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Why did my through-hull break off?

2015 March 1

Dear Boat Doctor, 

I was looking around my bilge and saw that the seacock on my shower drain looked odd. I grabbed it and the threaded portion of the through-hull broke off! I quickly plugged it with a wooden plug. This through-hull is in the forward section of my hull and had this occurred on a passage or while I was away the results could have been disastrous. 

I'm not sure what happened. The metal in the through-hull looks like it just corroded away and is an odd pinkish color. I am pretty sure this fitting was added later to the boat because the valve is different from others on the boat. 

What happened and how do I make sure it doesn't happen again? Richard Sidesinger Ft. Lauderdale, Florida

Dear Richard, 

I'm sorry to tell you that you were a victim of dezincification, but your quick thinking and handy wooden plug saved you. 

Metal plumbing fittings used in contact with seawater are typically bronze,which is an alloy of copper and tin, or silicon bronze, an alloy of copper and silicon. Bronze is a great marine material, durable and very galvanically compatible with stainless steel. Unfortunately it looks a lot like brass and is often used interchangeably with brass. Brass is fine in freshwater plumbing and gas systems, but is a poor material for raw water applications. 

In an active marine environment, the zinc can be dissolved or corroded from the brass alloy, leaving a punky matrix of copper behind. You'll see what looks like corroded metal with a telltale hue of pink copper. The resulting material is dramatically weakened. Additionally, ammonia, like those in household cleaners, will attack brass, making it brittle and weak. You may have used ammoniated cleaners in your shower compartment. 

I would give all the rest of your through-hulls a close inspection, but it is possible that this was a one-off assembly. Bronze fittings can be tricky to source, and many marine store clerks don't know the difference between bronze and brass. One source is Groco (410- 604-3800, www.groco.net). 

Brass alloy is often used in propellers and prop nuts, and it is vitally important to protect these with adequate shaft zinc anodes. If you allow your anodes to wear too far, the prop will be the next thing attacked. 

On another point, you mentioned that the through-hull broke at the threads. I assume the assembly used a valve that was just threaded onto the stem of the through-hull. It is a great practice to use a flanged seacock that seats down onto a backer board inside the hull. The hull itself will support the seacock making for a much more robust assembly.

Bob Pingel, a Coast Guard-licensed captain, runs Custom Line Splicing LLC. Send your questions to the Boat Doctor, at boatdoctor@sailingmagazine.net.