- Above board: On or above the deck, in plain view, not hiding anything.
- Above-water hull: The hull section of a vessel above the waterline, the visible part of a ship. Also, topsides.
- Act of Pardon, Act of Grace: A letter from a state or power authorising action by a privateer. Also see Letter of marque.
- Abaft: Toward the stern, relative to some object ("abaft the fore hatch").
- Abaft the beam: Further aft than the beam: a relative bearing of greater than 90 degrees from the bow: "two points abaft the port beam".
- Abandon ship!: An imperative to leave the vessel immediately, usually in the face of some imminent danger. It is an order issued by the Master or a delegated person in command. It is usually the last resort after all other mitigating actions have failed.
- Abeam: On the beam, a relative bearing at right angles to the centerline of the ship's keel.
- "Abel Brown": A sea shanty (song) about a young sailor trying to sleep with a maiden.
- Aboard: On or in a vessel (see also "close aboard").
- Absentee pennant: Special pennant flown to indicate absence of commanding officer, admiral, his chief of staff, or officer whose flag is flying (division, squadron, or flotilla commander).
- Absolute bearing: The bearing of an object in relation to north. Either true bearing, using the geographical or true north, or magnetic bearing, using magnetic north. See also "bearing" and "relative bearing".
- Accommodation ladder: A portable flight of steps down a ship's side.
- Admiral: Senior naval officer of Flag rank. In ascending order of seniority, Rear Admiral, Vice Admiral, Admiral and Admiral of the Fleet (Royal Navy). Derivation Arabic, from Amir al-Bahr("Ruler of the sea").
- Admiralty: A high naval authority in charge of a state's Navy or a major territorial component. In the Royal Navy (UK) the Board of Admiralty, executing the office of the Lord High Admiral, promulgates Naval law in the form of Queen's (or King's) Regulations and Admiralty Instructions.
- Admiralty law: Body of law that deals with maritime cases. In the UK administered by the Probate, Divorce and Admiralty Division of the High Court of Justice or supreme court.
- Adrift: Afloat and unattached in any way to the shore or seabed, but not under way. It implies that a vessel is not under control and therefore goes where the wind and current take her (loose from moorings, or out of place). Also refers to any gear not fastened down or put away properly. It can also be used to mean "absent without leave".
- Advance note: A note for one month's wages issued to sailors on their signing a ship's articles.
- Adviso: see aviso.
- Aft: The portion of the boat behind the middle area of the boat. Towards the stern (of the vessel).
- Afloat: Of a vessel which is floating freely (not aground or sunk). More generally of vessels in service ("the company has 10 ships afloat").
- Afternoon watch: The 1200–1600 watch.
- Aground: Resting on or touching the ground or bottom (usually involuntarily).
- Ahead: Forward of the bow.
- Ahoy: A cry to draw attention. Term used to hail a boat or a ship, as "Boat ahoy!"
- Ahull:
- lying broadside to the sea.
- to ride out a storm with no sails and helm held to leeward.
- Aid to Navigation: (ATON) Any device external to a vessel or aircraft specifically intended to assist navigators in determining their position or safe course, or to warn them of dangers or obstructions to navigation.
- All hands: Entire ship's company, both officers and enlisted personnel.
- All night in: Having no night watches.
- Aloft: In the rigging of a sailing ship. Above the ship's uppermost solid structure; overhead or high above.
- Alongside: By the side of a ship or pier.
- Amidships (or midships): In the middle portion of ship, along the line of the keel.
- Anchor:
- an object designed to prevent or slow the drift of a ship, attached to the ship by a line or chain; typically a metal, hook-like or plough-like object designed to grip the bottom under the body of water (but also see sea anchor).
- to deploy an anchor ("She anchored offshore.")
- Anchorage: A suitable place for a ship to anchor. Area of a port or harbor.
- Anchor's aweigh: Said of an anchor when just clear of the bottom.
- Anchor ball: Round black shape hoisted in the forepart of a vessel to show that it is anchored.
- Anchor buoy: A small buoy secured by a light line to anchor to indicate position of anchor on bottom.
- Anchor chain or anchor cable: Chain connecting the ship to the anchor.
- Anchor detail: Group of men who handle ground tackle when the ship is anchoring or getting underway.
- Anchor home: The term for when the anchor is secured for sea. Typically rests just outside the hawse pipe on the outer side of the hull, at the bow of a vessel.
- Anchor light: White light displayed by a ship at anchor. Two such lights are displayed by a ship over 150 feet (46 m) in length.
- Anchor rode: The anchor line, rope or cable connecting the anchor chain to the vessel. Also Rode.
- Anchor sentinel: A separate weight on a separate line which is loosely attached to the anchor rode so that it can slide down it easily. It is made fast at a distance slightly longer than the draftof the boat. It is used to prevent the anchor rode from becoming fouled on the keel or other underwater structures when the boat is resting at anchor and moving randomly during slack tide. Also called a kellet.
- Anchor watch: The crewmen assigned to take care of the ship while anchored or moored, charged with such duties as making sure that the anchor is holding and the vessel is not drifting. Most marine GPS units have an Anchor Watch alarm capability.
- Andrew: Traditional lower-deck slang term for the Royal Navy.
- Anti-rolling tanks: A pair of fluid-filled, usually water, tanks mounted on opposite sides of a ship below the waterline. Fluid would be pumped between them in an attempt to dampen the amount of roll.
- Apparent wind: The combination of the true wind and the headwind caused by the boat's forward motion. For example, it causes a light side wind to appear to come from well ahead of the beam.
- Arc of Visibility: The portion of the horizon over which a lighted aid to navigation is visible from seaward.
- Archboard: The plank along the stern where the name of the ship is commonly painted.
- Armament: A ship's weapons.
- Articles of War: Regulations governing the military and naval forces of UK and USA; read to every ship's company on commissioning and at specified intervals during the commission.
- ASDIC: A type of sonar used by the Allies for detecting submarines during the Second World War.
- Ashore: On the beach, shore or land.
- Astern: towards the stern (rear) of a vessel, behind a vessel.
- Asylum Harbour: A harbour used to provide shelter from a storm.
- ASW: Anti-submarine warfare.
- Athwart, athwartships: At right angles to the fore and aft or centerline of a ship
- Avast: Stop, cease or desist from whatever is being done. From the Dutch hou' vast (“hold fast”), from houd (“hold”) + vast (“fast”).
- Awash: So low in the water that the water is constantly washing across the surface.
- Aviso: (formerly also an adviso), a kind of dispatch boat or advice boat, survives particularly in the French navy, they are considered equivalent to the modern sloop.
- Aweigh: Position of an anchor just clear of the bottom.
- Axial fire: Fire oriented towards the ends of the ship; the opposite of broadside fire.
- Aye, aye (/ˌaɪ ˈaɪ/): Reply to an order or command to indicate that it, firstly, is heard; and, secondly, is understood and will be carried out. ("Aye, aye, sir" to officers). Also the proper reply from a hailed boat, to indicate that an officer is on board.
- Azimuth compass: An instrument employed for ascertaining position of the sun with respect to magnetic north. The azimuth of an object is its bearing from the observer measured as an angle clockwise from true north.
- Azimuth circle: Instrument used to take bearings of celestial objects.
Glossary of nautical terms
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