Depth sounder leak - quick fix?

[sildene]

I have a seepage (2 cups/d) at the base of a through-hull housing that holds the depth sounder transducer. This transducer is removable - can be extracted from the fitting like a paddlewheel speed transducer.

This housing is fitted with the usual large diameter nut that screws down to clamp the through-hull to the hull.

My proposed fix (from inside the hull):
- Undo the nut and backing pad (disc of plywood)
- Scrape away as much of the polyurethane sealant that was between the backing pad and the inner hull surface, as I can (not easy)
- Apply new sealant around the base of the through-hull
- Cover with a new backing pad, with bottom-facing edge chamfered
- Redo the nut

If anyone has attempted this type of fix, with either success or failure, I'd much appreciate input. Perhaps an external fix (remove the housing entirely and re-bed) is the only approach worth considering.

Further background:
I'm actually slipping the boat. But I can't risk removing the through-hull; it is plastic and was bedded with polyurethane, so it could crack or break if it is forced. The supplier (B&G) cannot provide a replacement housing (old model), and this sounder is part of a B&G suite on the boat that includes displays and other transducers (speed, wind etc.). If I could remove it without damage, re-bedding would be my preferred approach.

[GordMay]

I presume that you’re certain the leak is between the permanent transducer housing & hull joint, and not through the transducer & housing interface. Were it the latter, a simple O-Ring replacement might solve your problem.

Given your stated constraints, I think your proposed fix is a reasonable choice.

[Vasco]

Sildene,

I have seen seepage that is coming from beneath the plywood backing pad stopped by using fiberglass thread coated in sealant. Just start wrapping it around the pad where it meets the hull, it'll work a bit under the pad and seal the leak. This'll work for small leaks such as yours.

[GordMay]

Although buttering & stringing the perimeter edges of the backer may stop the leak, you may also be sealing moisture into an already “punky” plywood backing block.
If practicable, I’d replace the old plywood backer (as you proposed), whilst you are out of the water.

[sildene]

Much appreciate your replies and suggestions, Vasco and Gord.

The sealant-buttered string is indeed a handy trick to know when you have to do something while the boat's in the water.

[delmarrey]

Since it's already leaking, removing the nut and backing plate may cause you a slight increase especially if it starts to rotate, if it does rotate it can be pulled easily while it's out of the water.

But I'd suggest taking a small bucket with a large lip and grease that lip with some heavy grease. The dive down and place the bucket over the transducer. This will help prevent any catastrophic event while you in the middle of you fix.

My $P 1.0............................_/)

[Alan Wheeler]

Hi Sildene. I am assuming you have a glass hull??? The above advice is excellent, but.......
When you get her out, I suggest you try and remove the housing. I would apply a constant but carfully managed pressure to the fitting and push it out. It should "pop" out OK. I suggest you place your carefully calibrated foot on it and push down and see if it moves. You really need to ensure that the housing is sealed back in properly, or you are allowing water in to where it shouldn't be in the lamination.