"best value" core material

[rapscallion]

I have been looking into Core-Cell, Airex, nidacore, Rohacell, and balsa. I am a bit surprised at the prices of this stuff - if you were looking to build a 22' multi hull what core would provide the best value? Let's pretend the design allowed for such substitutions. Perhaps the Lindsay Lord approach with an aerorig would be the way to go. I could get my headstay tension and I could get a light hull for less cash.

[TeddyDiver]

Local timber

[marshmat]

If you ask me, surprisingly few of the foam cores are actually suitable for boat use. I would suggest, if you're having a hard time deciding, to do the following test (or some variation thereof):

Get some samples of each core you're considering. A square foot of each could suffice.

Cut each into three pieces- say a 12"x6" and two 6"x6".

Put one 6x6 piece of each in a bathtub or bucket of water and let it sit for a few days. Any that absorb a significant amount of water get cut from the list. Boats get wet. Boats bump into things. Boats flex. Boats have holes cut through them. Resins aren't impermeable. The core will get wet at some point, and you want one that won't disintegrate in the water. (End grain balsa is the odd one out here- while it'll soak up water parallel to the grain, it won't allow it to flow perpendicular to the grain within the wood structure, thus it's OK if the skins are bonded well. Foams aren't directional, so if they soak up water in this test, a small crack or blister could cause several square metres of core to saturate and fail.)

With the other 6x6 piece, you will try to beat the s**t out of the core. Use your fingernail at first- it shouldn't dent much if at all. Try a mallet. Then try a framing hammer. Then try the claw of a framing hammer. Remember that the core serves the same function in your hull as the web of an I-beam does; it has to be rigid and able to distribute any abuse evenly to the opposite skin. Any cores that don't inspire confidence after this get cut from the list.

Take the long piece (6x12, say) that remains and make a bridge between two bricks. This should be done out in the sun. Put a sheet of paper in a similar colour to the proposed hull over this, so that it will have a similar tendency to heat up in the sun. Put another brick at mid-span. Let sit for a while in the sun. Does it warp? Does it return to its original shape when the brick is removed? If the core is going to deflect under load in sunny weather, and stay that way, so will your hull/deck.

Now take a look at the ones that didn't get cut from the list and buy the cheapest of them.