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Foam Sandwich construction - Part 2

Foam strip planking:

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.

This method of installing foam over the molds is similar to strip planking with wooden strips.

foam sandwich boat building

There are no battens, the foam strips or planks are installed directly over the molds.

foam sandwich boat building, side planks

Warning: the picture above does not show all the molds or the strongbacks.

We can use plain rectangular planks or bead and cove strips:

foam sandwich boat bead and cove

Bead and cove foam strips

Bead and cove requires either pre shaped strips or strips with the bead and cove cut by the builder on a router or shaper. The second option is much more labor intensive. In addition to the increase in labor, there will be a waste of expensive material.

A faster, easier and less expensive method is to use plain rectangular section strips. We will discuss later how to line them up and fasten them to the mold.

Planking with foam strips has its own requirements. Foam is not as stiff as wood and will sag between widely spaced molds.
How much the foam will sag depends on the type, thickness and width of the foam strips.
Keep in mind that while the planks may look fair at rest, you will be pushing on them while wetting the fiberglass fabric with resin.
Wide and thick planks will be stiffer than thin narrow ones.
Hull shapes with little curvature in section will allow the use of wider (= stiffer) strips but that may not be sufficient to take care of the sagging problem.
In addition to good fastening between the planks, there are two ways to keep the foam strips from sagging
- reduce the spacing between molds
- make the strips stiffer

Molds spaced about 18" ( 45 cm) are generally sufficient to support foam 1/2" thick or more in strips 6" (15 cm) wide. This works well for moderate curvature shapes.

foam sandwich boat molds jig

The picture above shows a 10m (33') sail boat hull with a mold every 45 cm (18").

If the curvature requires narrower strips, a solution is to increase the stiffness of the strips.
This is done by laminating a first thin layer of glass to one side of the foam before cutting the strips. A 4 oz. woven fabric (120 gr) is ideal.
A foam sheet with such a thin layer of glass can easily be cut in strips on a table saw fitted with a blade for plastic or kitchen counter top laminates.
The fiberglass side will always be on the inside of the curve, this means on the inside skin of the hull.

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

  • CoreCell
  • Airex
  • DIAB products: Divinycell, Klegecell, Renicell
  • Dupont Nomex
  • Nidacore products
  • Raptor

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