John,

Plywood is much heavier than balsa for a given thickness unless you use Liteply which is a balsa layer between two thin sheets of soft veneer. Liteply may be a viable alternative to balsa but will need the grain filled in to a similar extent and is not that different to work with than medium hard balsa.

Glynn's design uses his normal method of construction which does depend on thickness of the balsa for a successful result. If you were to substitute plywood of the same thicknesses then the model could well turn out to be too heavy. If you use thinner plywood e.g. 0.75mm sheet instead of 3mm balsa and 1.5mm ply instead of 6mm balsa then you will not be able to make effective butt joints or fair adjacent pieces together by sanding across the joints. Put briefly, if you were building from ply you would not use Glynn's construction method but rather build the model on the basis of keel, bulkheads and ply skins much like the old Aerokit boats.

I am happy to use balsa on my models, once properly sealed and surface hardened using a number of methods it is a perfectly adequate material, especially for a small boat like Tyfone.

Colin