Sailing Glossary

288 terms and definitions from the RYA Coastal 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

Admiralty List of Lights: ALRS Volumes A–F — the definitive publication listing all navigational lights worldwide, kept corrected from Notices to MarinersVisual Aids to Navigation
Admiralty Sailing Directions (Pilots): Volumes describing coastlines, harbours, approaches, hazards, and local conditions in detail — essential for passage planningCharts & Publications
AIS (Automatic Identification System): Broadcasts vessel identity, position (GPS-derived), course, and speed. Invaluable for traffic awareness but not all vessels carry it. AIS positions share GPS vulnerabilities. Supplement with radar and visual lookout.Position, Course & Speed
AIS Class A vs Class B: Class A: mandatory on commercial vessels > 300 GT, higher power, more frequent updates. Class B: common on leisure craft, lower power, less frequent updatesRestricted Visibility
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
ARPA: Automatic Radar Plotting Aid — automatically tracks contacts and calculates their course, speed, CPA, and TCPARestricted Visibility

B

Backing: Wind shifting anticlockwise (e.g., W → SW → S)Meteorology
Bahamian moor: Two anchors set in opposite directions (up-tide and down-tide) so the boat swings in a narrow arc. Ideal for narrow channels with limited swinging room.Anchorwork
Bight: A loop in a rope — never stand in one under load.Ropework
Blue tint on chart: Indicates shallow waterCharts & Publications
Boat Safety Scheme awareness: Fuel, gas, electrical, ventilation, fire, escape, and carbon monoxide safety principles; supplements coastal seaworthiness checks.Safety
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
Breaking out a fouled anchor: Motor in a circle to change pull direction, shorten cable and use boat's buoyancy, or pull via a tripping line. Last resort: slip (abandon) the anchor.Anchorwork
Breton plotter: Rotating protractor aligned to chart grid — reads bearings directlyDrawing Instruments
Bridge clearance datum: Heights of bridges/cables measured above MHWS on UK chartsTides & Tidal Streams
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
Buys Ballot’s Law: Back to wind, low is to your left (N hemisphere). Slightly ahead-left due to friction.Meteorology

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
CBD cylinder: A vessel constrained by her draught displays a cylinder by day. By night: three all-round red lights vertically plus normal power-driven vessel lights.COLREGs
CBDR: Constant Bearing, Decreasing Range — means collision courseCOLREGs
CDMVT: Compass → Deviation → Magnetic → Variation → True (Cadbury Dairy Milk Very Tasty)Compass
Centre of gravity (G): The point through which a vessel's total weight acts downward — keeping G low (heavy items in the bilge) improves stabilitySafety
Chafe protection: Use chafe guards, leather wraps, or sacrificial tubing at fairleads, bow rollers, and dock edges. Reposition lines periodically to distribute wear.Ropework
Chart datum: The reference tidal level below which depths are measured — usually Lowest Astronomical Tide (LAT) in the UK.Drawing Instruments
Chart datum — WGS84: The horizontal reference system used by GPS. Older charts may use a different datum, causing positional errors if not correctedCharts & Publications
Chart Datum (CD): ≈ Lowest Astronomical Tide. Depths are below it, tidal heights above it.Tides & Tidal Streams
Chart housekeeping: Erase old workings before a new passage. Use a soft eraser with gentle pressure to protect printed detail. Keep the chart clean and reusable.Drawing Instruments
Chart labelling conventions: Courses: three-figure True bearing along the line (e.g., 045°T) with speed. Position lines: bearing at the end nearest the object. Times: four-figure 24-hour format (e.g., 1435).Drawing Instruments
Checking the anchor holds: Take a transit of two shore objects — if they move, you're draggingAnchorwork
Clearing bearing: Pre-plotted bearing defining a safe boundary — stay on the safe side to avoid a charted dangerChartwork
Clearing bearing: A compass bearing that defines a safe boundary to keep clear of a charted danger.Pilotage
Clearing bearing vs leading marks: Leading marks guide you ALONG a safe channel (steer to keep in line). Clearing bearings define the BOUNDARY of a danger (keep bearing NLT or NMT a value).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
COG vs Heading: COG (Course Over Ground) is the actual direction of movement across the seabed, derived from GPS. Heading is the direction the bow points, from the compass. They differ due to tidal stream, current, and leeway.Position, Course & Speed
Cold front: Sharper change, heavy rain/squalls, temp drops, vis improves afterMeteorology
Companionway: The entrance/stairway leading below deck from the cockpitNautical Terms
Compass check underway: In calm conditions with no cross-tide, compare compass heading (converted to true via CDMVT) with GPS COG. A consistent discrepancy means the deviation card needs updating.Compass
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
CPA / TCPA: Closest Point of Approach / Time to CPA — the nearest a contact will pass and when. CPA < 0.5 NM in open water warrants actionRestricted Visibility
CQR anchor: Plough type — reliable in sand and mudAnchorwork
Crew fatigue: A major safety risk on passage. Managed by structured watch rotation, enforced rest, shorter night watches, hot food and drinks, and the skipper monitoring crew alertness.Passage Planning
Crew overboard procedure: Shout 'Man Overboard', throw lifebuoy and dan buoy, designate a pointer, press MOB on GPS, then begin recovery manoeuvreSafety
Customs and local regulations: Check current official customs, immigration, reporting, harbour, TSS, firing range, and conservation requirements before departure.Passage Planning

