Sailing Glossary

185 terms and definitions from the RYA Day Skipper theory syllabus, covering nautical terminology, navigation, COLREGs, tides, and more.

1

1 knot: 1 nautical mile per hour. 'Knots per hour' is incorrect.Position, Course & Speed
1 minute of latitude: Equals 1 nautical mile (1,852 metres). Foundation of all chart distance measurement.Drawing Instruments
1 nautical mile: 1,852 metres = 1 minute of latitudePosition, Course & Speed
150N lifejacket: Standard coastal lifejacket — turns unconscious person face-upSafety

2

275N lifejacket: Offshore lifejacket — for heavy foul-weather clothingSafety

5

50-minute rule: HW occurs ~50 minutes later each day because the Moon advances in its orbitTides & Tidal Streams

A

Anchor bend: Knot specifically for securing anchor warp to the anchor ring. Tightens under load.Ropework
Anchor day shape: A single black ball displayed in the fore part of the vessel during daylight (COLREGs requirement).Anchorwork
Anchor watch: Monitoring the boat's position by transit, GPS, or depth sounder to detect dragging early.Anchorwork
Anchored vessel fog signal: Rapid bell ringing for 5 seconds every minuteRestricted Visibility
APEM: Appraise, Plan, Execute, Monitor — passage planning frameworkPassage Planning
Appraise phase: Gather weather, tides, charts, port info. Identify hazards. Assess crew.Passage Planning

B

Backing: Wind shifting anticlockwise (e.g., W → SW → S)Meteorology
Bight: A loop in a rope — never stand in one under load.Ropework
Blue tint on chart: Indicates shallow waterCharts & Publications
Bolt hole: Alternative harbour to divert to if conditions worsenPassage Planning
Bow: The front of the vesselNautical Terms
Bowline: Creates a fixed loop that doesn't slip. Easy to untie after loading.Ropework
Braided rope: Woven sheath over inner core. Flexible, runs smoothly through blocks, coil in figure-of-eight.Ropework
Breton plotter: Rotating protractor aligned to chart grid — reads bearings directlyDrawing Instruments
Bruce anchor: Claw type — good all-round, self-rights easilyAnchorwork
BST correction: UK tide tables are in UT. Add 1 hour during British Summer Time.Tides & Tidal Streams

C

CADET: Compass ADd East for True — add easterly errors going from Compass to TrueCompass
Cardinal clock mnemonic: N=12 (continuous), E=3 (3 flashes), S=6 (6+long), W=9 (9 flashes)Pilotage
Catenary: The natural sag in the anchor cable caused by its weight. Absorbs shock and keeps pull on the anchor horizontal.Anchorwork
CBDR: Constant Bearing, Decreasing Range — means collision courseCOLREGs
CDMVT: Compass → Deviation → Magnetic → Variation → True (Cadbury Dairy Milk Very Tasty)Compass
Chart datum: The reference tidal level below which depths are measured — usually Lowest Astronomical Tide (LAT) in the UK.Drawing Instruments
Chart Datum (CD): ≈ Lowest Astronomical Tide. Depths are below it, tidal heights above it.Tides & Tidal Streams
Checking the anchor holds: Take a transit of two shore objects — if they move, you're draggingAnchorwork
Clearing bearing: A compass bearing that defines a safe boundary to keep clear of a charted danger.Pilotage
Cleat hitch: Secures a rope to a cleat — most common way to make fast.Ropework
Clove hitch: Quick temporary hitch for fender lanyards to a guardrail. Can slip under intermittent load.Ropework
Cocked hat: Triangle formed by three position lines that don't meet exactly. Your position is inside it.Compass
Cocked hat: Small triangle from three bearing lines — position is inside itChartwork
Cockpit card: Waterproof card with key passage data for quick referencePassage Planning
Cold front: Sharper change, heavy rain/squalls, temp drops, vis improves afterMeteorology
Companionway: The entrance/stairway leading below deck from the cockpitNautical Terms
Compass rose — inner ring: Shows Magnetic bearings (includes local magnetic variation)Drawing Instruments
Compass rose — outer ring: Shows True bearings (aligned to True North)Drawing Instruments
Compass rose (outer ring): True North — aligned with chart gridCharts & Publications
Course to Steer (CTS): Compass course accounting for tide, leeway, and compass errors via vector triangleChartwork
CQR anchor: Plough type — reliable in sand and mudAnchorwork

