Reference

The glossary

253 terms in plain English — every word the courses use, defined the first time you meet it. In the beginner and core courses each new term pops up a definition on hover the first time it appears in a lesson; here is the whole vocabulary in one place.

A

abeam: Out to the side of the boat, at right angles to her length.
advection fog: Sea fog made by warm moist air over cold water — it ignores sunshine and arrives in good weather.
AED: An automatic defibrillator — turn it on and it talks you through; it only shocks when a shock is right.
aft: Towards the back of the boat.
afterdrop: The further fall in core temperature when cold limb blood returns to the heart on rewarming — why you warm the core, not the limbs.
air draught: The boat’s height above the waterline — what must fit under a bridge.
AIS: Automatic Identification System — transmitting vessels broadcast identity, position and course. A clear screen is not a clear sea.
almanac: The annual reference book of tides, ports, lights and radio details.
amidships: The middle part of the boat.
anaphylaxis: Severe, fast allergic reaction — swelling, rash, breathing trouble. The casualty’s adrenaline pen, now, and the radio.
angle of vanishing stability: The heel angle beyond which the boat no longer rights herself. Far beyond sailing angles on a ballasted cruiser.
anticyclone: A high-pressure system — generally settled weather, clockwise circulation.
arc to timeadvanced: Converting longitude to time for celestial work — the Earth turns 15° an hour, so 1° = 4 minutes and 1′ = 4 seconds of time.
assumed positionadvanced: The deliberate guess near your DR from which sight-reduction arithmetic starts.
astern: Behind the boat — or, of an engine, driving the boat backwards.
azimuthadvanced: The true bearing of a celestial body — computed, it also audits your compass.

B

backing: Wind direction changing anticlockwise (e.g. W to SW).
backstay: The wire from masthead to stern, balancing the forestay.
beam reach: Sailing with the wind square across the side — usually the fastest, most comfortable point of sail.
bearing: The direction of one thing from another, in degrees.
bearing away: Turning away from the wind.
Beaufort scale: The 0–12 wind scale. F4 pleasant, F6 a yacht’s "strong", F8 a gale.
bilge: The lowest space inside the hull, where any water collects.
blind sectoradvanced: The arc your own mast or superstructure hides from your radar antenna. Know yours.
blue board: A downstream barge’s request to pass starboard-to-starboard — shown with a flashing white light.
boom: The horizontal spar along the bottom of the mainsail. Mind your head in a gybe.
bow: The front end of the boat.
bower: The main anchor, carried on the bow.
bowline: The knot that puts a fixed loop in a rope’s end — strong, and unties after load.
broachadvanced: Being slewed beam-on to the seas while running — the event heavy-weather tactics exist to avoid.
broad reach: Sailing with the wind over the back corner of the boat.

C

cable: A tenth of a nautical mile, about 185 m — handy for harbour distances.
carbon monoxide: Odourless gas from heaters and engines whose symptoms mimic seasickness. Fit an alarm; ventilate on suspicion.
cardinal marks: Black-and-yellow marks naming the safe side by compass: pass north of a north cardinal.
CBDRadvanced: Constant bearing, decreasing range — if another vessel’s compass bearing holds steady while she closes, you are on a collision course and must act.
CEVNI: The European code for inland waterways — the rulebook that replaces much of the ColRegs on rivers and canals.
chart datum: The reference level charted depths are measured from — roughly the lowest the tide ever falls (close to LAT).
chronometeradvanced: The sacred clock: never reset at sea; its error and rate are logged and applied.
clearing bearing: A pre-drawn bearing you stay one side of to keep clear of a danger.
cleat: The deck fitting ropes are made fast to: full turn round the base, then figure-eights.
close-hauled: Sailing as close to the wind direction as the boat usefully can, about 40–45° off it.
clove hitch: The quick adjustable hitch — fenders on a rail. Not for anything that matters if it slips.
clutch: A lever jammer that holds a loaded line so the winch can serve other ropes. Load the winch before opening one.
clutteradvanced: Unwanted radar returns from waves or rain — the controls that remove it can remove small boats too.
cocked hat: The small triangle where three position lines almost meet — an honest picture of your fix’s uncertainty.
cockpit: The outdoor working/steering well, usually at the back.
COG: Course over ground, from GNSS — where you are actually going, tide included.
cold front: The sharp edge of returning cold air: squalls, a marked veer, then clearing colder air.
cold shock: The first minute in cold water: involuntary gasping. Float and breathe — do nothing else.
compass rose: The printed circle of bearings on a chart, with variation noted inside.
course to steer: The heading that, after the tide has pushed you, lands you on the track you wanted.
courtesy flagadvanced: The visited country’s flag, flown at the starboard spreader — cheap diplomacy, noticed when absent.
CPAadvanced: Closest point of approach — how near a radar contact will pass if nobody changes anything.