D

Danforth anchor: Fluke type — excellent in sand/mud, poor in weed/rockAnchorwork
Day shapes — anchored / NUC / RAM / CBD: Anchored: ball. NUC: two balls. RAM: ball-diamond-ball. CBD: cylinder.COLREGs
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 boat. Must be re-swung after any change to equipment near the compass.Compass
Dividers: Measure distance on the chart — transfer to latitude scaleDrawing Instruments
Doubling the angle on the bow: Running-fix shortcut: when relative bearing doubles, distance off = distance run between observationsChartwork
Draft: Depth of the hull below the waterlineNautical Terms
Drogue: Cone-shaped device trailed from the stern in heavy weather to slow the boat and keep the stern to the waves, preventing broachingSafety
DSC: Digital Selective Calling — sends automatic distress alert with MMSI and positionSafety
Dyneema/Spectra: Ultra-high-strength, very low stretch HMPE fibre. Best for high-performance halyards and sheets. Slippery — requires spliced terminations.Ropework

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
Echo sounder offset: Know whether depth is below transducer, below keel, or below waterline before comparing with charted depth plus tide.Position, Course & Speed
ENC (Electronic Navigational Chart): Vector-based digital chart produced to S-57/S-101 standard — can be zoomed, queried, and customised unlike raster chartsCharts & Publications
EPIRB: Emergency Position Indicating Radio Beacon — satellite distress alertSafety
Equinoctial springs: Largest spring tides of the year, near March and September equinoxesTides & Tidal Streams
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
Fetch: The unobstructed distance over water that the wind has been blowing — longer fetch produces larger wavesNautical Terms
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
Fluxgate compass: Electronic compass that senses the Earth's magnetic field digitally. Feeds heading to instruments/autopilot. Still reads magnetic north and needs calibration, but can be mounted low in the hull.Compass
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 strategy sequence: Fix position → reduce speed → lights ON → fog signals → post lookout → monitor radar/AIS/depth → consider anchoringRestricted Visibility
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
Free surface effect: Liquid in partially filled tanks sloshing to the low side as the boat heels, effectively raising the centre of gravity and reducing stabilitySafety
Freeboard: Height of the deck above the waterlineNautical Terms

