thru-hull corrosion

[djbeach]

The raw water brass thru-hull on my 1998 H376 has a greenish-blue corrosion problem.
Occassionally there are a few drops of water around the wooden part at the base of the
thru-hull. From what I have read up to this point this is something that can wait until the next haulout. But in my case the next haulout is not for at least another 2 years.
During my last haulout, about 7 months ago, I had the manager of the yard check it out.
He scraped some off and said it appeared only to be on the surface. At that time there had not been any water drops and the corrosion did not look quite as thick as now.
At this time, the thru-hull opens and closes normally.
How concerned should I be in the short-term?

 

[Rick]

The greenish blue residue on your bronze thru hulls is very common. I believe the term is "oxidation" and occurs on the paint of your car when you dont take care of it. It can be cleaned with several different "green compounds" and a good wire brush. See the archives here on cleaning bronze thru hulls. If left unchecked, it can cause problems long term. As far as your wood backing plate being wet, well that is not good. The seal is already compromised. The wood will eventually soften and cause the thru hull to fail. Probably when you exercise it. You can take your chances or pony up and get it hauled. One thing I would definately do is get a wooden plug that will fit the hole "when" it fails. You can drill a small hole near the top and with a small lanyard, tie it to something convenient near the thru hull. Great piece of mind.

Cheers

 

[RickSylvester]

and there's no way to properly fix it in the water. I just went through that scene.

Like most production boats I'll bet you've got the tapered threads of a valve screwed directly onto the straight threads of the mushroom. It's a thread mismatch. The ONLY way to properly fix it is to remove the entire assembly and put in a proper sea-cock or through-hull adapter assembly. Mainesail has a great website explaining it all.

The thread mismatch means that it's already tightened as far as it will go so that once it starts leaking it won't stop. Because you probably don't have a large flanged base on the through-hull, when you open and close it it puts large torquing forces on the whole assembly which is what causes the sealant to be dislodged.

One of mine leaked for quite a while but it made me so nervous that I went ahead and hauled and fixed it and a bunch of others.

Oh yeah, one more thing, don't EVER take yard personnel's advice as gospel. It's fine to ask, but ultimately you GOTTA do your own homework. I know this is water-under-the-bridge advice but I'm saying it because I've been there. I don't let ANYBODY work on my boat unless it's absoluteley necessary and even then I'm looking over their shoulder.

You can do this. It ain't rocket science but it IS a lot of work. I know it sucks but you'll be glad you did it.

 

[jcherubini]

DJ's corroding through-hull

I agree with Rick that the threads are probably mismatched. I am dumb enough that I actually considered just jamming them on and smearing them with 5200 till I figured out what the problem was. I am also cheap, so I was using 'house' fittings and not 'marine fittings-- when I bought Forespar elbows they worked fine.

If you are replacing the through-hull/seacock assembly consider Forespar Marleon. Talk about piece of mind-- it won't corrode or oxidize, won't freeze, won't seize, and, according to Forespar, won't break if you slid a 500-lb toolbox down the inside of the hull into it.

Under NO circumstances should you consider other plastics such as nylon or PVC.

All the underwater fixtures on Diana are Marelon. The only metal underwater is the keel and the rudder staft.

 

[Maine Sail]

If you are replacing the through-hull/seacock assembly consider Forespar Marleon. Talk about piece of mind-- it won't corrode or oxidize, won't freeze, won't seize, and, according to Forespar, won't break if you slid a 500-lb toolbox down the inside of the hull into it.

Under NO circumstances should you consider other plastics such as nylon or PVC.

All the underwater fixtures on Diana are Marelon. The only metal underwater is the keel and the rudder staft.

Marelon is a good option for lots of reasons however one must be aware that you MUST exercise (open and close) and lube them regularly or they WILL seize.

There are lots of reported failures of Marelon valves all over the boating forums, many members of this forums included, and sometimes even when cared for properly some of them still failed.

Marelon is tough stuff but won't likely withstand a tool box of 500 pounds "sliding" into it. The ABYC standard for strength is a 500 pound static load, meaning not moving, rested on the most inboard hard fitting (usually a hose barb), in the valves most vulnerable location for 30 seconds. Dropping or sliding a 500 pound tool box onto a valve is much more force applied than a 500 pound static load test.


The green color on your thru-hull fitting is most likely just verdigris. Normally this is a benign form of the bronze weathering due to exposure. It is wise however to clean off the verdigris from time to time and make sure it has not turned into something more. You will be looking for a uniform bronze color to the fitting. Any splotchy areas that are pinkish or coppery colored are signs that it is suffering. If it cleans up well and has a uniform color all is probably well unless of course it is leaking as you surmise it may be.

I agree with others about the wet wood backing block in that it's not a good sign..