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Foam Sandwich construction - Part 8

Inside structure, building the parts on a table:

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.

All flat parts like bulkheads, frames, sole etc. should be built outside of the boat. Cut the foam to shape, do the fiberglass work on a table and fit later.
This is a great opportunity to use vacuum bagging. Even if you don't, the glass content will be higher and the finish better if you use compression molding.
It is easy to do: build a table or use a flat floor covered with some Melamine panels, larger enough to take your largest part and build your flat parts on it. You will push the glass and resin down either with a vacuum bag or with another panel and weights.

vaccum bagging table

The table will first be coated with release agent like wax or covered with a polyethylene film.

Vacuum bagging is the subject of a different HowTo.

Inside structure, installation:

Foam sandwich is a very stiff material and requires much less framing than single skin fiberglass boats. In most cases, interior components are used as structural framing and very few if any additional stiffeners are required. Transversal and longitudinal bulkheads, stringers or engine girders, sole and deck will all become part of a monocoque structure.
The way those components are bonded to the hull is critical. A stiffener edge will never push directly on the inside skin or core. The loads will be transmitted by the skins.
Bulkheads must be installed on a trapezoidal foam pad.

faom pad under bulkhead

The foam pads are easy to cut from leftover foam and eliminate the need for the putty fillets used in stitch and glue.

When some parts of the hull require additional stiffeners, those are made from foam strips, trapezoidal section, covered by fiberglass tape.

foam stiffener

Engine beds, stringers and other girders are often made of a "top hat" section:

tophat stringer foam sandwich boat

In high performance hulls, those stiffeners may be capped with carbon fiber.

You can cut those parts yourself or buy pre-shaped stiffeners made from a core foam with dry fiberglass skins, ready to install like the Preforms sold by Prisma Composites.

foam sandwich boat Prisma beam

A stringer on a small boat, not used as engine bed, can be made of foam planks the same way as in plywood/epoxy/glass construction.

foam stringer outboard boat

Angular connections like a chine or transom to hull seam will benefit from a foam insert instead of a putty fillet:


foam sandwich chine corner

The builder must keep in mind that sharp angles always represent a potential weakness in fiberglass reinforced composites.
This means building nice round corners with corner strips and using foam pads under the framing.
In most cases, a radius equal to the thickness of the foam core is ideal.
Excessively large radii are not as strong.

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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