They are called Through Hulls. They are standard sizes and can be bought any any marine chandler.
They come in 4 different materials:
Nylon: Which is what you have. They are cheap and degrade in UV light and become brittle. They are only used above the water line.
Marelon: They do NOT degrade in UV and can be used underwater. This is highly preferred over nylon
Stainless Steel: They are expensive, look beautiful. They can rust when there is water introduced in the absence of air and also become brittle as well as potentially weep rust stains.
Bronze: Expensive, rugged, and will last a lifetime. They patina to a green color which is very nautical but not a look that is appreciated by everyone.
They are size by the Inner Diameter of the hole in the hull. Measure the hole, then buy your replacement. Use 3m 4000 which is UV resistant. This is typically a two person job. Otherwise you need the tool that will hold the through hull while you're tightening the nut on the inside. The tool is a pain and I prefer to do the job with two people.
https://defender.com/en_us/catalogse...hull%20fitting
3300 SCR - Looking for info
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