As Rico has said the thickness can vary in the area of the mast step. What you have is 3/8" thick end grain balsa with glass on both sides. The bottom layer is usually very thin say 1/16" and the top varies a lot as I am learning. I just recored the area under the mast step as part of a project I will post in my gallery after I finish a bit more and can photograph it. But the top layer of glass under my mast was over 1/8" thick. It took me 7 layers of 1708 biax to fill the core area and another 5 layers to bring it up to the surrounding deck area. While I was at it I recored the area where the hinges for the forward hatch bolt through the deck. Here is a photo of the recored area.
JERRY CARPENTER - C147
A man can succeed at almost anything for which he has unlimited enthusiam.