I am totaly restoring a 1975 25" and I am trying to get the best seal I can on my knot meter. The old system had the hull, sealant, a 3/4" plywood spacer, sealant and then the plastic nut. The bottom, inside is not totally flat. It leaked. Here's what I'm thinking. Hull, sealant, rubber washer, plastic spacer (I am using the 3/4 plastic material to mount my guages in the companionway) ground rounded at the base to align with the curviture of the hull, sealant, a rubber washer, sealant and then the plastic nut. Is this overkill? Do you think it will work? Once all my work is done (I have been on it since last October), I'll post this projects with photos. Thanks Ric "comes a time"