D

Danforth anchor: Fluke type — excellent in sand/mud, poor in weed/rockAnchorwork
Dead Reckoning (DR): Position from course and distance only — no tide or leewayChartwork
Deviation: Compass error from the boat's own magnetic field. Different on each heading.Compass
Deviation card: Table showing compass deviation for each heading — specific to your boatCompass
Dividers: Measure distance on the chart — transfer to latitude scaleDrawing Instruments
Draft: Depth of the hull below the waterlineNautical Terms
DSC: Digital Selective Calling — sends automatic distress alert with MMSI and positionSafety

E

East cardinal: Pass to EAST. Top: ▲▼ (base-to-base). Light: 3 quick flashes. Colours: black-yellow-black.Pilotage
Echo sounder in fog: Compare depth with chart to confirm position; follow depth contoursRestricted Visibility
EPIRB: Emergency Position Indicating Radio Beacon — satellite distress alertSafety
Error East, Compass Least: If variation/deviation is East, the compass reading is LESS than the true bearingCompass
Error West, Compass Best: If variation/deviation is West, the compass reading is MORE than the true bearingCompass
Estimated Position (EP): DR + tidal stream + leeway — marked △ with timeChartwork

F

F (Fixed): A continuous, steady lightVisual Aids to Navigation
Figure-of-eight: Stopper knot — prevents rope running through a block or fairlead.Ropework
Fire triangle: Fuel + Heat + Oxygen — remove any one to extinguish a fireSafety
First action in fog: Fix your position while you can still see landmarksRestricted Visibility
Fix: Position from observations (bearings, GPS). Marked ⊙ with time.Chartwork
Fl (Flashing): Single flash, dark longer than lightVisual Aids to Navigation
Fl(3) W 10s: Group of 3 white flashes, 10 second periodVisual Aids to Navigation
Fog rule of thumb: When in doubt: SLOW DOWN or STOPRestricted Visibility
Fog signal reliability: Sound over water is unpredictable — confirms proximity, not precise positionVisual Aids to Navigation
Fog visibility: Less than 1,000 metresMeteorology
Food waste discharge rule: Only permitted >12nm offshore, ground to <25mm piecesMarine Environment
Force 6: Strong breeze (22–27 kn). Large waves, spray. Time to reef.Meteorology
Freeboard: Height of the deck above the waterlineNautical Terms

G

Gybe: To turn the stern through the wind (boom swings across)Nautical Terms

H

Halyard: A rope used to hoist a sail up the mastNautical Terms
Hand-bearing compass: Portable compass for taking bearings. Hold away from metal. Gives compass bearings.Compass
Height over drying feature: Height of Tide minus Drying Height = depth of water over itTides & Tidal Streams
High pressure (N hemisphere): Settled weather, clockwise wind circulation, widely spaced isobarsMeteorology

I

Imminent / Soon / Later: Within 6h / 6–12h / 12–24hMeteorology
INT 1 (Chart 5011): Reference book for all nautical chart symbols and abbreviationsCharts & Publications
Iso (Isophase): Equal periods of light and darkVisual Aids to Navigation
Isobars close together: Steep pressure gradient = strong windsMeteorology
Isolated Danger mark: Black with red band(s), 2 black spheres, Fl(2) white. Over a small hazard, safe water around.Pilotage

K

Kellet (angel): A weight lowered down the anchor cable to improve catenary and holding power.Anchorwork
Kill cord: Engine cut-off lanyard — attached to helmsman, stops engine if they fall overboardSafety

L

Large-scale chart: Small area, great detail — 'zoomed in' (e.g., harbour approach)Charts & Publications
Latitude: N/S of Equator. 0°–90°. Measured on the side (vertical) scale of the chart.Position, Course & Speed
Leeward: Away from the wind (pronounced 'loo-ard')Nautical Terms
Leeway: Sideways drift from wind. Typically 3–10°. Port wind → starboard drift.Chartwork
Light height datum: Heights are above Mean High Water Springs (MHWS)Visual Aids to Navigation
Light sector (white): Usually indicates the safe approach channelVisual Aids to Navigation
Longitude: E/W of Greenwich. 0°–180°. Measured on the top/bottom (horizontal) scale.Position, Course & Speed
Low pressure (N hemisphere): Unsettled weather, anticlockwise wind circulation, closely spaced isobarsMeteorology

