Reference

The glossary

242 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.
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.
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.
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.

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.
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.
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.
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.

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.
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.
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 chain you lay out relative to depth — at least 4:1 with chain, so the pull 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.
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.
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 winch that hauls the anchor chain.
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.