Glossary
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TermDefinition
- Courses, RYA Inland Waterways CrewThe RYA Inland Waterways Crew course provides essential training for assisting with operations on canals, rivers, and other inland waterways.... More Details
- Courses, RYA Inland Waterways Helmsman'sThe RYA Inland Waterways Helmsman’s course is designed for those operating boats on canals and rivers. This two-day course covers boat handling,... More Details
- General Nautical Terminology, Safety INMARSAT
A global satellite communications organisation providing maritime, aviation, and land-based services for safety and operational...
More Details - General Nautical Terminology Inshore
The area of water close to the shore, typically within a few miles, where vessels operate in relatively calmer and shallower waters compared to...
More Details - General Nautical Terminology In StaysWhen the bow points directly into the wind with the sails flapping during the process of going about
- Courses, RYA Intermediate WindsurfingThe RYA Intermediate Windsurfing course bridges the gap between beginner and advanced skills. Participants learn techniques for improving speed,... More Details
- Courses, RYA Intermediate Windsurfing (Planing)The RYA Intermediate Windsurfing (Planing) course introduces participants to planing techniques, enabling them to harness the power of stronger... More Details
- General Nautical Terminology International Association of Lighthouse Authorities (IALA)
The International Association of Lighthouse Authorities (IALA) is an international organisation that aims to promote safe, efficient, and...
More Details - General Nautical Terminology International Certificate of Competence (ICC)
An official document that certifies a person's ability to operate a pleasure craft in countries that recognise it.
The International...
More Details - Flag / Pennant, General Nautical Terminology, Signals & Communication International Code of Signals (ICS)
A system of flags, codes, and symbols used by ships to communicate essential messages and ensure safety at sea, regardless of language...
More Details - General Nautical Terminology International Convention for the Prevention of Pollution from Ships (MARPOL)
Established by the IMO, MARPOL is the convention for preventing pollution from ships. It is the main international convention covering the...
More Details - Did you know?, General Nautical Terminology, Navigation International Date Line (IDL)
An imaginary longitudinal line located at approximately 180 degrees longitude serving as the demarcation where the calendar date changes by one...
More Details - General Nautical Terminology, Navigation International Hydrographic Organization (IHO)
An intergovernmental organisation dedicated to ensuring the uniformity and standardisation of hydrographic and nautical charting practices...
More Details - Organisation International Maritime Organisation (IMO)
A specialised agency of the United Nations headquartered in London and responsible for regulating shipping, the IMO has 175 Member States.
The...
More Details - General Nautical Terminology, Safety International Maritime Satellite Organisation (INMARSAT)
A global satellite communications organisation providing maritime, aviation, and land-based services for safety and operational...
More Details - Flag / Pennant, Signals & Communication International Maritime Signal Flags
A system of flags ships use to communicate messages, signals, or important information visually. Flags often represent letters, numbers, or...
More Details - Did you know?, Navigation International Meridian Conference
A diplomatic meeting held in 1884 in Washington, D.C., where representatives from 25 nations convened to establish a standard prime meridian for...
More Details - Morse Code, Signals & Communication International Morse Code
A standardised system of encoding text characters as sequences of dots (short signals) and dashes (long signals) used for communication,...
More Details - General Nautical Terminology International Regulations for Preventing Collisions at Sea (IRPCS)
- Organisation, Signals & Communication International Telecommunication Union (ITU)
A specialised agency of the United Nations responsible for coordinating global telecommunication operations and services, including radio...
More Details - General Nautical Terminology, Weather (Meteorology) Irish Sea
Sea area of the Met Office Shipping Forecast. Covers the Irish Sea between Ireland and Great Britain.
Between Ireland and Great Britain, the...
More Details - General Nautical Terminology IRPCS
- Organisation ISAF
World Sailing, formerly known as the International Sailing Federation (ISAF), is the governing body for the sport of sailing...
More Details - Light Characteristic Iso
A light where the duration of light and darkness are equal.
Isophase describes a navigational aid that emits a light signal with equal periods...
More Details - General Nautical Terminology IsobarOn a synoptic chart, a line joining points of equal pressure
- General Nautical Terminology Isolated Danger Mark
Buoy or pillar positioned on a dangerous underwater obstruction.

