You remember the rudder correctly. In that case, the foam simply provided a form over which to lay the glass. I'm not positive, but I strongly suspect that if now through some magic I could go remove the foam, the strength would be undiminished.

I *can* see where the foam between the thin layers of glass would transfer some load from one layer to another and add some strength that way, in which case the expanding foam would probably be effective... Somebody should take a trashed hull and try both methods to see if they can discern a difference.