I had bronze thru-hulls on a previous
boat. Boat was about 15 years old. Every one of them leaked.
That tapered
plug inside the valve body had pitted....electrolysis the likely culprit. ???.
I'm not sure...I had two large bronze grounding plates on the
hull for a ham/ssb ground and I never could visibly tell whether or not either one of them had deteriorated.
In addition, whenever I parked the boat, be it at
anchor or at a dock, I would throw a sacrificial zinc fish" over the side which was tied into the boat's grounding system. The wear on this sacrificial " fish" was noticeable.
There could have been stray voltage from other
boats or even the dock
power system.
But even closing the bonze valve, water would work it's way around the tapered
plug.
At a haul-out I pulled each tapered plug and spent many hours trying to sand those tapered plugs smooth. When I re-inserted the plugs I made sure to slather them with a good quality waterproof grease and gave each nut an extra few tightening increments.
It was all in vain, the thru-hulls leaked as before. Off course, I could not effectively clean the valve body itself, though I tried, and likely the valve body had pitted as well as the tapered plug.
The whole thing was an exercise in futility and at a subsequent haulout, at great expense and effort I removed all the bronze fittings and replaced them with marelon, a plastic thru-hull.
The leaking stopped ! Off course, I could have replaced the old bronze valves with new bronze valves, but decided the plastic would likely give me better protection, and yes, it was the cheaper option.
Interestingly, all the bronze valves were tied into a grounding system, which consisted of a grounding wire tied into one of the bronze thru-hull fittings.
Whether this was part of the problem, I cannot say, but whatever was eating one thru-hull was also eating all the others.
Modern plastics have come a long way and I would not hesitate to recommend the marelon variety of thru'hulls. Those things are built bulletproof imo.
Subsequent to their
installation, I never had not had a
single problem with them.
Off course, to each his own. One can read any number of " opinions" on this subject and this is simply my opinion.