An isolated danger mark is a navigation aid used at sea to indicate a...
More Details - Light Characteristic Isophase (Iso)
A light where the duration of light and darkness are equal.
Isophase describes a navigational aid that emits a light signal with equal periods...
More Details - Organisation, Signals & Communication ITU
A specialised agency of the United Nations responsible for coordinating global telecommunication operations and services, including radio...
More Details - General Nautical Terminology, Safety Jackstay
A length of webbing (or on rare occasions a length of wire) which run from the bow to the stern of the vessel. They have sufficient slack in them...
More Details - Flag / Pennant, Signals & Communication Code Flag Juliett (J)

The International Maritime Signal Flag Juliett.
Meaning
I am on fire and have dangerous cargo on board: keep well clear of me.
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OR
I am... - General Nautical Terminology Jib
A triangular sail set ahead of the foremast, typically attached to a stay, and used to improve a vessel's manoeuvrability and balance.
The jib...
More Details - General Nautical Terminology Jib SheetsLines attached to clew of headsail to control sail shape.
- Flag Semaphore, Signals & Communication Letter J (Flag Semaphore)

This signal is also used to transition from letters to numbers when communicating a message.
- Morse Code, Signals & Communication Letter J (Morse Code)
International Morse Code sequence representing the letter J.
- General Nautical Terminology, Safety Jonbuoy
A specialised man overboard recovery device designed to provide buoyancy and visibility to a person in the water.

A Jonbuoy is a...
More Details - Did you know?, Phonetic Alphabet, Signals & Communication Juliett
Pronounced: JOO-lee-ETT
The term for the letter J in the NATO Phonetic Alphabet.
More Details - General Nautical Terminology Jury RigA temporary but effective device that replaces lost or damaged gear
- General Nautical Terminology, Weather (Meteorology) Katabatic Wind
A gravity-driven wind that flows downhill from elevated terrain, such as glaciers or high coastal cliffs, typically bringing colder, denser air...
More Details - Flag / Pennant, Signals & Communication Code Flag Kilo (K)

The International Maritime Signal Flag Kilo.
Meaning
I wish to communicate with you.
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If appended by a Numeral Pennant:
I wish to... - General Nautical Terminology KedgeA technique for moving or turning a ship by using a relatively light anchor known as a kedge.
- General Nautical Terminology Kedge Anchor
A small, lightweight anchor used to reposition or manoeuvre a vessel in confined waters or to assist in anchoring when the main anchor is...
More Details - General Nautical Terminology KeelA structural element that sometimes resembles a fin and protrudes below a boat usually along its centre line
- Courses, RYA Keelboat Club RacingThe RYA Keelboat Club Racing course prepares sailors to participate confidently in club-level keelboat races. Participants learn race rules,... More Details
- Courses, RYA Keelboat Day SailingThe RYA Keelboat Day Sailing course teaches participants how to plan and execute safe and enjoyable day trips in keelboats. This course covers... More Details
- Courses, RYA Keelboat Level 1 - Start SailingThe RYA Keelboat Level 1 course is an entry-level program for those new to sailing. Participants learn basic skills such as rigging, steering, and... More Details
- Courses, RYA Keelboat Level 2 - Basic SkillsThe RYA Keelboat Level 2 course builds on Level 1 by teaching participants to sail independently. Practical sessions focus on tacking, gybing, and... More Details
- Courses, RYA Keelboat Level 3 - Better SailingThe RYA Keelboat Level 3 course refines sailing techniques and enhances confidence in challenging conditions. Participants practice advanced... More Details
- Courses, RYA Keelboat Performance SailingThe RYA Keelboat Performance Sailing course is tailored for sailors looking to optimize performance in keelboats. Participants focus on advanced... More Details
- Courses, RYA Keelboat Regional RacingThe RYA Keelboat Regional Racing course is designed for sailors competing at a regional level. The program focuses on advanced racing tactics,... More Details
- Courses, RYA Keelboat Sailing with SpinnakersThe RYA Keelboat Sailing with Spinnakers course introduces sailors to spinnaker handling. Participants learn how to rig, hoist, trim, and gybe... More Details
- Courses, RYA Keelboat Seamanship SkillsThe RYA Keelboat Seamanship Skills course focuses on building practical skills for handling keelboats in various conditions. Participants practice... More Details
- Courses, RYA Keelboat Start RacingThe RYA Keelboat Start Racing course introduces sailors to the fundamentals of competitive keelboat racing. Participants learn essential racing... More Details
- General Nautical Terminology KetchA two-masted fore-and-aft rigged sailboat with the aft mast (the mizzen) mounted (stepped) afore (in front of) the rudder.
- General Nautical Terminology Kicking StrapLine or tackle to pull the boom down to keep it horizontal
- Phonetic Alphabet, Signals & Communication Kilo
Pronounced: key-loh
The term for the letter K in the NATO Phonetic Alphabet.
- Flag Semaphore, Signals & Communication Letter K (Flag Semaphore)

