Seacock

A seacock is a valve on the hull of a boat that can be opened or closed. A seacock may either permit water to flow in to the boat, such as for cooling an engine, or for a salt water faucet; or out of the boat, such as for a sink drain or a toilet.

Many seacocks are left open most of the time, although this may vary depending on the application. Seacocks feeding into or out of a closed system, like the engine cooling system, are almost always left open. Seacocks connected to something open, such as a sink drain, might be opened up in port but closed when at sea. The reason for this is that when the boat is level, the drain or other opening will always be above the waterline, and so water will only flow out. At sea, when the boat may lean over or roll in the waves, the opening may sometimes be below the waterline. If the seacock is open when that occurs, seawater could flow the wrong way -- into the boat.