D

dan buoy: The tall flagged float thrown to mark a person in the water.
danger line: A dotted line drawing attention to a danger, or enclosing an area unsafe to navigate.
day shapes: The black balls, cones, diamonds and cylinders vessels hoist by day to declare their status.
dead reckoning: Position worked up from course steered and distance run only.
deck log: The written hourly record of position, course and events — the backup navigator that never loses power.
declinationadvanced: The latitude of a celestial body’s ground position.
depression: A low-pressure system — the Atlantic’s travelling wind machine, circulating anticlockwise.
depth contour: A charted line joining points of equal depth — a useful invisible handrail.
deviation: Your own boat’s compass error, caused by her metal and electronics — different on each heading.
dipadvanced: The correction for your eye’s height above the sea: the visible horizon sits below the true horizontal.
dipping rangeadvanced: The distance at which a light of known height first lifts above (or dips below) the horizon — a circle of position.
doubling the angleadvanced: Run until the bow angle to an object doubles: distance off then equals distance run.
DR-ABC: The primary survey order: Danger, Response, Airway, Breathing — then act, radio call in parallel.
draught: How deep the boat reaches below the waterline — the depth she needs to float.
dredged channel: A channel maintained to a stated depth by dredging, charted with its dredged depth.
drogueadvanced: A towed drag device that slows a running boat and holds her stern square to following seas.
drying height: How far a bank or rock that covers and uncovers stands ABOVE chart datum (underlined on the chart).
DSC: Digital Selective Calling — the radio’s data "doorbell", including the red distress button.

E

echo sounder: The instrument that measures depth — check WHERE its zero is set (keel, transducer or waterline).
ED: Existence Doubtful — a charted danger that may not be there; give it the benefit of the doubt.
ensignadvanced: Your vessel’s national flag, worn at or near the stern.
EPIRB: The boat’s registered satellite distress beacon — floats free and transmits your identity and position for days.
equation of timeadvanced: The few minutes by which the real sun runs fast or slow of clock noon through the year.
estimated position: Dead reckoning improved with the tide’s push and leeway — the best position without a fix.

F

fairway: The main navigable channel of a harbour or river.
fender: The inflatable cushion hung between hull and berth.
fire triangle: The three things every fire needs at once — heat, fuel and oxygen. Remove any one and it goes out.
First Point of Ariesadvanced: The sky’s own Greenwich: the reference point star positions are measured from.
Fl: Flashing — a light on for less time than it is off (e.g. Fl.R.4s).
forestay: The wire from masthead to bow; the headsail flies from it.
Foul: Foul ground — safe to sail over but never to anchor or trawl in; wreckage or debris below.
freeboard: The height of the deck edge above the waterline — how far the hull stands out of the sea. It is the extra you add to draught and water depth when working out how much rode the bow roller is above the seabed.

G

galley: The boat’s kitchen.
geographical positionadvanced: The point on earth directly beneath a celestial body at an instant. Abbreviated GP.
GHAadvanced: Greenwich hour angle — how far west of Greenwich a body’s ground position lies.
give-way vessel: The vessel required to keep out of the way — early, boldly and visibly.
GMadvanced: Metacentric height — the height of the metacentre above the centre of gravity, a measure of initial stiffness: a big GM is stiff and snappy, a small one tender and easy.
GMDSS: The worldwide rescue system knitting radios, satellites and coast stations together.
GNSS: Satellite navigation in general — GPS and its siblings.
grab bag: The pre-packed abandon-ship bag: beacon, radio, flares, water, tablets, papers.
great circleadvanced: The genuinely shortest track across the globe — it bows poleward of the straight chart line.
GRIB: Raw computer-model weather data files — useful, but not a human forecast.
ground track: The path actually made over the seabed, tide included.
gybe: Turning the stern through the wind — the boom crosses fast, so it needs respect.
GZadvanced: The righting arm — the lever between a heeled boat’s buoyancy and her weight that rights her; it grows, peaks, then falls to zero at the angle of vanishing stability.

