A - Comprehensive Guide to Nautical Terms: Letter A

The nautical world has a rich history and a unique vocabulary that has evolved over centuries. As a result, many terms and phrases have developed that are specific to the maritime industry. In this article, we'll explore some of the most commonly used and interesting nautical terms beginning with the letter A. By the end of this guide, you'll have a better understanding of these terms and their meanings, enhancing your knowledge of the fascinating world of sailing and navigation.


An A-Frame is a lifting apparatus commonly found installed on the stern of pipe-laying vessels, cable-laying vessels, and offshore construction vessels. It is designed in the shape of the letter 'A' and is used to lift heavy objects or equipment on and off the vessel.


Abaft is a term used to describe the position of an object or area on a ship relative to the vessel's centerline. It refers to a location that is towards the stern (rear) of the ship, behind another reference point. For example, the phrase "abaft the fore hatch" means that the location is behind the forward hatch.

Abandon Ship

Abandon Ship is an urgent command given to the passengers and crew of a vessel when they need to leave the ship immediately due to an emergency, such as the vessel being on the verge of sinking. This command indicates that all onboard must evacuate the ship as quickly and safely as possible.

Able-Bodied Seaman

An Able-Bodied Seaman (also called an Able Seaman or simply AB) is a member of the Deck Department on a ship. They typically have more than two years of experience and are expected to be well-versed in their job responsibilities. These duties include standing watch, keeping a lookout, cleaning and maintenance tasks, conducting security rounds, supervising lower-ranking deck crew members, and assisting with operations such as docking, undocking, line handling, and the loading and unloading of cargo.


Abeam refers to any object, direction, or location that is at a right angle to the ship's centerline or keel. It is used to describe the position of something in relation to the ship, such as another vessel or a navigational waypoint. For example, "two points abaft the port beam" means that the object is located two points (22.5 degrees) behind the port (left) side of the ship at a right angle to the centerline.


Aboard is a term used to describe being on or inside a vessel. It can also refer to a location close to a ship, as in the phrase "close aboard." When someone is aboard a ship, they are considered to be part of the ship's crew or a passenger and are subject to the rules and regulations of the vessel.


The Accommodation area on a ship is the part of the vessel where the crew's living quarters and facilities are located. This includes the cabins, the crew mess (the dining area and space where crew members can relax when they are not working), and other amenities such as bathrooms and recreational spaces.

Accommodation Ladder

The Accommodation Ladder is a portable set of stairs, complete with handrails, that is attached to the side of a ship. It allows for easy access between the dock or a smaller boat and the vessel. The ladder is designed to ensure that no matter the angle of inclination, the steps remain horizontal for safe use.

Active Fin Stabilizers

Active Fin Stabilizers are devices used to reduce the rolling motion of a ship in rough seas. They provide increased stability and safety while minimizing the negative impact on speed and fuel performance in adverse conditions. The fins are typically located near the middle of the ship on both the port (left) and starboard (right) sides and can be either retractable or non-retractable.

Active Fin Stabilizers have long been a feature on passenger ships, such as cruise liners. However, they are becoming increasingly popular on commercial cargo and container ships, which also benefit from improved stability and reduced fuel consumption.

Added Mass

When a vessel is in motion, the water surrounding the hull creates a hydrodynamic force. Due to the complex shape of a ship, an additional quantity of water is added to the water that has been displaced by the vessel's movement. This added water accounts for the added mass and is factored into calculations related to the ship's overall performance and stability.


Admeasurement is the formal process of measuring a ship to determine its capacity or tonnage. This measurement is essential for calculating the ship's cargo-carrying capacity, stability, and other performance-related factors.


In the context of shipping, Administration refers to the government of the state under whose authority a vessel is operating. A ship flies the flag of a specific state, and the Administration is the government of that state. This entity is responsible for ensuring that the vessel complies with all relevant rules, regulations, and safety standards.


When a vessel is adrift, it is floating without being steered or moored. This can occur when a ship loses power or control, or when it is intentionally set adrift for a specific purpose, such as to drift with the current. In a broader sense, the term can also apply to any unsecured equipment, lines, or other items onboard a ship that are not properly stowed or secured.


A vessel is considered afloat when it is floating on the water's surface, as opposed to being in dry dock on land or run aground. A ship that is afloat is able to freely move and navigate, subject to the conditions of the water and the skill of its crew.

