Sea chest

A sea chest is a recessed box built into the hull below the waterline that takes in seawater for the cooling, ballast, fire, and sanitary systems and acts as the common suction for the seawater pumps. It carries a hull grating to keep out debris, a strainer, and a vent, and most ships fit a high sea chest and a low sea chest so the crew can switch between them in shallow, silted, or ice-laden water. A steam or compressed-air blow-through connection clears marine growth and ice, and cathodic protection or anti-fouling guards against corrosion at the opening.