The H 376 appears to be deck stepped... so spartite or any other seal won't be the issue. Is there any evidence of collapse or cracking in the deck in the mast area? Are there fittings (ie turning blocks, mast cables, instrument wires, etc) passing through the deck in that vicinity? This water has to be getting into the boat through some such route.
It's also very common for leaks "inside' to be a long way from their original source as water runs along liners before finding somewhere to leak below. I think you need to go over your deck fittings with a critical eye for some candidates here.
One other thought.. if the cables and wires in the mast enter the boat in the mast step, maybe your mast's drain holes are plugged, and bottom of the mast is filling enough to come down that way... the wires may turn down the liner rather than go down the compression post...
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"
Last edited by Faster; 11-24-2010 at 11:10 PM.