Bronze versa 316 s/steel as through hull fittings and seacocks.

[Graham_Wright]

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Plastic, usually Nylon of one sort or another, is fine - I haven't heard of any problems.

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Er no;- nylon absorbs water.

 

[JonJon]

I have had two "plastic" thru hull fittings fail on me - fortunately I do not have them below the water line. They just seem to crack with behind the flange.