G

GDOP (Geometric Dilution of Precision): An overall DOP value considering all dimensions (latitude, longitude, altitude, and time). High GDOP means poor satellite geometry and unreliable positioning across all axes.Position, Course & Speed
Geographical range: Maximum distance a light can be seen based on the curvature of the earth, height of the light, and observer's height of eyeVisual Aids to Navigation
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
HDOP: Horizontal Dilution of Precision — GPS accuracy measure based on satellite geometry. Low = good.Chartwork
HDOP (Horizontal Dilution of Precision): A measure of satellite geometry quality in the horizontal plane. HDOP near 1.0 = excellent fix. Above 4 = degraded accuracy. It multiplies the base ranging error to give actual position uncertainty.Position, Course & Speed
Heaving-to: Heavy weather tactic: back the jib, deeply reef the main, lash helm to leeward — boat settles 50–60° to wind and drifts slowly to leewardSafety
Heeling error: Additional compass deviation caused by the boat heeling — ferrous masses shift relative to the compass. Worst on N/S headings. Corrected with a vertical magnet in the binnacle.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
HM Coastguard: Coordinates UK maritime search and rescue and broadcasts MSI; call early with clear identity, position, problem, POB, and assistance required.Safety

I

Imminent / Soon / Later: Within 6h / 6–12h / 12–24hMeteorology
INT 1 (Chart 5011): Reference book for all nautical chart symbols and abbreviationsCharts & Publications
Inverted barometer effect: Low pressure raises sea level (~1 cm per 1 mb below 1013 mb)Tides & Tidal Streams
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

Katabatic wind: Cold dense air draining downhill from high ground. Can be strong and gusty.Meteorology
Kedging: Using an anchor to move the boat — row the kedge out by dinghy, drop it, and haul the boat towards it. Used to free a grounding or move against an adverse tide.Anchorwork
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

Land breeze: Night-time: land cools → air flows offshore. Weaker (F1–2).Meteorology
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
Leading marks: Two marks aligned in transit that define a safe channel. Steer to keep them in line — if they separate, you are off track.Pilotage
Lee shore: A shore onto which the wind blows — dangerous because the vessel is driven towards itNautical Terms
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: An arc over which a light shows a particular colour. White = safe approach; red/green = straying towards danger.Pilotage
Light sector (white): Usually indicates the safe approach channelVisual Aids to Navigation
Light sectors: Coloured arcs (W, R, G) radiating from a light that guide vessels on safe tracks; crossing a boundary changes the observed colourVisual Aids to Navigation
Log speed vs SOG: Log usually measures speed through water; GPS SOG is over ground and includes tide/current. Calibrate and cross-check.Position, Course & Speed
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
Luminous range: Distance at which a light is visible based on its intensity adjusted for actual atmospheric visibility (derived from luminous range diagrams)Visual Aids to Navigation

M

Making good: The actual course or speed achieved over the ground after accounting for tide and leewayNautical Terms
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
Motor-sailing cone: Cone with apex pointing downward, displayed forward. Indicates vessel under sail AND engine. Vessels under 12m are exempt.COLREGs
Multi-hour CTS: Stack hourly tidal vectors end-to-end, swing total distance arc from cumulative end to get one averaged courseChartwork
Multipath error (GPS): Occurs when satellite signals reflect off nearby surfaces (cliffs, buildings, large vessels) before reaching the antenna. The longer path causes the receiver to overestimate range, corrupting the position fix. Worst in harbours and confined waters.Position, Course & Speed

