Fractured through-hull sounder sender

[davidbfox]

My boat is out of the water for the first time in several years. The NASA sounder has never worked for the three years I have had the boat.
When checking the sender from inside the hull, the threaded plastic section came off in my hand flush with the hull (no tools involved). On checking outside, the same thing happened. All that is left in place is the middle section of the threaded part of the sender where it passes through the hull.

I'm guessing it must have been overtightened on installation and partially sheared both inside and outside the hull. Is there another likely cause (deterioration of the plastic maybe)?.

It's a NASA target sounder, maybe 10 years old. No history of leaks.

Two queries:
1) When fitting the new one, am I right to think Sikaflex and not too tight?
2) Is there a known history of disintegrating sender units potentially sinking boats?

 

[damo]

Personally I would fit an in-hull transducer, and do without the hole at all. Otherwise sikaflex, unless the manufacturer specifies something different, do it up loosely so as not squeeze out the gunk, then tighten after it has gone off.

I hate plastic through-hulls /forums/images/graemlins/frown.gif

 

[DanTribe]

I have had two Nasa log through hull fittings fail just as you describe.First one came off in my hand as I was withdrawing the paddlewheel.
One was set in Sikaflex, the other in silicone.
My sounder transducers are set inside the hull so not at risk.
If you read the Nasa installation instructions, they do say to glass them in and not rely on the fitting itself.
Bit frightening though, it leaves a large hole in the boat.

 

[Kurrawong_Kid]

Nasa specify a silicone sealant, I believe.

 

[hightech]

What's your hull made of? Plastic is fine with a grp or steel hull, but bronze is needed with a wood hull.

Fracturing like you have happens in wooden huls as the wood swells and the transducer gets crushed.

If you are not wooden then the plastic may have been attacked by something in the bilge or antifoul causing it to go brittle, unusual though.

 

[DanTribe]

I don't believe it is unusual. As I said, I've had two fail in a short period and other people have had similiar experiances.
When I contacted Nasa they were not at all surprised and sent me another FOC, as if it was a regular occurance.
The point of failure is very very thin, just a few millimetres.
The fittings should be regarded as a former for glassing in only. Don't rely on the strength of the fitting to keep the sea out.

 

[vyv_cox]

Solvent-based sealants react with some plastics, leading to stress-corrosion cracking. This is especially a problem with polycarbonate, although I doubt if your transducer is made from this. That is the reason that silicone sealant is specified - it has no hydrocarbon solvent.

 

[Stemar]

My NASA sounder works fine through the best part of an inch of GRP. Get a big dob of blutak and stick the sounder down in different places until you find one where it works well. Mine's held in with half a tube of Gosport pound shop's version of No More Nails