Rolf

When Styrofoam is used in boats they call it EPS. It is very strong in compression and makes good bulkheads. While the foam is closed cell, it will absorb or trap water. I say trap because if you stand it up, it will drip out over time. How much water the foam traps depends on the foam. The really light stuff (2-3 lb per cubic foot) like what is used in surf boards, traps water badly and is hard to get out. the heavier stuff (6+ lb per cubic foot) will trap only a little and will dry if it does get wet. Styrofoam is used because it is really cheap and easy to buy. There are other foams, PVC and polyurethane are fairly common. Rigid polyurethane is used for building insulation and I buy PVC at a upholstery supply company (I have no idea what they use it for)

If you are worried about water. Make the skin a little thicker and dissolve out the foam with gasoline when you are finished. For carbon /foam boards more than 90% strength is in the skin. There are a lot of little tricks to foam core. Coating the core to strengthen it before you start really working on it or is when the foam gets too thin, you stop it. It is solid past that point.


In terms of joining hollow core wood. The are many ways. I know of one that works but it may not be the best. In general you build up the edges with additional layers of wood . When sand the joining surfaces flat you have 6-10mm wide surfaces to glue together. It is also a good idea to add a central spar. The spar has to be fitted so it just touches both sides. The details are actually more important than the general theory where the wood gets thin like the trailing edge. Everybody has their secret method. I saw an old rudder once where the trailing edge was made separately and the skin cut to fit in the trailing edge. It looked a lot like the Marstrom except the trailing edge was some dense wood.