H

halyard: A rope that hoists a sail up the mast.
handbearing compass: A small hand-held compass for taking bearings of objects.
HAT: Highest Astronomical Tide — the highest the tide can ever reach from sun and moon alone (no weather). Overhead clearances — bridges, power cables — are measured DOWN from HAT, the worst case, so you always have at least the charted clearance.
HDOPadvanced: Horizontal dilution of precision — how satellite geometry sharpens a GNSS fix: satellites spread round the sky give a low HDOP (sharp), bunched ones a high HDOP (smeared).
headsail: The sail flown from the forestay at the front — a jib, or a bigger genoa.
heaving-toadvanced: Parking under sail: headsail backed, helm lashed, the boat lying quietly 40-odd degrees off the wind.
heeling: The boat leaning over under wind pressure.
helm: The steering position (tiller or wheel) — or the person steering.
HELP position: Heat Escape Lessening Posture: knees up, arms hugged in, protecting groin and armpits in cold water.
high water: The moment (and height) of the tide’s peak. Abbreviated HW.
HRU: Hydrostatic release unit — a pressure-triggered link that frees a stowed liferaft as the boat sinks, so it floats up, inflates and is not dragged under.
hull: The watertight body of the boat.
hypothermia: Dangerous cooling of the body’s core. Handle casualties gently, horizontally, rewarm slowly.

I

IALA: The international buoyage authority. Region A (Europe and most of the world): red to port entering harbour.
ICC: The International Certificate of Competence — the licence many European waters ask of visiting skippers.
index erroradvanced: A sextant’s own zero offset — measured against the horizon, recorded, and applied to every sight.
interceptadvanced: The miles between the altitude you measured and the altitude your assumed position predicts — towards or away from the body.
IRPCS: The international collision regulations — the rules of the road at sea. Also called the ColRegs.
Iso: Isophase — equal periods of light and dark.
isobars: Lines of equal pressure on a weather chart — close together means wind.
isolated danger mark: Black with red bands, two black balls on top: a small danger with safe water all round.

J

jackstays: The webbing lines along the deck you clip a harness tether to.

K

katabatic wind: Cold air draining suddenly downhill from high ground — the mountain-anchorage surprise.
kedge: The lighter second anchor.
keel: The fin under the hull that resists sideways slide and carries ballast to keep her upright.
kill cord: The lanyard that stops an outboard dead if the driver goes overboard. Worn, not draped.

L

land breeze: The sea breeze’s gentle night-time opposite: air drifting offshore as the land cools.
LAT: Lowest Astronomical Tide — the lowest the tide can ever fall from sun and moon alone. Chart datum is set at or near it.
lateral marks: The channel-edge buoys: red cans to port, green cones to starboard, entering from seaward (Region A).
latitude: Position north or south of the equator, in degrees and minutes. One minute = one nautical mile.
leading lights: Two lights that line up, one above the other, when you are on the safe approach line.
lee shore: A shore the wind is blowing you onto — the dangerous one to be near.
lee-bowingadvanced: Tacking so the stream pushes on the lee bow, lifting you to windward — the tide doing your beating for you.
leeward: The side or direction the wind is blowing TOWARDS — away from the wind.
leeway: The sideways slide a boat makes downwind of the course she is steering.
LFl: Long-flashing — a single flash of two seconds or more each period; safe-water marks use it.
LHAadvanced: Local hour angle — the body’s GHA adjusted for YOUR longitude.
liferaft: The inflatable last resort. Step UP into it — leave only a boat that is truly sinking.
light characteristic: A light’s signature rhythm, colour and period — e.g. Fl(2)G.10s — by which you identify it at night.
longitude: Position east or west of the Greenwich meridian, in degrees and minutes.
low water: The moment (and height) of the tide’s trough. Abbreviated LW.
luffing: Turning towards the wind; a sail "luffs" when its front edge trembles.

M

mainsail: The big sail set on the mast and boom.
MARPAadvanced: The radar’s automatic target-plotting aid — it computes CPA for you; the eye still checks it.
mast: The vertical spar that carries the sails.
masthead light: The white forward-facing light a power-driven vessel adds — sail carries none.
Mayday: The distress call: grave and imminent danger to vessel or life. Outranks everything on the air.
meridian: A north–south line of longitude on the chart.
meridian passageadvanced: The body’s daily crossing of your meridian — for the sun, local noon, when latitude falls out of one subtraction.
MHWN: Mean High Water Neaps — the average height of high water at neap tides (lower than springs).
MHWS: Mean High Water Springs — the average height of high water at spring tides. Heights of land features and lighthouse elevations on the chart are measured UP from here.
MMSI: Your radio’s nine-digit identity number, from the ship’s radio licence.
Mo(A): Morse “A” (· —) flashed by a light — used by safe-water marks and some landfall lights.
motor-sailing: Sails up with the engine driving — legally a power vessel, cone point-down by day.

