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You can use okoume. It's accessable in Europe.

I was wondering why nobody uses plywood to get additional strenght?

ok, but why okoume? maybe is hard wood but heavy 400-600kg m3
balsa 120-190kg m3 2800 psi
paulownia 260-290kg m3 5700 psi
Western Red Cedar 390 kg m3 ~6000 psi

NO. When balso is used as a composite core material, it is used in the "end-grain" direction. So you would cross-cut let's say 100x100x6mm lengths of your balsa and then arrange them side-by-side into a core. Done this way it is stronger than high tech foams, but not water resistant if you get a skin puncture.

alden wrote:NO. When balso is used as a composite core material, it is used in the "end-grain" direction. So you would cross-cut let's say 100x100x6mm lengths of your balsa and then arrange them side-by-side into a core. Done this way it is stronger than high tech foams, but not water resistant if you get a skin puncture.

What you describe is certainly the classic definition of core material in a sandwich. the balsa is aligned to maximise its compressive and shear properties.

truth is, the flexural properties for balsa are so much better than most other core materials that the balsa is frequently aligned length-wise to minimise the need for additional reinforcement.
cross-grain compressive strengh is still exceptional.

one of tthe most popular construction methods is to fabricate "sandwich" skins using 1/8" balsa planks for the core. The sandwich is laminated top and bottom over a very light core such as 1# EPS.
Similar to composite sandwich construction in surfboards and windsurfers.

lots of ways to skin a balsa tree.

regards,
-bill

vol wrote: balsa 120-190kg m3 2800 psi
paulownia 260-290kg m3 5700 psi
Western Red Cedar 390 kg m3 ~6000 psi

How about Cedar? I have been considering it as a core material and was wondering if anybody had some feedback?

thanks!

If you can get it and you want to use a wood core I'd go Cedar - has one of the best strength to weight ratios going.