Mike
A search for masthead AND buoyancy brings up many topics – most relevant are these:
http://wayfarer.org.uk/forum/viewtopic.php?t=163&highlight=masthead+buoyancy
http://wayfarer.org.uk/forum/viewtopic.php?t=273&highlight=masthead+buoyancy
http://wayfarer.org.uk/forum/viewtopic.php?t=417&highlight=masthead+buoyancy
David