First I don't know anything about this, but... I have an idea. The Marblehead class is 50.7" give or take, you can look it up to be sure. [link=http://gbmy.com/]GBMY[/link] sells ballast for Marbleheads and it ranges from 6.6 lbs to 7.7 lbs. So with a boat the same size that uses that range of ballast, you can at least narrow the field down. That said, it is all speculation on my part because I have no experience building or sailing yet. I am still building my boat. The ballast is $30.00 or you could just make your own.

Good luck and post some pics.