How to build a foam sandwich composite boat

Foam Sandwich Manual from bateau2.com tutorials

Inside fiberglass;

Roll the hull over which ever way you want. Since you will need a cradle to support the hull while upright, it makes sense to build it now and use it to roll the hull over.

This is not difficult thanks to the light weight of the hull at this stage.

The shape of the roll over cradle and how it is fastened to the jig will depend on the boat but two external frames parallel to the molds with bracing are sufficient.

One or two persons can turn over a large hull this way:

• lift one side with jacks until the hull is balanced on a corner of the cradle.
• use a hoist (come along) to control the roll until the hull is at 90 degrees to the floor
• repeat for another 90 degrees until the hull is resting keel down, open side up.

Some builders build a cradle with rounded corners to facilitate the roll.

With the hull right side up, add more support frames as necessary and check diagonals and level.

Once the outside is well supported, before removing the molds, transfer some important measurements to the core: baseline (or a waterline) and some main station locations. Those marks will be visible through the resin. If necessary we will repeat them every 2 or 3 layers. This will help us align frames, bulkheads, sole etc.

Prepare for inside skin:

Remove the interior framing.

Remove cheap foam from single skin areas, taper core foam at single skin transition.

Experience and lab tests shows that the ideal taper ratio of length to thickness is 3 to 1.

Fill gaps between core foam planks if any with epoxy putty.

Single skin areas:

Hardware installed on the hull like chainplates, rudder fittings or rudder ports, may require high density foam inserts or a transition to single skin fiberglass. We describe how to install those components in the paragraph about hardware but you must plan for it at this stage.

Where needed, replace the hull foam with high density foam or grind down the core foam to make a transition to single if necessary.

Single skin areas are made of the combined in and outside skin. In the keel area, the overlap of the two skins from the two sides are often sufficient for power boats but sailboats may require extra layers to receive keel bolts.

For example, a stub keel or inside ballast:

Unballasted boats may use a double sandwich (club sandwich) at the keel to prevent damage when stored on the hard or on a trailer: Some designers use a plywood core in that area.

Y
our plans will show those local reinforcements and specify the lamination schedule.

Build the inside skin.

This is done the same way as the outside skin.

Next: Inside structure

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