N

nautical mile: 1,852 m — one minute of latitude. The sea’s unit of distance.
navigable semicircleadvanced: The less-deadly half of a tropical storm — the side to be on if avoidance has failed.
Navtex: The receiver that prints scheduled marine safety information — forecasts and warnings — offshore.
neaps: The fortnightly small tides after the half moons — the gentle half of the cycle.
no-go zone: The arc either side of the wind direction where sails stall and the boat cannot sail.

O

Obstn: Obstruction — something dangerous on the bottom whose nature is unknown or unspecified.
Oc: Occulting — a light on for more time than it is off; the opposite of flashing.
occluded front: The cold front having caught the warm front — both compressed into one band of cloud and rain.
OSR: The World Sailing Offshore Special Regulations — the public standard for offshore equipment, training and the categories 0–4 that grade how far offshore you go.

P

PA: Position Approximate — the charted position is not accurately determined.
painter: The bow rope of a dinghy or liferaft.
Pan-Pan: The urgency call: safety at risk, but no immediate danger to life.
parallel indexingadvanced: Keeping a radar echo sliding along a pre-drawn line to hold a safe track in poor visibility.
PD: Position Doubtful — reported in differing positions; treat the whole area with suspicion.
pilot book: A guide book of harbours and coastlines with local directions.
pilotage: Eyeball navigation in confined waters, from a prepared plan of marks, bearings and depths.
PLB: A pocket-sized personal satellite distress beacon, registered to a person.
plotting sheetadvanced: A blank latitude grid you scale for your latitude — mid-ocean chartwork paper.
point of no returnadvanced: The moment on a passage when turning back stops being the cheaper option.
port: The left-hand side of the boat when facing forward. Marked red.
position line: A line you are known to be somewhere along — one bearing, one transit, one depth contour.
post-rescue collapse: Cardiac collapse during or just after lifting a cold casualty from the water — the reason to recover them horizontal and gently.
preferred channel markadvanced: A banded lateral mark at a fork showing which side the main channel lies; light Fl(2+1).
prolonged blast: A 4–6 second sound signal — the fog and "attention" blast.
prop walk: The sideways kick a turning propeller gives the stern, strongest going astern — a right-handed prop walks the stern to port.

Q

Q: Quick-flashing — roughly 50–60 flashes a minute; cardinal marks use Q and VQ.
Q flagadvanced: The plain yellow flag: “my vessel is healthy and I request clearance” — flown arriving where customs formalities apply.

R

raconadvanced: A radar beacon on a mark that answers your pulse with a Morse letter painted on screen.
radiation fog: Land fog from clear calm nights; slides down rivers at dawn and burns off in sun — unlike sea fog.
recovery position: An unresponsive but breathing casualty on their side, airway draining downhill, wedged against the boat’s roll.
reefing: Reducing sail area so the boat copes better in stronger wind. Done early, it is easy.
refractionadvanced: The atmosphere bending light upward, making everything read high — worst near the horizon.
restricted visibility: Fog, mist, heavy rain — conditions where nobody is stand-on and everyone slows down.
rhumb lineadvanced: The constant-heading track — the straight line on a Mercator chart, slightly longer than the great circle.
riding turn: A rope turn jammed over another on a winch drum — prevented by feeding the rope fair, cleared with care.
righting moment: The leverage of keel ballast that pulls a heeled boat back upright — growing with heel, up to a point.
rode: The whole length of chain and/or warp that connects the boat to her anchor.
rolling hitch: The knot that grips along a loaded rope — used to take the strain off a jammed sheet.
rudder: The underwater blade, turned by tiller or wheel, that steers the boat.
rule of twelfths: Quick mental model of the tide’s rise: 1, 2, 3, 3, 2, 1 twelfths of the range per hour.
running fixadvanced: A fix from ONE object: a bearing, a measured run, a second bearing, and the first line transferred along the track.