M

MARPOL: International Convention for the Prevention of Pollution from Ships — sets global pollution standardsMarine Environment
MAYDAY: Distress call for grave and imminent danger to life or vessel (VHF Ch 16)Safety
MCZ: Marine Conservation Zone — protected area with restrictions on activitiesMarine Environment
Measuring distance on a chart: Always use the LATITUDE scale (side). Never longitude.Position, Course & Speed
Mercator projection: Chart projection where scale increases with latitude — always measure distance at the same latitude as your position.Drawing Instruments
MHWS: Mean High Water Springs — highest average tide levelTides & Tidal Streams
Minimum distance from whales: 100 metres — do not approach closerMarine Environment
MLWS: Mean Low Water Springs — lowest average tide levelTides & Tidal Streams
Monitor phase: Track progress against plan. Monitor weather. Have a Plan B.Passage Planning

N

Nautical Almanac: All-in-one reference: tide tables, lights, port info, radio detailsCharts & Publications
Nav log entry: Time, course, log reading, wind, pressure, position — at least hourlyPassage Planning
Neap tides: Sun and Moon at 90° (quarter moon). Smallest tidal range.Tides & Tidal Streams
Nominal range: Distance visible in clear conditions (met. visibility 10nm)Visual Aids to Navigation
North cardinal: Pass to NORTH. Top: ▲▲ (both up). Light: continuous quick flash. Colours: black above yellow.Pilotage
Notices to Mariners: Weekly publications for correcting nautical chartsCharts & Publications
NUC lights: Two all-round RED lights verticallyCOLREGs
Nylon (Polyamide): High elasticity, shock-absorbing. Used for mooring lines and anchor warps.Ropework

O

Oc (Occulting): Steady light with regular dark periods (light > dark)Visual Aids to Navigation
Oil in bilge: Use absorbent pads; dispose ashore. Never pump oily water overboard.Marine Environment

P

PAN PAN: Urgency call — serious situation but no immediate danger to lifeSafety
Parallel rulers: Hinged rulers that walk across the chart maintaining a constant angleDrawing Instruments
Plan phase: Plot route, waypoints, courses. Mark bolt holes. Brief crew.Passage Planning
Plastics at sea: NEVER discharge — prohibited under all circumstancesMarine Environment
Polyester (Terylene): Low stretch, UV-resistant. Used for sheets and halyards.Ropework
Polypropylene: Floats, cheap, UV-sensitive. Used for safety/heaving lines.Ropework
Port: Left side when facing forward (red)Nautical Terms
Portland plotter: Rotating protractor plotter — reads bearings from chart grid lines without walking to compass rose. Same principle as Breton plotter.Drawing Instruments
Power vessel fog signal: 1 prolonged blast every 2 minutes (making way)Restricted Visibility

Q

Q (Quick): Rapid continuous flashing (50–79 per minute)Visual Aids to Navigation

R

RAM lights: Red-White-Red all-round lights verticallyCOLREGs
Reciprocal bearing: Bearing ± 180°. Used to plot bearings from charted objects.Position, Course & Speed
Red parachute flare: Long-range distress signal — reaches 300m, visible 40+ secondsSafety
Reef knot: Ties two ends of the SAME rope (e.g., around a reefed sail). Not for joining different ropes.Ropework
Region A lateral: Port: Red, can shape, even numbers. Port side when entering from seaward.Pilotage
Region A lateral: Starboard: Green, conical shape, odd numbers. Starboard side when entering from seaward.Pilotage
Rolling hitch: Grips another rope under tension — used to take load off a jammed sheet or halyard.Ropework
Rule 12 (Sailing): Port tack gives way to starboard tack. Same tack: windward gives wayCOLREGs
Rule 13 (Overtaking): Overtaking vessel ALWAYS gives way — overrides all other rulesCOLREGs
Rule 14 (Head-on): Both power-driven vessels alter to starboard — pass port-to-portCOLREGs
Rule 15 (Crossing): Vessel with other on her starboard gives way. Give-way vessel acts early.COLREGs
Rule 18 Hierarchy: Power-driven → Sailing → Fishing → NUC → RAM (most to least burdened)COLREGs
Rule 19: Conduct in restricted visibility — replaces Rules 12–18 when not in sightRestricted Visibility
Rule 5: Look-out — maintain proper look-out by sight AND hearing at all timesCOLREGs
Rule 7: Risk of Collision — if in doubt, risk EXISTS. Use compass bearings (CBDR)COLREGs
Rule 8: Action to Avoid Collision — must be positive, early, and large enough to be obviousCOLREGs
Running rigging: Ropes actively used to control sails (halyards, sheets)Nautical Terms