Aframax Tanker

An Aframax tanker is an oil tanker with a deadweight (DWT) capacity of between 80,000 and 120,000 metric tonnes. The name "Aframax" is derived from the Average Freight Rate Assessment (AFRA) system, which was developed by Shell Oil in 1954 to standardize the terms of shipping contracts.

Aframax tankers are best suited for short- to medium-haul journeys due to their size. However, their size also allows them to serve the majority of the world's ports, particularly those in regions that are unable to accommodate larger VLCC (Very Large Crude Carriers) and ULCC (Ultra Large Crude Carriers) tankers due to smaller port facilities or the lack of offshore oil terminals.


Aft is a term used to describe a location or direction that is towards or near the stern (rear) of a ship. For example, the phrase "the engine room is located aft" means that the engine room is situated towards the rear of the vessel.

Aft Peak

The Aft Peak is a compartment located towards the rear (aft) of the ship, just forward of the aftermost watertight bulkhead.

Aft Peak Bulkhead

The Aft Peak Bulkhead is the first main transverse bulkhead located forward of the stern. This bulkhead must be watertight to maintain the ship's structural integrity and prevent flooding in the event of damage to the hull.

Aft Peak Tank

The Aft Peak Tank is a water tank located in the rear (aft) section of a ship. This tank can either be designated as a freshwater tank, used for storing potable water, or as a ballast water tank, which is used to adjust the ship's trim and stability.

Aft Perpendicular

The Aft Perpendicular is an imaginary vertical line that is drawn between the forward side of the stern and the summer load waterline on a ship. Depending on the ship's design, the Aft Perpendicular can be drawn through the aft side of the rudder post or through the centerline of the rudder pintles. The Length Overall (LOA) is the distance between the fore and aft extreme points of the vessel, and this line serves as the reference point for hydrostatic calculations.


The Afterbody is the section of a ship's hull that lies aft of the midships area. It encompasses the entire rear half of the hull, from the upper deck to the keel. This area is important for the ship's stability, as it helps to counterbalance the weight and force exerted by the forward sections of the vessel.

Afternoon Watch

The Afternoon Watch is the period of duty onboard a vessel that takes place between 12:00 and 16:00 hours. During this time, crew members are responsible for various tasks, such as maintaining the ship's course, monitoring weather conditions, and ensuring the safety and security of the vessel and its passengers.

Agency Fee

An Agency Fee is a sum of money charged to a ship by its agent to compensate for any services received while the vessel was in port. These services can include arranging for pilots, coordinating cargo operations, and providing supplies or provisions to the ship.


A ship is considered to be aground when it is resting on or touching shallow ground, such as a sandbar, reef, or the sea floor. This can occur due to navigational errors, extreme weather conditions, or mechanical failures. When a vessel is aground, it is at risk of damage or even sinking, and efforts must be made to free it and return it to deeper water as quickly and safely as possible.

Aids to Navigation

Aids to Navigation (AtoN) are any devices, markers, or systems designed to assist navigators in determining their position or safe course, or to warn them of dangers or obstructions to navigation. Examples of AtoN include beacons, lighthouses, buoys, lights, and radio beacons.

Aids to Navigation Service Vessel

An Aids to Navigation Service Vessel is a ship or boat that is equipped with specialized equipment used for servicing, maintaining, and repairing navigation aids, such as automatic lighthouses, buoys, beacons, and lights. These vessels play a critical role in ensuring that navigational aids remain functional and effective in guiding ships safely through various waterways.

Air Cushion Vehicle (ACV)

An Air Cushion Vehicle (ACV), commonly known as a hovercraft, is a type of vessel that is supported primarily or entirely by a cushion of air. ACVs are buoyed both while in motion and at rest, meaning that air must be continuously generated to maintain their lift. Hovercraft are used for a variety of purposes, including transportation, military operations, and search and rescue missions.

Air Draft

The Air Draft, or Air Draught, is the vertical distance from the surface of the water (the waterline) to the highest fixed point on a vessel. This measurement is important for determining whether a ship can safely pass under bridges, power lines, or other overhead structures.

Air Lock

An Air Lock is a protected, enclosed area typically found on a gas carrier vessel that enables safe passage between a hazardous gas zone and a gas-safe space. This area is designed to minimize the risk of explosions or leaks by isolating the potentially dangerous gas from the rest of the ship.