S

safe water mark: Red-and-white striped mark: safe water all round — often a landfall or mid-channel mark.
SART: A liferaft transponder that paints a distinctive trail on rescuers’ radar or AIS for the final miles.
scope: How much anchor rode you lay out relative to depth, measured from the bow roller — at least 4:1 all-chain, or 6:1 for a chain-and-warp mix, so the pull on the anchor stays horizontal.
SD: Sounding Doubtful — the charted depth is unreliable.
sea anchoradvanced: A parachute-like drag streamed from the bow to hold her head to the seas.
sea breeze: The afternoon onshore wind a sunny coast brews for itself.
sea state: The defined wave-height vocabulary: smooth, slight, moderate, rough, very rough, high… each a numeric band.
secondary port: A port whose tides are given as differences from a standard port.
secondary survey: The calm head-to-toe check for the injuries the obvious one hid, once the casualty is stable.
sector light: A shore light showing different colours in different directions — white usually marks the safe approach.
Sécurité: The safety call: navigational or weather information for all vessels.
semi-diameteradvanced: Half the sun’s width (~16′): added back when you measured to its lower edge.
SHAadvanced: Sidereal hour angle — a star’s fixed offset west of Aries. GHA★ = GHA Aries + SHA.
sheets: The ropes that trim the sails in and out.
shrouds: The wires holding the mast up from the sides.
sidelights: Red (port) and green (starboard) lights showing from ahead to just abaft the beam.
slack water: The short pause when the stream barely runs, around its turn.
sloop: The standard modern rig: one mast, one headsail.
SOG: Speed over ground, from GNSS — tide included, unlike the log.
soundings: The charted depth figures, measured below chart datum — the depth of water at the lowest tides, so there is almost always more than the chart shows.
special marks: Yellow marks for non-navigational things — check the chart for what.
spring tides: The fortnightly big tides after full and new moon — highest highs, lowest lows, fastest streams.
springs: The diagonal mooring lines that stop a berthed boat surging fore-and-aft — and lever her out when leaving.
stand-on vessel: The vessel that holds course and speed — until it is clear the other is not acting.
standard port: A port with full tide tables and its own tidal curve.
starboard: The right-hand side of the boat when facing forward. Marked green.
steady bearing: A converging vessel whose bearing does not change while range closes — you are going to meet.
stern: The back end of the boat.
sternlight: The white light showing astern.
synoptic chart: The pressure map of highs, lows and fronts that explains the forecast.

T

tacking: Turning the bow through the wind so the sails fill on the other side.
TCPAadvanced: Time to closest point of approach.
tidal diamond: A lettered magenta diamond on the chart keyed to a table of stream direction and rate, hour by hour.
tidal gate: A place or time the tide controls — a bar, a sill, a race — passable only in its window.
tidal range: The height difference between a high water and the next low water.
tidal standadvanced: A long flat-topped high water (Solent-style) where local geography stretches the curve — special local curves apply.
tidal stream: The horizontal flow of the tide — the moving water that carries the boat sideways.
tidal stream atlas: A booklet of hourly chartlets showing the streams across a whole area.
tiller: The steering arm fixed to the rudder. Push it away from where you want the bow to go.
topmark: The shape on top of a buoy (cones, spheres, ✕) that encodes its meaning.
tourniquet: The last-resort band for catastrophic limb bleeding — applied high, time written down, never covered.
traffic separation scheme: One-way shipping lanes (magenta on the chart). Cross on a heading at right angles; do not impede.
transferred position lineadvanced: A position line slid forward along your track to be crossed with a later one.
transit: Two fixed objects seen exactly in line — a position line of perfect accuracy, no compass needed.
tripping line: A light line from the anchor’s crown to a small buoy — pulls a fouled anchor out backwards.
TRSadvanced: Tropical revolving storm — hurricane, cyclone, typhoon. Avoided by calendar and routeing first.

U

under-keel clearance: The water left beneath the keel: charted depth plus height of tide, less your draught and a safety margin.

V

variation: The angle between true north and magnetic north, printed in the chart’s compass rose. Changes with place and year.
veering: Wind direction changing clockwise (e.g. SW to W).
VQ: Very-quick-flashing — roughly 100–120 flashes a minute.

W

warm front: The leading edge of warm air: thickening cloud, steady rain, wind veer as it passes.
warp: A rope used as part of the ground tackle — an anchor rode of rope (often with a length of chain at the anchor) or a mooring line. Rope warp stretches to absorb snatch loads where all-chain would jerk. (British usage; US sailors more often call the rope rode the anchor “line”.)
water track: The path made through the water — the heading line, before the tide moves the water itself.
waypoint: A chosen position a GNSS set guides you to — picked off dangers, never on top of a busy mark.
winch: The geared drum that multiplies your pull on a loaded rope. Turns go on clockwise.
windlass: The deck winch — hand or electric — that hauls the anchor chain and breaks the anchor out of the ground.
windward: The side or direction the wind is coming FROM.
Wk: Wreck — the abbreviation beside a wreck symbol on the chart.

X

XTE: Cross-track error — how far you sit left or right of the straight line between waypoints.

Z

zenith distanceadvanced: 90° minus the corrected altitude — your distance from the body’s ground position, in degrees.

Meet the terms in context in the courses— or ask the tutor on any lesson.