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
NAVTEX: Automatic text MSI broadcast on 518 kHz. Weather, gale warnings, nav warnings.Meteorology
Neap tides: Sun and Moon at 90° (quarter moon). Smallest tidal range.Tides & Tidal Streams
Night vision: Takes 20–30 min to develop; destroyed instantly by white light. Use red lighting to preserve it. Essential for spotting unlit marks and judging distance at night.Pilotage
Nominal range: Distance visible in clear conditions (met. visibility 10nm)Visual Aids to Navigation
Nominal range: Based on light intensity alone, assuming meteorological visibility of 10 nautical miles — the range printed on chartsVisual 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
Notices to Mariners (NMs): Weekly Admiralty publications listing corrections to charts — permanent changes in violet ink, T&P notices in pencilCharts & Publications
NP5011 (INT1): The international reference for all chart symbols and abbreviations — the UK edition of INT1 published by the UKHOCharts & 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
Parallax error: Occurs when your eye is not directly above the ruler edge, making it appear aligned with a different line. Avoid by looking straight down at the chart.Drawing Instruments
Parallel rulers: Hinged rulers that walk across the chart maintaining a constant angleDrawing Instruments
Pilot book (sailing directions): Contains harbour approach details: leading lights, dangers, depths, tidal streams, anchorages, port signals, and local regulations. Supplements the chart.Pilotage
Pilot vessel lights: White over red all-round lights at or near the masthead, plus sidelights and stern light when making way.COLREGs
Pilotage plan: Detailed plan for navigating confined or complex waters (harbour approaches, rivers, channels). Includes clearing bearings, transits, depths, turning points — distinct from the broader passage plan.Passage Planning
Pilotage plan: A step-by-step approach guide prepared in advance: courses, transits, clearing bearings, depths, lights, VHF, tides, and contingencies — written on a cockpit card.Pilotage
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
Portland plotter advantage: Reads bearings directly from chart grid lines — no walking to compass rose. Faster and less prone to slipping, ideal under exam pressure or on a moving boat.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

Radar fix: A position fix obtained using radar ranges and/or bearings to charted features. Radar ranges (using VRMs) are more accurate than bearings. Two or more well-separated ranges give a reliable fix, especially in poor visibility.Position, Course & Speed
Radar reflector: Passive device hoisted aloft to increase a vessel's radar cross-section, making it visible to other vessels' radar at greater rangeRestricted Visibility
RAIM (Receiver Autonomous Integrity Monitoring): A GPS feature that cross-checks redundant satellite measurements to detect faulty signals. Requires ≥ 5 satellites to detect a fault, ≥ 6 to exclude it. Provides an integrity warning to the navigator.Position, Course & Speed
RAM lights: Red-White-Red all-round lights verticallyCOLREGs
Rapid barometer fall: >5 mb in 3 hours = severe weather approaching fastMeteorology
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
Rising/dipping distance: The geographical range at which a light appears above or disappears below the horizon — provides a reliable distance-off position lineVisual Aids to Navigation
Rolling hitch: Grips another rope under tension — used to take load off a jammed sheet or halyard.Ropework
Rope inspection: Check at start and end of every passage. Look for glazed fibres, flattened sections, broken filaments, and UV discolouration at chafe points.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 19 (Restricted Visibility): Applies when NOT in sight of each other. Safe speed, use radar. Do NOT alter to port for vessel forward of beam. Do NOT alter toward vessel abeam or abaft beam.COLREGs
Rule 2 (Responsibility): No vessel is exempt from consequences of neglecting the rules or ordinary practice of seamen. Departure from rules is required when necessary to avoid immediate danger.COLREGs
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
Rule of thirds (fuel): Divide total fuel into thirds: one third for the passage, one third for return/next leg, one third as reserve for adverse conditions, diversions, or emergencies.Passage Planning
Running fix: Fix from two bearings of one object at different times, transferring the earlier position line forward by DR/EP movementChartwork
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
Sea breeze: Daytime: land heats → air rises → sea air flows in. F3–4, peaks mid-afternoon.Meteorology
Sea clutter (radar): Wave reflections near your vessel that create noise on the radar display, potentially masking small targets at close rangeRestricted Visibility
Sea fog: Usually warm moist air cooled over colder sea; watch dew point spread, low stratus, and nearby visibility reports.Meteorology
Seagrass and anchor wash: Never use anchor wash over seagrass — the powerful water jet scours the seabed and destroys fragile root systemsMarine Environment
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
Sewage discharge in UK waters: No untreated sewage discharge within 3nm of land. Use holding tanks inshore and pump out at shore facilities.Marine Environment
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
Shore contact / CG66: A responsible person ashore who holds your vessel details, crew list, passage plan, and check-in schedule. The CG66 form can also be filed with HM Coastguard. They raise the alarm if you fail to report.Passage Planning
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
Splice advantage: Retains 90–95% of rope breaking strength vs 50–70% for a knot. Streamlined, runs smoothly through blocks and clutches.Ropework
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 motoring on known headings and comparing compass readings with a transit or charted reference bearing. Produces the deviation card.Compass

