I just looked more closely at your post and at your picture.

So you have 7 layers of 1/4 inch heavy timber. Does it really need to be that heavy?

We worked on the assumption that as long as the front layer and the back layer were reasonable hard, the centre layers could be balsa. So we had a type of sandwich construction with the WRC providing some resistance to damage, the glass providing strength and the balsa adding bulk with very little weight.

We never tested a paddle to destruction, so I don't know if our theory was sound. Al I can say is that they were strong enough.

I still don't have an answer to your original question.