Plumbing on a sailboat delivers fresh water from the tanks, expels sewage from the heads, and moves seawater both through the engine and out of the bilges. Important parts of the plumbing are the thru-hull fittings and their seacocks, the hoses and the pumps. There might also be filters, check valves or other valves, and vents.
Thru-hullsEvery opening or thru-hull below the waterline has a seacock, which is a quarter-turn valve used to open or close it. The handle lines up with the thru-hull when open and is perpendicular to the direction of flow when closed. Ordinary gate valves are sometimes found and must be promptly replaced. Proper seacocks have ball valves or tapered plugs. They are made of bronze, sometimes stainless steel on metal hulls, or a strong plastic called Marlon. There should be a backing plate or adapter between the seacock and the thru-hull and the hose connected to the seacock should have two clamps.
Tapered
plug seacockThe traditional tapered plug seacock is simple and effective but requires periodic maintenance. An adjusting nut, a lock nut, and a washer secure the tapered plug in the seacock body. The shaft that these fit onto has a flat that aligns with a flat in the washer. The adjusting nut is tightened down only enough to prevent leaks, but not too tight, and the lock nut is then tightened while holding the adjusting nut in position.
One or two drain plugs are the only other parts of this simple device. A layer of waterproof grease on the tapered plug seals out water. It should be removed with solvent and replaced with fresh grease annually. Sometimes a zerk fitting in one of the drain plug holes allows refreshing the grease with a grease gun. If fresh grease does not provide a waterproof seal, the mating surface between the tapered plug and the seacock body can be cleaned and reconditioned with lapping compound. If the plug is difficult to remove, try tapping the shaft with a hammer, using the nuts to protect the end of the shaft from damage. If this fails, try a careful application of heat to the body. When the boat is left in the water during winter, and if ice will form down to the thru-hull, each seacock must be closed and drained to prevent it from freezing and cracking.