Copyright 2017 - Woods Designs, Foss Quay, Millbrook, Torpoint, Cornwall, PL10 1EN, UK
  • home built Flica 37

  • plywood Romany 34

  • lightweight 14ft Zeta mainhull

  • Strike 15 trimaran at speed

  • 28ft Skoota in British Columbia

  • 10ft 2 sheet ply Duo dinghy

  • 24ft Strider sailing fast

  • 36ft Mirage open deck catamaran

Over the years I've built several foam sandwich boats, but always as an amateur rather than professional builder. That means I haven't had access to lifting gear, a large work force, sophisticated heated workshops etc and I only use tools that anyone might have at home.

There are several ways to build a foam sandwich boat.

The conventional way is to set up complete frames upside down, add timber battens at approx 150mm intervals. Then add the foam, glass the outside and finish it. Then release from the frames turn over and glass the inside. This is easy and quick. However the unsupported hull will be floppy until the inner skin is made, so can easily distort. Also glassing the inside can be tricky.

So another building option is to make a split mould, with a join on the hull centreline (along both keel and deck, as the deck can be included). Using this method the inner skin is laid up first, then the half hull/deck released from the mould, outer layer glassed (which is easy to reach) and the two halves joined. The disadvantage of this method is making the two halves exactly the same and making the glass joint inside, especially near the bow. You have the same length of joints to make, so there is no weight saving.

The third system which high tech race boats use is to make a complete male mould including a skin. Then this mould is smoothed, covered with release agent and the inner skin laid up. Then the foam core added using a vacuum bag and finally the outer skin laid up. Without doubt this last method gives the best results, but is also the most labour intensive and most expensive. It is also the only method that requires the use of a vacuum pump.

All three methods can be used with the plans I supply and the finished boat will be equally strong and approximately the same weight whichever system you use. (The vacuum bagged method will probably be the lightest as it tends to use the least resin. But we are only talking about a few Kgs, so any saving is academic really, especially when you consider that the shell weight is probably only half the all up weight.)

Airex foam is still the only sensible choice for double curved areas, eg the hull bottoms as it can bend at room temperature. But it softens in even moderate heat (eg from the sun). So a rigid pvc foam is necessary for the decks and topsides. I have found Divinycell to be the best foam, partly because it has smaller "pores".

Divinycell make a lightweight bonding paste for their foam, but other companies make similar products which I have also used. The main problem with foam sandwich construction is that although you can stick glass to foam its hard to stick foam to glass. Because of the huge area you need lots of pressure.

People tried sandbags etc to weigh the foam down, but it doesn't work as you can't get an even pressure. Its also lots of heavy sand to move! That's why about 25 years ago people started using vacuum bags. But they found there were problems in getting all the air out over the whole panel. It was possible to suck the air out near the outlet and then the polythene sheet would stick down hard and so prevent any more air being sucked out. So builders then started laying plastic mesh fencing between the laminate and polythene. That made channels for the air to run through. But it was messy as the resin stuck to the fencing. So people then developed bleed cloth which seemed to do it all. But of course its expensive as its a one time use only material. Also it's time consuming to stick down.

Using a vacuum does suck up some of the resin and remove most of the little air bubbles. That makes a stronger, lighter laminate but to be honest I don't think the savings really amount to very much and are only important on a racing boat. And the bond is still only on the surface of each foam skin.

Instead of using a vacuum bag, I prefer using "contour foam" or scored foam, which is like end grain balsa in the sense that it is cut into small squares (about 11/2") and stuck to a thin glass backing. The squares mean that not only will the foam fold round a curved hull but also it can be laid down one row of squares at a time. Thus each square is in effect put down individually. Furthermore the bonding paste oozes out through the squares so you can see that its properly bonded, and also the squares increase the bonding area. I admit the weight is a bit higher, because of the extra bonding area, but its quick, reliable and needs no disposable materials. You can also do it alone. All the Sagittas, Elves, Flicas and Banshees were built that way. It's also how I built my Gypsy and Eclipse. I haven't had any reported problems with core failure over the last 17 years.

I hope the above helps explain the alternative "low tech" foam technique I prefer.

The glass I like to use is +/45 deg "Biaxial" glass bonded to a chopped strand mat backing for stability and inter-layer adhesion. That is, instead of the glass strands running along and across the roll ie 0 deg and 90 deg (0/90) they run at 45 deg to the length. Its harder to make and thus more expensive, but has three advantages. One is that it isn't woven, but two layers laid one on top of the other. That makes its stronger as the woven glass tends to try and straighten the rovings which is weaker. Two, the bond to the substrate is better and it looks neater as the glass is flat. Finally, the impact strength is increased. That's because the energy from an impact runs the length of the glass strands and then "explodes", as it were, at the ends. So the longer the strands the better. That's one reason chopped strand glass isn't very strong. Obviously the longer the glass lengths the better. Glass at 45 deg is 40% longer than at 0 or 90 deg.

If you build in epoxy resin instead of polyester then you don't need to use the mat backing. However I suggest making a few test samples as some glass cloths are more "drape-able" than others. That is a good thing on small complex mouldings but a real pain to lay down onto a big hull.

The newest construction technique is to use resin infusion. I am not going to say any more about it here. In part because I feel that if you need to know the basics about it then probably you should chose an easier building system. In other words, it is for experienced builders.