I can't remember exactly but you get a general idea by looking around the edges of the boat - the hatch for example. The builder may have added some additional layers around the bow but not much. I've done enough sanding to realize you can practically see through the topsides. But ultimately the length of bolt will depend on the thickness of your backing block.

:: I am going to put a bow pulpit on my Santana. I need to
:: purchase new bolts to hold it on. How thick is the deck
:: in the bow (not allowing for the thickness of the
:: pulpit's base or the nut for the bolt)?