The thru-hull fitting under the sink in the head of my boat has a slight leak no matter whether the valve is in the open or the closed position. Is there some kind of putty or sealer that would stop this leak?
Tom Wood responds:
Thru-hulls and seacocks are generally low-maintenance items, but they require vigilance because the failure of one can have very dramatic consequences.
First of all, keep in mind that thru-hulls and the valve attached to them are two separate parts. The thru-hull is the threaded fitting that passes through the skin of the boat—some have mushroom-style heads on the outside while others are flushed into the hull. Unless there is a seacock inside the hull bolted to the skin of the hull, you'll find that most thru-hulls have a nut holding them in place. Thru-hulls used for below-the-waterline applications should be made of bronze or nylon materials. If it is truly the thru-hull that is leaking, you are right to be concerned and should haul the boat immediately and replace it.
Usually, any minor weeping or dripping from the assembly is due to the seacock, ball valve, or gate valve instead of the thru-hull. Whether plastic or bronze, these valves often develop small leaks around the handle seals. Such leaks are normally of no immediate concern, but should be repaired when you have your next haulout. It's not hard to take a seacock apart and re-seat and grease it with a good quality waterproof grease. A gate valve should be replaced, preferably with a proper seacock or at least a good-quality ball valve.
If the leak is at one of the joints, either at the threads between the thru-hull and the valve, between the valve and the tailpipe, or at the joint between the hose and the tailpipe, then we're talking about another kettle of fish. The joint will need to be taken apart and resealed with proper compound.
Nothing that you can apply to the outside of the thru-hull or valve assembly will stop these leaks. If the job involves anything short of removing the thru-hull, it is possible to perform many of these tasks with the boat in the water, although this requires two trips into the water—one to insert a plug in the thru-hull from the outside, and a second one to remove it. This operation is fraught with danger and is best avoided unless an emergency exists.
On a side note, sink drains often have even more serious problems. In boats with modest freeboard and deep bilges, the bottom of the sink is often uncomfortably close to the waterline. This allows the level of the salt water to sit just at the level of the drain's connection to the sink, a part often made of inexpensive metal for household use. When this fitting corrodes away, the hose connection simply falls off, sinking the boat if the seacock is open. We have one friend who lost her boat this way, and it was a very nice Cape Dory.
One of the first modifications we normally make to a new boat is to replace the metal sink drain and basket with one made of nylon. And we never leave the boat for any length of time with the sink seacock open. Good luck to you.