I have a small (21') sailboat with a bilge pump hose that outputs to a through-hull that, depending on the weight distribution in the boat, is sometimes slightly below the waterline (not a great design IMO, but I definitely am not able to change the fitting location right now). It has a plastic through-hull fitting similar to this one, and it is just barely leaking. There's no noticeable droplets, but I can see a wet sheen below the fitting and a small amount of water pools in that cabinet over the span of a couple of days.

The cabinet that the bilge pump hose runs through is also where the electrical fuse panel and switches are, so I'd really like to get this fixed to avoid creating extra opportunity for corrosion there.

My question is, how can I fix this leak without hauling the boat out? I am tempted to just try tightening the plastic nut from the inside, but I'm afraid of cracking it and turning a minor problem into a major one. I considered applying some silicone to the inside and I think this might work short term, but it feels like a very temporary solution.

1 Answer 1


Correctly applied silicone of the right grade is a (semi) permanent solution.

Like any other submerged seal it requires regular maintenance and inspection, but if you get a silicone that's designed for continuous submersion in water then it will hold.

  • What's the best way to apply it in this case? Considering that the fitting is partially below the water line so I can't remove it.
    – Ian
    Apr 26, 2021 at 18:26
  • @Ian if you want to seal from the outside then silicone that sets underwater does exist, you'd be paying a premium for it but it would get you through the season. However for a proper job you would have to get the boat out and dry.
    – Separatrix
    Apr 27, 2021 at 7:13
  • All silicone sets underwater Apr 28, 2021 at 11:13

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