This shape represents the Letter K (Flag Semaphore) and the Number 0 (Flag Semaphore).
- Morse Code, Signals & Communication Letter K (Morse Code)
International Morse Code sequence representing the letter K.
- General Nautical Terminology, Navigation Knot
A unit of speed used in maritime navigation, equivalent to one nautical mile per hour.
A knot is a crucial measurement in marine navigation,...
More Details - General Nautical Terminology, Knot Knot
A deliberate intertwining of rope or line designed to bind, secure, or attach objects.
Knots are essential in marine environments for various...
More Details - General Nautical Terminology, Weather (Meteorology) Land Breeze
A local wind that blows from the land towards the sea, typically during the night, caused by the differential cooling rates of land and...
More Details - General Nautical Terminology Landfall
The event of a vessel sighting or reaching land after a voyage at sea.
Landfall is a term used in the maritime world to describe the moment...
More Details - General Nautical Terminology LanyardA short length of line used to secure an object such as a knife
- General Nautical Terminology Lash DownTo secure firmly with a rope or line
- General Nautical Terminology, Navigation LAT
The lowest level that sea tides can reach under normal meteorological conditions, based on astronomical influences such as the gravitational pull...
More Details - General Nautical Terminology Lateral MarkBuoy, or post used to mark Port and Starboard sides of a channel
- General Nautical Terminology, Navigation Latitude
The angular distance of a location on Earth's surface, measured in degrees(°), minutes('), and seconds('') north or south of the equator which is...
More Details - General Nautical Terminology Lazarette
A storage compartment located in the stern (aft) of a boat, typically used for stowing gear, equipment, or provisions.
A lazarette is a...
More Details - General Nautical Terminology Lazy JacksA network of cordage rigged to a point on the mast and to a series of points on either side of the boom that cradles and guides the sail onto the... More Details
- Flag / Pennant, Signals & Communication Code Flag Lima (L)

The International Maritime Signal Flag Lima.
Meaning
Stop immediately.
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If appended by a Numeral Pennant, Latitude (the first 2 digits... - General Nautical Terminology Leading LightsLights positioned to assist entry into harbour or safe passage through a channel. Usually one light above another marks the correct course.
- General Nautical Terminology LeadlineAn instrument used in navigation to measure water depth; a line attached to some lead. Usually the line is marked with knots at regular intervals.
- General Nautical Terminology LeagueA unit of length, normally equal to three nautical miles.
- Courses, RYA Learn to WingsurfThe RYA Learn to Wingsurf course introduces participants to the basics of wingsurfing. Participants learn how to control the wing, balance on the... More Details
- General Nautical Terminology LeeThe direction towards which the wind blow
- General Nautical Terminology LeechThe trailing edge of a triangular sail
- General Nautical Terminology Lee-oh!Command given by helm when tacking as the bow passes through the wind. Tells the crew to reset the sails on the new course.
- General Nautical Terminology, Weather (Meteorology) Leeshore
A shoreline that lies downwind of a vessel, posing a potential hazard as the wind pushes the vessel towards it.
A leeshore is a concept in...
More Details - General Nautical Terminology Leeward
The side of a vessel or an object that is sheltered from the wind, opposite to the windward side.
Leeward is a crucial concept for...
More Details - Flag Semaphore, Signals & Communication Letters to Follow (Flag Semaphore)

