Yo James

I built my guillemot s&g out of 4mm which I thought was the same as 1/8. Must measure my scraps. But the thin ply was easy to work and really floppy until I stitched the panels together. By the time I had fibreglass cloth sheathed both sides of the hull bottom it was rock hard tough. the deck sheathed onto but not underneath was hard, but flexible.

If you are going to sheathe 1/8 should be good enough.

Good Luck

Pete