Details, attachments, fittings:

This file is an excerpt from the foam sandwich boat building instructions supplied with our plans.
The instructions supplied with the plans are more detailed and contain information specific to the boat.
For faster loading, we divided the smaller online version in several sections.

Chainplates, rubrails, cleats, winches and other hardware should be installed on high density foam inserts or use sleeves or with epoxy putty inserts. High density foam inserts is the preferred method. In some rare cases, hardware is installed on single skin fiberglass.

In addition to the high density foam inserts, all hardware is installed on wide backing plates.

Rubrails are best fastened to high density foam. If the plans show such a rubrail, the plank closest to the sheer should be made from high density foam or the hull to deck seam should be made in single skin.

Seacocks are not structural and do not compress the foam if installed correctly. High density foam inserts are optional.

Seacocks can also be installed through the hull core bedded in epoxy putty. Drill a hole slightly larger than the diameter of the seacock, seal the hole with resin and after it cures, install the seacock with adequate sealant and without excessive pressure.

Hull portholes, windows, hatch framing are installed directly on the hull or deck foam, no need for inserts.

Rudder ports or rudder gudgeons should be installed on high density foam inserts, same as the cleats.

Trailerable boats will have U bolts on the transom and bow. Those must be installed on high density foam inserts.

Alternate methods are SS sleeves or epoxy putty inserts. In all cases, use wide backing plates. Evan's Gatehouse rule of thumb for fiberglass and aluminum backing plates is a thickness equal 0.4 or 0.5 times the fastener diameter for solid glass backing pads or .3 to 0.4 times diameter for aluminum.
For example, if the specified bolts are 10 mm (3/*) diameter, then the aluminum backing plate should be between 3 and 4 mm thick (3/16”).

Sail boat chainplates fastened to the hull are rare nowadays but if your plans call for them, you should make a transition to single skin where the chain plates are fastened.

Lifting plates on power boats are sometimes installed the same way than chainplates.

The end, for now.
This file will be updated and more material added as builders ask questions on our message board.

The information above is based on Jacques Mertens experience with foam sandwich construction since 1977 and on technical literature from:

Thanks to Evan Gatehouse for the technical proof reading and suggestions.

Thanks to our builders who volunteered to proof read for comprehension and grammar, in particular Glover Housman.

Copyright 2007 Jacques Mertens