which thru hull fittings, plastic, s/s , bronze ?

[srp]

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Was it a nylon fitting at the waterline? Peculiarly prone to ultraviolet deterioration on the upward facing inside, leading to cracking in situ with no forces applied.

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No - this was just behind the bilge keel on my Corribee - well out of the way of UV light. Perhaps it had been overtightened when it was installed originally. I've only owned the boat for a year, and its been out of the water since September. I have no idea what it was intended for - it was blanked off internally with a screw-on cap. The two cockpit drains very close by are both bronze fittings, so why this was plastic who knows.
I must agree that Marelon fittings are in a different class - my vented loop for the engine cooling system is a Marelon on. Someone is now going to tell me 'ah, it's fine EXCEPT for engine cooling water fittings' !!
And whoever said this was a useful thread is absolutely spot on - exactly the reason why this forum is so good.

 

[Caer Urfa]

The one point not mentioned here is if your boat is doing say 6 knots and you hit a log then the chances of shearing off a plastic fitting is far higher than a bronze fitting and sometimes the plastic one's stick out further.
Most fittings get sheared off at or near the waterline level due to wood floating!
This is not guess work but experiance handed to me by the Coxwein of the local lifeboat who has seen this many times.