A Close Call with Underwater Corrosion
For years, we have harangued sailors about the need to do a careful survey of all underwater hardware every time the boat is hauled. Now we'll show you why. This through-hull fitting was removed from a Tartan 40 while it was hauled for bottom paint at the CMO yard in Puerto La Cruz, Venezuela. The boat had a previous history of eating zincs.
Most of the underwater through-hulls on this boat are recessed below the surface of the hull to reduce drag, then faced off with polyester or epoxy putty. On this fitting, which was not puttied over, the owner noticed severe pitting of the exposed surface flange. He managed to remove both the seacock and through-hull without destroying them—no mean feat, in our experience.
While the seacock itself showed no apparent degradation, the through-hull fitting was wasted to the point where you could see daylight through the threads, and the flange was almost crumbly. There is little doubt that a good thunk on the seacock would have broken it free on the inside of the boat, resulting in a very large hole under the water. Needless to say, the owner spent some time carefully looking at his other through-hull fittings.
While there are a number of possible causes for radical corrosion in a boat just over a decade old, the end result is the same: significant risk of fitting failure and subsequent sinking.
If your boat is hauled out now, take a couple of hours to carefully examine all pieces of underwater metal. Scrape the bottom paint off anything that looks even remotely suspect, since a few coats of paint could easily mask significant deterioration.