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In sailing ships the toilet was placed in the bow for two reasons. First, since most vessels of the era could not sail directly into the wind, the winds came mostly across the rear of the ship placing the head essentially downwind. Secondly, if placed somewhat above the water line, vents or slots cut near the floor level would allow normal wave action to wash out the facility. Only the captain had his private toilet near his quarters at the stern of the ship in the quarter gallery.
In many modern boats, the heads look similar to a seated, flush toilets but use a system of valves and pumps that brings sea water into the toilet and pumps the waste out through the hull in place of the more normal cistern and plumbing trap to a drain. In small boats the pump is often hand operated. The cleaning mechanism is easily blocked if too much toilet paper or other fibrous material is put down the pan.
Submarine heads face the problem that at greater depths higher water pressure makes it harder to pump the waste out through the hull. As a result early systems could be complicated, with the head fitted to the US S class being described as almost taking an engineer to operate. Making a mistake resulted in the waste being expelled back into the body of the submarine. The toilet on the World War 1 British E class was considered so poor by the captain of HMS E35 that he preferred the crew to wait to relieve themselves until the submarine surfaced at night. As a result many submarines used the heads as an extra storage space for provisions.