.

In the best case scenario, that same mold will be used for all deck parts, the roofs and sometimes for the cockpit or cabin sides.

In other cases, it will be necessary to fabricate a complete superstructure mold. If you kept the cuts from the hull molds, they will provide a good start with the correct camber and width.

You can also use the top of the hull molds to build a deck mold but keep track of the baseline.

Some builders prefer to build the superstructure on a female mold. Either method is valid but the female mold has one big advantage: the outside skin can be applied with the deck on the hull allowing for overlaps on the hull which create a much stronger deck to hull joint.

The second method uses the existing hull as a mold.

Battens are installed on top of the bulkheads and that structure is used as a mold.

In this picture, we see the deck of a small power boat. No bulkheads but temporary deck frames. What you do not see are battens under the foam and polyethylene sheets to keep the deck from bonding to the hull.

The upper side of the foam will be fiberglassed, the foam removed, flipped and the other side glassed.

During the fabrication of the deck, it is essential to plan for the installation of hardware and use high density foam inserts wherever necessary: under winches, cleats, windlass, pulpit, stanchions etc. This is only required where we expect high compression loads. There is no need for high density foam around hatches for example. See the details chapter for more information about hardware installation.

Deck to hull bond.

The picture above shows a simple and clean hull to deck seam.

In the picture below, the sheer clamp and the edges of deck and hull are made of high density foam. This allows the installation of a rubrail and stanchions with mechanical fasteners.

The strongest boat will be the one with a deck tabbed to the hull with inside and outside tape.

Here we show a hull to deck seam with single skin corner filled with a high density insert. This is the strongest possible hull to deck joint, ideal for commercial boats but not necessary for most pleasure boats.

The outside tabbing will produce a ridge that is difficult to fair and the inside tabbing will be problematic unless the hull is rolled on it's side. For that reason, decks are often simply epoxy glued to a sheer clamp with a thin tape of glass on the outside.

Keep in mind that most production boats have their decks simply fastened to the hull sheer clamp with mechanical fasteners and a sealant. The best production boats have their deck epoxy glued to the hull with a sheer clamp but very rarely tabbed to the hull.

The deck should also be epoxy glued to cleats on frames and bulkheads. Inside cabins, those cleats are sometimes made of laminated wood to mimic wooden roof frames, in other places, we will use the same triangular section of high density than for the hull to deck seam.

In this picture of a superstructure section, you see roof beams under roof and triangular cleats under gunwale, made from high density foam or laminated wood. Sheer clamp not shown.

Next: Details, attachments, fittings

Jacques Mertens
www.bateau.com
Vero Beach, April 2007