This flag semaphore signal indicates a change from numbers to letters (Letters to follow). This switches back from numbers to the default...
More Details - Courses, RYA Level 1 Start Powerboating (PB1)The RYA Level 1 Start Powerboating course introduces beginners to the basics of powerboat handling. Participants learn low-speed maneuvers, safety... More Details
- Courses, RYA Level 2 Powerboat Handling (PB2)The RYA Level 2 Powerboat Handling course is a comprehensive two-day program that teaches participants the skills needed to operate powerboats... More Details
- General Nautical Terminology, Safety Life Jacket
A buoyant, wearable safety device designed to keep a person afloat in the water and reduce the risk of drowning.
A life jacket, also known as...
More Details - General Nautical Terminology LifelinePersonal safety device attached to harness on lifejacket and to vessel strongpoint. Designed to keep you on board
- General Nautical Terminology, Safety Liferaft
An inflatable or rigid floating device designed to provide emergency refuge and safety for individuals forced to abandon a vessel at sea.
A...
More Details - General Nautical Terminology Lighthouse
A tall, often cylindrical structure equipped with a powerful light and other navigational aids, built near coastlines or on islands to guide...
More Details - Phonetic Alphabet, Signals & Communication Lima
Pronounced: LEE-mah
The term for the letter L in the NATO Phonetic Alphabet.
- General Nautical Terminology Line
A rope used on a boat for various purposes, such as securing the vessel to a mooring (mooring line), adjusting sails (halyard or sheet), towing...
More Details - General Nautical Terminology, Navigation Lines of Position (LOP)
Navigational lines drawn on a chart, each representing a possible location of a vessel based on a single observation or measurement.
A line...
More Details - General Nautical Terminology ListA permanent lean to one side or the other
- General Nautical Terminology Listing
The condition of a vessel leaning to one side due to an imbalance in weight or external forces.
A vessel lists when it tilts to port or...
More Details - Flag Semaphore, Signals & Communication Letter L (Flag Semaphore)

Flag semaphore indicating the letter L.
- Morse Code, Signals & Communication Letter L (Morse Code)
International Morse Code sequence representing the letter L.
- General Nautical Terminology LNM
A Local Notice to Mariners (LNM) is a publication issued by maritime authorities to provide important information to mariners about navigational...
More Details - General Nautical Terminology Local Notice to Mariners (LNM)
A Local Notice to Mariners (LNM) is a publication issued by maritime authorities to provide important information to mariners about navigational...
More Details - General Nautical Terminology Lock
A gated section of a waterway, such as a canal or river, designed to raise or lower vessels between stretches of water at different levels.
A...
More Details - General Nautical Terminology, Parts of a Boat Locker
A storage compartment or enclosed space on a boat, typically used for stowing equipment, gear, or provisions securely.
A locker on a boat is a...
More Details - General Nautical Terminology Logbook
An official record maintained aboard a vessel that documents the ship's daily activities, including navigational data, weather conditions, and...
More Details - General Nautical Terminology Log Line
A device used historically by mariners to measure a vessel's speed through the water.
The log line consists of a wooden board, known as the...
More Details - General Nautical Terminology Log ReadingThe reading of distance travelled through the water usually taken every hour from the log and recorded in the deck log
- General Nautical Terminology, Navigation Longitude
The geographic coordinate that specifies the east-west position of a point on the Earth's surface, measured in degrees(°), minutes(') and...
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