The head is a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

's toilet

Toilet

A toilet is a sanitation fixture used primarily for the disposal of human excrement, often found in a small room referred to as a toilet/bathroom/lavatory...

. The name derives from sailing ship

Sailing ship

The term sailing ship is now used to refer to any large wind-powered vessel. In technical terms, a ship was a sailing vessel with a specific rig of at least three masts, square rigged on all of them, making the sailing adjective redundant. In popular usage "ship" became associated with all large...

s in which the toilet area for the regular sailors was placed at the head or bow

Bow (ship)

The bow is a nautical term that refers to the forward part of the hull of a ship or boat, the point that is most forward when the vessel is underway. Both of the adjectives fore and forward mean towards the bow...

 of the ship.

Design

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, below the poop deck

Poop deck

In naval architecture, a poop deck is a deck that forms the roof of a cabin built in the rear, or "aft", part of the superstructure of a ship.The name originates from the French word for stern, la poupe, from Latin puppis...

.

In many modern boats, the heads look similar to a seated, flush toilet

Flush toilet

A flush toilet is a toilet that disposes of human waste by using water to flush it through a drainpipe to another location. Flushing mechanisms are found more often on western toilets , but many squat toilets also are made for automated flushing...

s but use a system of valve

Valve

A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...

s and pump

Pump

A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...

s that brings sea water into the toilet and pumps the waste out through the hull

Hull (watercraft)

A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

 in place of the more normal cistern

Cistern

A cistern is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. Cisterns are distinguished from wells by their waterproof linings...

 and plumbing trap

Trap (plumbing)

In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as the S-bend invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend...

 to a drain. In small boats the pump is often hand operated. The cleaning mechanism is often easily blocked if too much toilet paper

Toilet paper

Toilet paper is a soft paper product used to maintain personal hygiene after human defecation or urination. However, it can also be used for other purposes such as blowing one's nose when one has a cold or absorbing common spills around the house, although paper towels are more used for the latter...

 or other fibrous material is put down the pan.
Submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 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

United States S class submarine

The United States' S-class submarines, often simply called S-boats , were the first class of submarines built to a United States Navy design....

 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

British E class submarine

The British E class submarines started out as improved versions of the British D class submarine. All of the first group and some of the second group were completed before the outbreak of World War I....

 was considered so poor by the captain of HMS E35

HMS E35

HMS E35 was a British E class submarine built by John Brown, Clydebank. She was laid down on 20 May 1916 and was commissioned on 14 July 1917.-Service history:...

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.