Don’t ignore little things - through hull failure

[Syrah]

We had been getting a hint of an unpleasant smell from the front head, which I had put it down to perhaps being a minor leak in the holding tank or the plumbing associated with it. The front head is rarely used.

We were getting ready for a major cruise (just pre COVID) and the boat was coming out for antifouling. I asked the marina shipwright manager to inspect all through hulls and to take a particularly good look at the front holding tank while the boat was out of the water.

The elbow, which sits below the shut off valve and is directly connected through the 2 inch diameter hole in the hull, disintegrated in his hands as he was removing it. I’d hate to think what could have happened if the valve was roughly opened out in open sea.

I never got to see the parts (due to lockdown), only this photo.



It looks to me to be galvanic corrosion. I’m guessing the through hull fitting was bronze and the elbow was brass, but I’ll never really know. (i.e. connected dissimilar metals sitting in an electrolyte)

The replacement fittings are now glass reinforced nylon (Trudesign).

I’m posting this just to remind everyone to always err on the side of safety. It may be a bigger problem than you think.

Regards Wayne

[Charlie-Bravo]

Lucky you !

the pink colour in your photos would suggest that the fitting is old, and the zinc in the alloy has gone the same way as your anodes do, the material then becomes very brittle and weak. How many years old do you think they were?

There is is a lot of info out there on connecting the hull fittings to the anodes, .... old school , some say it's a good idea, others say the through hulls are better isolated, depending on where the anodes are placed in relation to the skin fittings, all probably valid opinions.
All I know is that an awful lot of water comes in when they let go, and if your not there to pump it out the result is not good.
At least with the new non metallic fittings you can rest with a little peace of mind.

Although DZR fittings should last 3 to 5 years, and some survive less and some longer, I replaced all ours last year with DZR as I was haemorrhaging money at the time , and plan to change to truedesign next time.

CB

[uli057]

Hello.
We have discussed this topic here several times. The fittings are not made of bronze, and as the predecessor already wrote, the fittings are pricked out over the years and become brittle. Renewing all of them is the best measure. Bronce is the better choice for me because it is not so demanding. Trudesign is good but too space demanding. And I still have a galvanic isolation in the boat for shore power (220v), it is from Sterling!

Regards
Uli

[Syrah]

Bronze is primarily an alloy of Copper and Tin and Brass is an alloy of Copper and Zinc

I believe that the through hull was probably Bronze and the elbow was probably Brass. I’m not making negative comments about either of these alloys. I’m only suggesting that they were very likely dissimilar.

Two dissimilar metals connected together and sitting in an electrolyte (sea water) is a galvanic cell. A small current will flow in this situation and the most reactive metal will become a sacrificial anode.

It looks like the zinc within the Brass elbow has been sacrificed. This has concentrated the percentage of Copper (hence the colour change), and made it more porous and brittle in the process. This is a totally self contained system. It is not connected to any of the other metallic surfaces in the boat and so will not be protected by a galvanic isolator.

Connecting boats to shore power can introduce a whole range of electrochemical problems, depending on how the earth system of the marina is configured. In its simplest form the shore power earth connects multiple boats, each that have a range of different (dissimilar) metal components and all sitting in the same electrolyte (the sea).

Zincs are fitted because if one metal is going to be sacrificed, it’s much better that it be the one designed to be sacrificed first.

Galvanic protection devices appear to raise the voltage slightly on the boats earth system, to the point where a sacrificial anode no longer exists. Advertising for these devices seems to suggest that protection of zincs is one of the prime purposes.

[mikebz]

Brass skin fittings/valves/tails may only last 2-5 years but proper DZR should last a lot longer than that.  Good article on bronzes/brasses/dezincification here:  coxeng.co.uk/metallurgy/brasses-and-bronzes/

[no3l]

Hi,

I would fully endorse this warning.

I discovered the same problem with the elbow on the raw water intake for the engine. The elbow had a furry appearance and disintegrated when we applied pressure when removing it for replacement. The best way to describe it would be that it had the consistency of chocolate. All the other fittings look ok but we will be doing replacements in the winter service.

What worries me is that there wasn't any sign of the problem when we launched in March, just before the lockdown, but it was very apparent when we finally got back to the boat.

Regards
Noel

[sitara]

Same problem with the engine raw water intake on an 8 year old SO36i. Symptom was a very minor water leak from near the 90 degree elbow, probably about a teaspoon after the boat had been left for a week. The fittings had discoloured but looked OK when cleaned up, but decided to replace it as a safety measure. The hose tail disintegrated on first pull when we tried to remove the hose! The original hose tail must have been very thin, and the remaining material crumbled very easily. The tap and through hull were sound although discoloured.

All replaced with Tru Design plastic and back in the water tomorrow. My lesson is to be very suspicious of any small leak around through hulls.

[rxc]

Do any of you who done this replacement know whether the thru hulls have NPS threads or BSP threads? I have a US marketed boat, but am not sure which threads to spec.

[sitara]

Hi rcx,

I replaced the whole unit, partly to avoid this problem. I have seen NPT thread converted to BSP by running a die over the thread. As I understand it the pitch is the same on both but the shape of the peaks and valleys is different. I would guess a French buit boat would be BSP while a US built boat would probably be NPT.

I hope someone with more expertise replies.

Rob

[Charlie-Bravo]

Even if you arm yourself with thread gauges you are unlikely to determine the difference on an old through hull, as they do look very similar , 5 degrees on thread angle, and BSP has a rounded peak and trough on the thread form, NPT has flattened peak and trough, but being 'plumbing' rather than aerospace the fittings can be a bit varied in quality,  
a reasonable thing to do is to pop to your local plumbers merchant and buy a couple of sockets one BSP one NPT (cheap things) in what ever size through hull you have, clean up the threads best you can, and then screw on each socket by hand, the one that only goes on about 2 1/2 turns is not the thread and the other one will likely go on much further and feel nicer and you have found the correct thread.
Being USA it could well be NPT but you can't really go with a guess, and not doing through hulls correctly can well be a false economy.

The suggestion above of replacing the through hull and seacock not only solves your thread question, but also you will then know for certain that it is new and unlikely to give you any problems for many years to come.

CB