How to build a foam sandwich composite boat

Foam Sandwich Manual from bateau2.com tutorials

Planking the jig.

Planks width:

The width of the foam planks depends on the curvature of the hull. In flat areas, very wide planks can be used, in others, the planks must be narrow to produce a smooth curve. Your designer may specify the width of the planks.

Plank fastening to molds:

The strips will be fastened to the molds from outside with nails or screws on buttons and we will use copulas between them, along their edges.

Copulas are small plastic barbed nails made by Raptor.

The nails are removed before fiberglassing.

If the molds are too thin to take a fastener, small cleats installed flush with the molds should be used.

Plank to plank fastening:

Some builders glue the foam strips together with spots of hot glue, others tie them to the molds with straps (masking tape) and “spot weld” with resin in between. We prefer to use copulas.

The copulas stay in the core.

If copulas are not available, the builder can use toothpicks. In addition to the copulas, the planks can be spot welded with small amounts of fast cure epoxy glue.

Planking sequence:

This will depend on the hull shape but the most common method consist of planking from the sheer up to where the chine curvature starts

and then start at the keel, proceeding towards the chine.

The planks between those two areas must be cut to fit as you progress.

This is the most common way to lay out the planks but other methods are valid; it doesn't really matter.

In most cases, the transom will be left open to provide access to the inside of the jig but it is not an absolute requirement. In outboard powered boats, the transom is almost always installed on the jig at this stage.

The gaps between the planks are filled with a resin slurry then faired. As we stated on the first page, we exclusively use epoxy resin, no other putty or glue.

At this point, the building progress is almost identical to the building of a plywood cored boat. We will apply the outside skin, let it cure, flip it over and build the inside skin.

Books or articles about foam sandwich will recommend applying a coat of bonding putty to the foam before the first layer of glass. This is important with polyester to avoid core separation but it is not necessary with epoxy.

Compared to plywood cored composite, the fiberglass skins will be much thicker. We usually start with a priming coat or tack coat of resin over which we apply the layers of glass specified by the designer.

We prefer to work wet on wet. This is applying one layer glass and epoxy over the other while the first one is still wet or green. Green means not yet hardened or cured.

Wet on wet has many advantages. In particular this method will produce a laminate with a higher glass content (this means stronger) and we avoid potential amine problems and sanding between coats.

There are two problems that could keep us from working wet on wet: exotherm and flexing of the planking.

Exotherm problems result from heat produced by the epoxy cure. Fortunately, epoxy has a much lower exotherm than polyester and only very thick laminates require you to do the layup in several steps. For those hulls, the lamination will require several days anyway.

Flexing of the core while applying the fiberglass may happen on any boat built with the foam planking method.

If, at the start, the planking flexes between molds, start by applying a skin coat: the first layer of the lamination schedule and let it cure before proceeding wet on wet with the next layers. This will add enough stiffness to the hull in order to properly squeeze the excess resin out of the following layers.

If the first layer is heavy material, use a thin veil like a 4 oz. woven cloth as skin coat.

We prefer to use full lengths of glass and work wet on wet. On large hulls, it may not be physically possible to apply the fiberglass lengthwise. In that case, the glass can be laid up across the hull but pay attention to the fiber orientation.

If wet on wet is not possible, depending on the resin type and temperature, you may have to scrub the resin amine blush between layers of epoxy.

How and when to scrub amine between cured layers is described in our epoxy How To files.

Often, we use a finishing veil to reduce the need for fairing. This is a last layer of glass, a very thin one, installed after a rough fairing of the previous layers.

All the hull fairing should be done while the hull is upside down. See our tutorials about fairing, sanding and priming.

Next: Inside fiberglass

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