Step by step:

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.

The techniques described below apply to all method: wide sheets contoured or not, planks shaped or not.
If the builder choose to plank with wide sheets on slats, less molds will be required.
We will focus on the foam plank method with plain foam, no bead and cove, no skin on the inside.
The jig is made of molds spaced maximum 18”. It is then covered by Divinycell H80 strips 6” wide, 1/2” thick.

Strongbacks:

The first step is to build a strongback to support the molds. All boat building books describe how to build a strongback and level it.

The strong back is a ladder type frame usually made from heavy wooden beams but some builders use steel framing as in the picture below.

On the strong backs we will mount our molds.

The strong backs must be level. This is extremely important but easy to achieve. A hose water level is the best tool to level a strongback. It is a simple clear plastic tube filled with colored water (food coloring works well):

To use such a water level is simple: attach one end to a wall with the water approximately at the level of one corner of the strongback. Put the other end against the corner of the strongback and measure the difference. Let's say that your strongback is 2” below the colored water level. All other corners must be adjusted to be at the same 2” below. This is much more accurate than any mason level or laser system.

The strongbacks must not only be level but square. Compare the diagonals, if they are equal, the strongback frame is square.

Molds:

Molds can be made from particle boards, framing lumber or cheap plywood.

In some cases, CNC cut molds are available. For all our foam sandwich boats, a DXF file showing the stations full size is available.
A DXF file is a Drawing eXchange Format file, a CAD file compatible with all systems. All blue print shops and plotting services can read DXF files.
Our DXF files can be used by a plotting service to print full size patterns. Mylar should be used for patterns because paper is not dimensionally stable.

You can also cut your own molds from the dimensions given on the plans. All our plans for foam sandwich show all the molds spaced 18” or less and the dimensions for each mold. This is rare. Most boat plans show the traditional 10 stations. Ten stations are not sufficient to build a jig with closely spaced molds. Additional molds located between the ten stations are necessary. If the dimensions for these molds are not on the plans you may have to loft the complete lines plan full size and extract molds dimensions from the lofting. This is not required with our plans.

Fabricating the molds:

If you do not use full size patterns extracted from our DXF files, you must draw the molds on a table or floor made from plywood sheets.

Definitions:

Before we proceed, let's in introduce some notions that we will use: stations, waterlines and buttocks. Those are sections through the hull used to represent the 3 dimensional shape in 2 dimensions.

The waterlines (WL) are just what the name says: lines produced by sections parallel to the water line:

The stations are transversal cuts:

The buttocks are longitudinal sections perpendicular to the waterline (vertical):

We will use the stations lines to draw the molds.

Next: Drawing the molds

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