Air Lubrication

Air Lubrication is a technique used to reduce hull friction and improve a ship's efficiency by creating a layer of air between the underside of the vessel and the surface of the water. This can result in a reduction in drag of up to 15%, leading to increased fuel efficiency and reduced emissions.

Air Resistance

Air Resistance refers to the resistance experienced by a ship due to the air circulating around the part of the vessel that is above the waterline. This resistance can affect the ship's speed, stability, and fuel consumption, and must be taken into account when designing and operating a vessel.

Air Trunks

Air Trunks are compartments within a ship's hull that either house air ducts and other lines, such as pipes and cables, or serve as conduits for the circulation of air. These trunks are essential for maintaining proper ventilation and air circulation throughout the vessel, helping to ensure the comfort and safety of the crew and passengers on board.

Aldis Lamp

The Aldis Lamp, also known as a Signal Lamp or Morse Lamp, is a handheld electric lamp typically found on the bridge wing of a vessel. It is used to send Morse code messages via flashing light between ships of all types, including naval and commercial vessels. This form of communication is important for coordinating ship movements, relaying distress signals, and exchanging information with other vessels.


An Alleyway is the term used to describe any corridor onboard a ship that connects one area of the accommodation to another. These passageways are essential for allowing crew members and passengers to move freely and safely between different parts of the vessel.


Allision is the term used to describe the act of a moving ship striking or colliding with a stationary object, such as a dock or another vessel that is not underway. This is distinct from a collision, which typically refers to two vessels that are both in motion colliding with one another. Allisions can result in significant damage to both the ship and the stationary object, and may require extensive repairs or even the replacement of the affected structures.


Amidships, or midships, refers to the middle portion of a ship, along the line of the keel. This central area of a vessel is important for maintaining stability and balance, as it is typically where the ship's center of gravity is located. In shipbuilding, the amidships area is often used as a reference point for the placement of various components and systems.


An Anchor is a heavy, hook-like device that is attached to a ship by a line, chain, or cable and is designed to grip the seabed in order to hold the vessel in place. Anchors come in various shapes and sizes and are used to secure a ship in position, regardless of wind and current conditions.

Anchor Ball

The Anchor Ball is a round black ball that is displayed at the front of a vessel to indicate that the ship is anchored. This visual signal informs other vessels in the area that the anchored ship is not underway and that they should take appropriate precautions to avoid colliding with it.

Anchor Chain

The Anchor Chain, also known as the anchor rode, cable, or rope, is the line that connects the ship to the anchor. The chain is typically made of metal links, which provide strength and durability to withstand the forces exerted by the ship and the anchor as they interact with the sea and the seabed.

Anchor Handling Tug

An Anchor Handling Tug is a type of tugboat that is specifically designed to move anchors and tow other vessels, such as drilling rigs, lighters, and barges. These specialized vessels are equipped with powerful engines and winches, allowing them to handle the heavy loads and forces associated with anchor handling and towing operations.


An Anchorage is a suitable location for a ship to anchor, providing a safe and secure place for the vessel to remain stationary. Anchorages are typically found in protected bays or harbors, where the water is calm and the seabed is suitable for the anchor to grip. Ships may anchor at an anchorage for a variety of reasons, such as to wait out a storm, to load or unload cargo, or to conduct maintenance or repairs.

Anchor Watch

The Anchor Watch is a duty performed by a crew member or officer while a ship is at anchor. The individual on anchor watch is responsible for monitoring the ship's position, ensuring that the anchor is holding, and keeping an eye out for other vessels that may be drifting or sailing too close to the anchored ship. Anchor watch is particularly important during rough weather and at night when visibility is limited.

Anti-Fouling Paint

Anti-Fouling Paint is a type of paint that contains specific agents to prevent the growth and attachment of organisms, such as algae, barnacles, and other marine life, on a ship's hull. By reducing the buildup of these organisms, anti-fouling paint helps to maintain the ship's performance and fuel efficiency, as well as reduce the risk of spreading invasive species between different bodies of water.

Anti-Heeling System

An Anti-Heeling System is a device or system used during cargo operations, such as the loading and unloading of freight, to ensure that the ship's heeling angle is kept to a minimum. Excessive heeling can result in damage to cargo, jammed containers, or twisted ramps. The anti-heeling system works by adjusting the distribution of ballast water within the ship's tanks, counteracting the forces that cause the vessel to heel.

