Martin Johnson, owner of "Sunflower" in Duluth, MN wrote an article in the MSOG website about the cavity below the cabin sole, and the keel of his '76 M-17. Seems he had CB trunk leaks that threatened further freeze-related damage. So he put inspection holes in the cabin floor next to the CB trunk, and filled the cavity with antifreeze in winter. Here's my question - Does anyone know how big the cavity is below the floor? I plan on drilling a hole and filling the cavity with two-part closed cell foam to exclude any water intrusion - even though I have no evidence of a problem at this time. It would be nice to know how much foam to mix... Thanks, Mark House 1977 M-17 # 232 "Seranita"