Foam Sandwich construction - Part 5

Setting up the jig:

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.

boat building one molds up on jig

There is more than one good method to install the molds on the strongbacks but think of how you will align the molds during the set up.
You will have to move your molds in 3 directions: back and forth, sideways and up and down.
The fore and aft alignment is easy: mark the location of the molds on the strongbacks and check with a plumb line. We like to put some of the strongback cross beams at a mold location. The centerline alignment can be checked with a plumb line dropped from a steel cable running above the jig or better, with an inexpensive laser pointer. The same laser method can be used to level all molds to the baseline.
Drill small holes to check alignment; a laser beam must run through all the holes if the molds are correctly set up.

boat building laser holes in molds

One hole on the centerline at the same height for all molds is a good start but two additional ones on the same waterline and at the same distance from the centerline are perfect. If your laser beam passes through the 3 holes, your jig is perfectly lined up.
We have found that a 3/8" diameter (10 mm) hole is fine for a precise alignment.

Molds should be braced after alignment.

Planking:

We will describe the planking with wide foam strips but before we start, let's look at our plans to check if the designer specified foam core everywhere.
Depending on the design, there are parts of a foam sandwich boat that are made of single skin glass and others where we will need high density inserts.
The standard foam sandwich does not have the compressive strength to take concentrated loads such as under a winch or cleat, at a mast step or for keel bolts. The designer also considers loads on the sandwich when the boat runs aground, is stored on the hard or lifted. A keel or chine may be considered as a girder.
For those reasons, some parts of the boat are made of a single skin or have a high density insert. Some examples:

foam sandwich keel

A single skin keel as in one of our power boats. This is used in moderate and deep vee hulls only, not in shallow vee hulls.

foam sandwich keel 2

Another way to stiffen the keel: a double sandwich made from high density foam (club sandwich).

foam sandwich hull to deck

A high density foam inwale on a single skin transition will receive a screwed through rubrail and toe rail.

foam sandwich cleat insert

A through bolted cleat: the dark area is high density foam.

There is more, we will examine inserts later when discussing details.

A common high density insert is the first plank at the sheer and or at the keel. This is simple: start your planking with the high density foam:

foam sandwich sheer plank

In some cases, like the keel mentioned above, there is a transition from sandwich to single skin.

Below is a sailboat keel or stub keel: the hull is foam sandwich, the keel is single skin.

foam sandwich sail boat keel

To make the transition from sandwich to single skin, after turning the hull over, the foam will be removed with a grinder in the single skin area. The foam is tapered from full thickness to zero then covered by the inside skin and maybe some additional layers up to the thickness specified on your plans.

Since we will remove the foam, there is no need to use expensive sandwich foam in the keel area. Instead, we will plank that part with cheap insulation foam of approximately the same thickness.

foam sandwich boat keel

In the picture above, the dark area shows our sandwich foam, the lighter area is cheap insulation foam that will be removed.

Now that we have decided where to use high density foam and cheap sacrificial foam, let's proceed with the planking.

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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.