T

T&P Notice: Temporary or Preliminary notice — applied in pencil on charts and erased when cancelled or supersededCharts & Publications
Tack (verb): To turn the bow through the windNautical Terms
Tandem anchoring: Two anchors on the same cable, one ahead of the other. Roughly doubles holding power in a single direction — used in strong conditions.Anchorwork
TBT (tributyltin): Toxic anti-fouling substance — now banned internationallyMarine Environment
The Green Blue: Joint programme by British Marine and the RYA promoting environmentally responsible boating — best-practice guidance at thegreenblue.org.ukMarine Environment
Three-strand laid rope: Twisted construction — coil clockwise. Cheaper, easy to splice, can kink.Ropework
Tidal anomaly: Local effect such as double high water, tidal stand, bore, or unusual stream pattern; use almanac notes and local curves.Tides & Tidal Streams
Tidal diamond: Charted point giving tidal stream set (direction) and rate (speed) hourlyTides & Tidal Streams
Tidal gate: A point or passage where tidal streams are so strong it can only be safely navigated at certain states of the tideNautical Terms
Tidal gate: Passage point where stream is strong enough that timing is criticalTides & Tidal Streams
Tidal gate: Time window when tide allows safe passage (depth/stream)Passage Planning
Tidal observation: Live height or stream data from a buoy, beacon, gauge, or current meter; compare with predictions and check datum/update time.Tides & Tidal Streams
Tidal range: HW height minus LW heightTides & Tidal Streams
Transferred position line: A position line advanced by the vessel's course and distance made good (including tide) to combine with a later observationChartwork
Transit: Two charted objects in line giving an exact position line without needing a compassNautical Terms
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
Tropical maritime air: Warm moist air; often cloudy, drizzle or rain, and moderate-poor visibility around UK coasts.Meteorology
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
VMCA: Voluntary Marine Conservation Area — ecologically sensitive site where best practice is to minimise impact, though restrictions are voluntary not statutoryMarine Environment

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
Warm front cloud sequence: Ci → Cs → As → Ns (high cloud to low, then rain)Meteorology
Warm sector: Between warm and cold fronts. Overcast, drizzle, moderate-poor visibility.Meteorology
Watch system: Organised rotation of crew for on-duty and off-duty periods. Common patterns: 4-on/4-off or 3-watch rotation. Essential on passages over a few hours to manage fatigue.Passage Planning
Waypoint: Lat/Long position for navigation. Used by GPS/chart plotters.Chartwork
Weather shore: A shore from which the wind blows, providing sheltered water in its leeNautical Terms
Weather window: A period when forecast wind, sea state, and visibility are within safe limits for the vessel and crew for the full duration of the planned passage.Passage Planning
Weatherfax / satellite: Adds synoptic charts and observed cloud evidence; check issue/valid time and compare with official forecasts.Meteorology
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
Williamson turn: Recovery manoeuvre when a crew overboard casualty is lost from sight — turn 60° off course then reverse helm to reciprocal heading, returning along the original trackSafety
Wind against tide: Creates steep, short, breaking waves — dangerous at headlands and racesTides & Tidal Streams
Wind direction: Forecasts give direction wind blows FROM, not TOMeteorology
Windward: Towards the windNautical Terms

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