S

Safe speed (Rule 6): Adapt speed to visibility so you can take effective avoiding actionRestricted Visibility
Safe Water mark: Red/white vertical stripes, 1 red sphere, long flash / Mo(A). Navigable water all around.Pilotage
Sailing vessel fog signal: 1 prolonged + 2 short blasts every 2 minutesRestricted Visibility
Sailing vessel lights: Sidelights (red/green) + stern light only — NO masthead lightCOLREGs
SART: Search and Rescue Transponder — shows on radar to guide rescuersSafety
Scope (chain): Minimum 4:1 ratio — cable length to depth (including tidal range)Anchorwork
Scope (rope + chain): Minimum 6:1 ratioAnchorwork
Seagrass beds (Zostera): Critical marine habitat — never anchor on them; damage takes years to recoverMarine Environment
Secondary port: Uses time and height differences from a standard portTides & Tidal Streams
SECURITÉ: Safety message — navigational or meteorological warningSafety
Semi-diurnal: Two high waters and two low waters per ~25 hours (UK pattern)Tides & Tidal Streams
Sheet: A rope that controls the angle of a sail to the windNautical Terms
Sheet bend: Joins two ropes of different diameter. Use double sheet bend for extra security.Ropework
Shipping Forecast times: BBC Radio 4: 0048, 0520, 1201, 1754Meteorology
Small-scale chart: Large area, less detail — 'zoomed out' (e.g., passage planning)Charts & Publications
SOG: Speed Over Ground — actual speed relative to the seabed (includes tidal effect)Chartwork
South cardinal: Pass to SOUTH. Top: ▼▼ (both down). Light: 6 quick + long flash. Colours: yellow above black.Pilotage
Special mark: All yellow, various shapes, yellow light. Exercise areas, cables, outfalls etc.Pilotage
Speed-Distance-Time: S = D ÷ T, D = S × T, T = D ÷ SPosition, Course & Speed
Spring tides: Sun and Moon aligned (new/full moon). Largest tidal range. Occur 1–2 days after.Tides & Tidal Streams
Standing rigging: Fixed wires supporting the mast (shrouds, forestay, backstay)Nautical Terms
Starboard: Right side when facing forward (green)Nautical Terms
Stern: The back of the vesselNautical Terms
Swinging circle: The area your boat can cover as it swings around the anchor with wind/tide changes. Must be clear of other vessels and shore.Anchorwork
Swinging the compass: Process of determining deviation by comparing compass readings with known bearings on multiple headingsCompass

T

Tack (verb): To turn the bow through the windNautical Terms
TBT (tributyltin): Toxic anti-fouling substance — now banned internationallyMarine Environment
Three-strand laid rope: Twisted construction — coil clockwise. Cheaper, easy to splice, can kink.Ropework
Tidal diamond: Charted point giving tidal stream set (direction) and rate (speed) hourlyTides & Tidal Streams
Tidal gate: Time window when tide allows safe passage (depth/stream)Passage Planning
Tidal range: HW height minus LW heightTides & Tidal Streams
Transit: Two fixed objects in line — gives a precise position line without instruments.Pilotage
Tripping line: Line attached to the anchor crown — used to recover a fouled anchor by pulling it out backwards.Anchorwork
True bearing format: Three figures, clockwise from True North. E.g., 005°T, 090°T, 180°T.Position, Course & Speed

U

Underlined number on chart: Drying height — height above Chart Datum that dries at low tideCharts & Publications

V

Variation: Difference between True North and Magnetic North. Found on chart compass rose.Compass
Vector triangle: Tidal stream + water track = ground track. Plot tide first, then swing speed arc.Chartwork
Veering: Wind shifting clockwise (e.g., SW → W → NW)Meteorology

W

Walking dividers: Set to a known span (e.g., 5nm), walk along the route, count spansDrawing Instruments
Warm front: Gradual deterioration, steady rain, poor vis, temp rises, wind veersMeteorology
Waypoint: Lat/Long position for navigation. Used by GPS/chart plotters.Chartwork
Weighing anchor: Motor forward while retrieving cable. Pull vertically to break free.Anchorwork
West cardinal: Pass to WEST. Top: ▼▲ (point-to-point). Light: 9 quick flashes. Colours: yellow-black-yellow.Pilotage
Whipping: Binding the end of a rope with twine to prevent fraying. Sailmaker's whipping is the most secure.Ropework
White flare: NOT a distress signal — used to indicate position and avoid collisionSafety
Why not use the longitude scale?: A minute of longitude varies in length with latitude — only the latitude scale gives consistent nautical miles.Drawing Instruments
Why use pencil?: So the chart can be erased and reused — always plot in pencilDrawing Instruments
Wind direction: Forecasts give direction wind blows FROM, not TOMeteorology
Windward: Towards the windNautical Terms

X

XTE: Cross-Track Error — how far off the intended track you areChartwork