Anti-Roll Tank

An Anti-Roll Tank is a tank that is fitted to a ship in order to improve its response to roll motion. The tank is designed to slow the rate of water transfer from the port side of the tank to the starboard side, helping to maintain a greater quantity of water on the ship's higher side. This increased water weight helps to counteract the forces that cause the ship to roll, providing greater stability and comfort for the crew and passengers on board.


Appendages are any parts of a ship that protrude from the hull below the waterline. These can include propellers, rudders, shafts, bilge keels, struts, and sonar domes. These structures play important roles in the ship's propulsion, steering, and overall performance, but can also create additional drag and resistance as the vessel moves through the water.


The Apron is an area directly in front of or behind a wharf shed, onto which cargo is lifted. Cargo is unloaded from a ship onto the front Apron and then loaded into trucks or railroad cars from the rear Apron. This process is critical for the efficient transfer of goods between the ship and the shore, and proper organization and management of the Apron area are essential for smooth cargo operations.

Archimedes' Principle

Archimedes' Principle states that the upward buoyant force exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. This principle explains why a ship is able to float on the water, as the vessel's weight is less than the weight of the water it displaces.

Asylum Harbor

An Asylum Harbor is a harbor that provides shelter from a storm. These harbors are typically found in protected bays or inlets, where the water is calm and the surrounding land provides a barrier against the wind and waves. Ships may seek refuge in an asylum harbor during extreme weather conditions to protect the vessel and its crew from damage or injury.


Athwartships is a term used to describe a direction or position that is at right angles to the ship's centerline or keel. This can refer to the location of an object on the ship, such as a cargo hold or a piece of equipment, or the direction of a force acting on the vessel, such as the wind or a wave.


The atmosphere is the layer of gases that surrounds the Earth, extending from the Earth's surface to the edge of space. This layer of gases plays a critical role in the Earth's climate and weather systems, as well as providing the air we breathe. In the context of sailing and navigation, the atmosphere influences factors such as wind speed and direction, air pressure, and temperature, all of which can have a significant impact on a ship's performance and the safety of its crew.


An Auto-Container is a type of shipping container that is specifically designed for the transportation of cars and other vehicles. These containers are equipped with specialized fixtures and fittings to secure the vehicles in place during transit, helping to protect them from damage and movement.

Auto-Mooring System

An Auto-Mooring System is a technology that automates the process of mooring and unmooring a ship, reducing the time spent in port and, thus, fuel consumption and emissions. The system is controlled remotely from the ship's bridge or from a shore-based location, and uses specialized equipment to secure the vessel to the dock or mooring buoy.

Automatic Identification System (AIS)

An Automatic Identification System (AIS) is a navigational aid and automated tracking system that is used for the identification of vessels and navigational marks. AIS displays other vessels in the vicinity of a ship, as well as the ship on which it is installed, to other vessels equipped with AIS in the area.

AIS is a broadcast transponder system that operates in the VHF mobile maritime band, autonomously and continuously transmitting and receiving information about the ship's position, course, speed, and other relevant data.

Automatic Radar Plotting Aid (ARPA)

An Automatic Radar Plotting Aid (ARPA) is a marine radar technology designed to replace the manual plotting of targets on a radar screen. The ARPA uses radar contacts to calculate the course, speed, and closest point of approach (CPA) of various tracked objects, allowing the ship's crew to monitor the screen and take appropriate action if there is a potential risk of collision with the tracked object.


An Autopilot, or Automatic Pilot, is a self-steering device system used for automatic navigation. Autopilots come in varying degrees of complexity and sophistication, with basic models able to maintain a vessel on a pre-set compass course, while more advanced systems can gather data from the ship's instruments or connect to GPS receivers for more precise navigation.

An Autopilot can help to reduce crew workload, improve fuel efficiency, and enhance the overall safety and performance of a vessel during its voyage.


Auxiliaries are any secondary systems, equipment, or machinery on a ship that support the primary systems, such as the main engines, propellers, and steering gear. Examples of auxiliaries include generators, pumps, winches, and ventilation systems. These systems play a vital role in the overall operation and safety of the vessel, and must be properly maintained and monitored to ensure optimal performance.

In conclusion, understanding the meaning and usage of various nautical terms is essential for anyone involved in the maritime industry. These terms not only help to ensure clear communication between crew members and other professionals in the field, but also contribute to the rich history and tradition of sailing and navigation. This comprehensive guide to nautical terms beginning with the letter A is a valuable resource for anyone looking to expand their knowledge of the fascinating world of sailing and navigation.

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