Corroded pipe nipple

Yes it is beneath the waterline. Working through everything related to that and another one on the other side with the same issue. We had some kind of electrolysis event over the Summer and need to sort it out. Could be a neighbor at the Marina.

I say check your running gear, if that corrosion on the outlet is the result of one summer. Get your boat and surrounding area metered is you think you or a neighboring boat is emitting stray a/c current, and don't put your feet in the water until that's resolved.

2 issues

That cap is no doubt added after the original thru hull was cast.
It's probably copper soldered on.

The reaction between the bottom paint (all kinds of metal) and the copper cap has probably helped in its demise also.

When original, it may have been at or above the water line also.

yep, probably less than a Summer actually. Shafts are fine, Zincs were nearly gone and props got hit a little. The props are at the prop shop getting inspected. Clearly we need to pay much more attention to this situation on a more regular basis.

this is the other side of the boat near the transom.

On both sides kind of enforce my thought. If one neighbor was leaking DCv current, IMO; only that side would be damaged.
I'll bet a cup of cheap coffee (da good stuff), the rest of your boat is fine.

I'm going to call this typical.

What bottom did you or yard last use? Were the thru hull fittings sanded and not sealed?

What shape is your internal bonding in?
Back transom corners are always wet on the inside. Bonding fails quick back there.

Internal bonding (so far) is really clean and the boat is super dry inside. But as you know it only takes one thing. We think its our neighbor across the dock opposite our transom but we're still going over everything. To be continued.

There is no bronze pipe, and again only the Groco close nipples. Buck Algonquin doesn't even have them . Bronze is cast. Brass and copper is not . No other nipple length are made only close .
It stinks , I know. Longer bronze nipples would come in handy at times.

Looks to me, it's possible whatever bottom paint you used last year is reacting to the bronze, possibly.
How long do you normaly get out of your zincs ?

Anyway, also what Beau said, keep out yourself and others out of the water till is gets sorted out. Can be stray current from the marina electric near by...who knows.

http://www.farmerscopper.com/bronze-pipe.html

Lol...you better be able to handle 21 foot lengths...or many fittings . I don't think they are interested is single piece order. I'm not talking bulk orders. But it's good to know if you want to Repipe your yacht in all bronze .

If Buck, groco, perko don't have it , your out of luck. . Bronze won't end up on most boats . The guy who installed the AC unit or units for the boat builder or owner will still possibly use brass , like in the OP.

Also that site talks about silicone bronze. That's soft stuff. Not hard and Soild like cast bronze. Like silicone bronze screws etc...soft .

There are also strong nylon etc materials to consider?

As an aside it may be time to have that old hard surface bottom paint soda blasted and replaced with some barrier coats and ablative bottom paint?

You have a serious electrolysis issue going on. How are your zincs? If they're good check your shorepower connections, and then start checking your bonding system.

The bottom paint is Seahawk Biocop light blue. Its a wood boat as well if I failed to mention it. Probably going to deal with the bottom next fall when the boat is down south for the winter.

Personally that damage does not appear to be paint related to me. I agree that if that damage happened within a single season you have a serious electrolysis issue, either on or off the boat Those thru hulls definitely need to be bonded . Do you have a zinc divers plate on the transom tied into a #8 (?) green coated single strand copper wire that runs thru the boat?

The fact that you had zincs left on your shafts while those thru hulls show so much sacrifice is interesting....

The boat is out for the winter. I will be up to do some more digging next week so I'll report back. (there are no plates on transom and the zincs were mostly depleted). Next season I'm going to be sticking a GoPro down there regularly when at the dock to monitor it more carefully.

Where would you put the Go Pro? For what purpose?

I do that too when I want to inspect running gear or bow thruster without getting wet. I have a boat hook with a GoPro mount bolted to the tip

ditto. Shurhold makes a mount that goes right on the end of the scrub brush pole