What you are describing is a disaster guarenteed to happen.

Why?
The mahogany board weighs 15 pounds. The oak board should weight about 23.
Even if I hadn't just lost 7 pounds, what severe problems will the 8 pounds
cause?

I am not being argumentative; I concede I don't know anything about boat
building, and little more about sailing. But I have sailed this with my 95
pound son in it, and it is fine (if a little sluggish); why would 8 pounds
of board matter?