Scarab 18 foot folding trimaran

We built two of these boats Both of them were built using foam sandwich construction. The panels were made by joining the foam sheets together the full length of the boat and then covering with glass. Building a foam sandwich flat panel boat is similar to building a boat in plywood. Covering the panel with peel ply makes it smooth so it is easier to fill the boat.

The Main Hull

A temporary frame is made plywood or MDF thick enough not to distort. Both of these boats have the permanent bulkheads in the same position as the temporary bulkheads. The panels are joined along the edges using soft metal strapping if necessary.

We make our own foam sandwich panels. If you have space it is good to make them the length of the boat. Making foam panels.

Main hull

main hull

The main-hull is lifted off the frame and turned over. This boat was designed with a flat panel on the bottom so it can be rest on a piece of plywood or carpet on the floor eliminating an extra building frame. It also makes the boat low enough so it is easy to work inside. The joints were filleted and taped.

main hull

The next job to complete on this boat is the beam supports that hold the forward and aft beams. To make these beams foam is cut to the right shape and layers of foam are glued together. Many layers of glass are added to the foam shapes until all the laminates stipulated in the plans are added. The edges are smoothed off (a sanding disc in a grinder works well). Double bias tape is added to the edges to hold all the foam and glass together. The pivot holes for the folding system are drilled at this time. It is easier to drill plumb holes at this stage. A drill press, if available, works well.

Beam support

Main hull

The beam supports were taped in. The centreboard case and the settee risers are taped in place and the top of the centreboard case is glued in place. An inspection port (used to install the centreboard control lines) is cut before the bunk top/shelf is installed because it is hard to get at later.

Main hull

main hull

The hatch is removed and glassed on the under-side. The cabin top is removed and finished on the inside. It is much easier to glass the roof at this stage and the cabin can be finished now if desired.

main hull

The cabin sides are attached to the frame and the other cabin panels with metal straps. Foam is bent over the frame and attached to temporary pieces of foam attached to the cabin sides. Glass cloth is laid over the foam and wetted out with resin. We use peel ply as well to cut down on finishing later. The edges of the cabin sides and roof are then taped.

main hull

The Floats

float float

The Beams

To see how these beams were built see Building Trimaran beams.

Scarab 18