Port and starboard are nautical terms which refer to the left and right sides, respectively, of a ship or aircraft as perceived by a person on board facing the bow (front). At night, the port side of a vessel is indicated with a red navigation light and the starboard side with a green one.
The starboard side of most naval vessels the world over is designated the "senior" side. The officers' gangway or sea ladder is shipped on this side and this side of the quarterdeck is reserved for the captain. The flag or pennant of the ship's captain or senior officer in command is generally hoist on the starboard yard.
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The origin of the term starboard comes from early boating practices. Before ships had rudders on their centerlines, they were steered by use of a specialized steering oar. This oar was held by an oarsman located in the stern (back) of the ship. However, like most of society, there were many more right-handed sailors than left-handed sailors. This meant that the steering oar (which had been broadened to provide better control) used to be affixed to the right side of the ship. The word starboard comes from Old English steorbord, literally meaning the side on which the ship is steered, descendant from the Old Norse words stýri meaning "rudder" (from the verb stýra, literally "being at the helm", "having a hand in") and borð meaning etymologically "board", then the "side of a ship".
An early version of "port" is larboard, which itself derives from Middle-English ladebord via corruption in the 16th century by association with starboard. The term larboard, when shouted in the wind, was presumably too easy to confuse with starboard[1] and so the word port came to replace it. Port is derived from the practice of sailors mooring ships on the left side at ports in order to prevent the steering oar from being crushed.
Larboard continued to be used well into the 1850s by whalers, despite being long superseded by "port" in the merchant vessel service at the time. "Port" was not officially adopted by the Royal Navy until 1844 (Ray Parkin, H. M. Bark Endeavour). Robert FitzRoy, Captain of Darwin's HMS Beagle, is said to have taught his crew to use the term port instead of larboard, thus propelling the use of the word into the Naval Services vocabulary.
Vessels at sea do not actually have any "right of way"—they may be, correctly, in the position of being the "stand on vessel" or the "give way" vessel. Therefore, at no time should any vessel actually navigate its way into a collision, and the regulations are clear that no one in command of a vessel may assume a "right of way" up to a point of collision.
Consider two ships on courses that intersect. The ordinary rule is that the ship on the left must give way. The stand on vessel sees the green light on the starboard (right) side of the ship on the left. The give way vessel sees the red light on the port side of the stand on vessel. If the courses are intersecting, the helmsman usually gives way to a red light by going around the stern of the stand on vessel.
There are other rules governing which is a stand on vessel, such as the wind based rules for sailing vessels, powered ships giving way to sailing ships, and all other ships giving way to powered vessels that are constrained by their draft or restricted in their ability to maneuver. Therefore the green light does not mean an unqualified go, but rather it means proceed with caution subject to other rules applying. The earliest railway signals went red/green/white (as per the stern light) for stop/caution/go following this naval practice and were only later changed to the more familiar red/yellow/green.
The very simple application of red light and green light is that if the helmsman sees a red light, the helmsman should make sure that the other vessel can see his green light, which usually means giving way. If he sees a green light, he should stand on, but without getting into a collision situation.
The sailing rule that dictates that a sailing vessel on starboard tack is the stand on vessel is as old as any other regulation. Likewise, if on the same tack, a sailing vessel that is upwind of another is the give way vessel.
There are a number of tricks used to remember which side port and starboard each refer to:
A port buoy is a lateral buoy used to guide vessels through channels or close to shallow water. The port buoy is one that a vessel must leave to port when passing upstream. If in International Association of Lighthouse Authorities area A, the port buoys are red. If in IALA area B (Japan, the Americas, South Korea, and the Philippines) then the "handedness" of buoyage is reversed, and a vessel leaves black or green buoys to port.
Mnemonic devices for buoys in IALA area B: