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The advice given on this site is based upon individual or quoted experience, yours may differ.
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Frank, only places I can think of are the keel bolts themself's or the two through hulls at the head and galley sink. Although they are above the water line, during a heel they will be under. Have you installed any through hulls like a knot meter or depth finder? Are you 100% sure its sea water?
Yes, it's seawater, and yes I've installed another above the water line thru hull: a Y valve in the head discharge line to give me the option of holding tank or direct-to-sea discharge. It may have been open when we were sailing. Next time out I'll be sure the line's shifted to the holding tank.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.