Beat me to it !! My marina's store is a distributor for Buck Algonquin and they stock almost any nipple you might need up to something like 2.5 " or more. Ironically I just got off the phone with them for a different matter and asked them if they had a bronze source ! Prolly just need to find a local B.A. distributor.
No kidding...that's a great idea. I've got one on the boat , lol I think that's where it is. Haven't used it in a while. I used it for skiing , mounted on my helmet . Ya'll would be proud of me! I'll try it under water strap it to my boat hook.
For some reason, I've never seen a wood boat with a transom plate. I do not know why. But may be you have a 12 volt source getting into the bilge/bilge water causing this ...a bilge pump or pump wire , old wire in general hanging loose in the bilge etc. As far as bonding goes , to bond or not to bond ? Some wood boats and glass are not bonded at all so not to carry stray current to all fitting and let it eat up just one , I belive is the though behind this . Anyway it is an art to each individual boat .
The boat is cold molded. There is no transom plate. I am not sure why but I have heard that you need to be careful to not over zinc a wooden boat and also someone told me "they don't look good on sportfishing boats" go figure (no swim platform to hide it). The bilge areas in this boat are usually bone dry. Anyway, we'll continue to hunt it down as it hasn't been a problem every before.
The theory of the bonding system is that at its end there is a sacrificial zinc or other less noble metal which protects all the attached fitting. Once the fittings are properly connected they become one unit with the need for only one zinc source. Even with a/c current in the water the fitting won't be harmed as long as the less noble zinc is there. Imo leaving bare fittings to their own is courting disaster? Many older wooden boats have a keel located ground, some times in the form of copper tubes or zinc bars . I believe the more modern ones, like Jarret use the divers plate which can also be attached to the bottom. The transom is only one location. Someone has to do a full and competent survey of the galvanic and other induced currents flowing both in and outside that boat.
You can compare a cold molded boat to a wooden boat. Completely different. A cold molded hull is closer to a fiberglass hull except that the glass is replaced by thin strips of mahogany
Overzincing doesn’t apply to cold molded. You need anything having saltwater through it bonded, to a zinc.
It is a classic case of dezincification of a brass nipple: http://www.werc.com/2016/07/28/dezincification-failure-of-brass-components/ Brass fittings have no place on marine applications used in raw water flow.
I had an issue with zincs not lasting and galvanic corrosion on a yacht. It was explained to me that overzincing could be a problem and if overzinced by quite a bit, could create electrolysis because of that. Some hull materials are more susceptible to being overzinced and some manufacturers like Volvo with their IPS and Yamaha outboards are foregoing zincs altogether and using Aluminum as the sacrificial anode instead of zincs.
Here’s some reference material on the “over zinging” discussion: http://www.seashieldmarine.com/zincs-boat/ The material (zinc) is not the issue, the amount becomes the problem.