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Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.

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Meteorology for Navigators
Warm Front
Warm air sliding up over colder air ahead of a depression
Marked on charts by red semicircles pointing in the direction of travel. Its shallow slope, around 1 in 150, produces a long lead of cloud: cirrus, then cirrostratus, altostratus and finally nimbostratus with continuous rain over many hours. Pressure falls steadily, wind backs and freshens ahead of it, then veers as it passes, and visibility often deteriorates markedly in the warm sector behind, sometimes to sea fog.
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Passage Planning
Watch System and Crew Briefing
Arranging who is on watch, and briefing everyone beforehand
Planning includes the human element: a watch rota giving adequate rest for the length of passage, clear allocation of tasks, and a briefing so that every crew member knows the route, the main hazards, the contingency ports, the stowage of safety equipment and the drill for man overboard, fire or engine failure. Fatigue is the commonest root of poor decisions offshore, so the rota should be fixed before departure rather than improvised at sea.
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Dead Reckoning & Estimated Position

Water Track (Course Through the Water)

The path the boat cuts through the water once leeway is included

The water track is the course steered corrected for leeway, representing the vessel's actual motion relative to the water mass. It is the first leg drawn in an EP construction, its length set by the distance logged, and it carries a single arrowhead. Under sail on a beat this may lie 5 to 10 degrees downwind of the heading. Adding the tidal stream vector to the end of the water track produces the ground track.

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Wind, Waves & Sea State

Wave Period (T)

The time between successive wave crests passing a fixed point

Measured in seconds, and the key to wave speed and length in deep water. Wavelength in metres is approximately 1.56 times the period squared, and wave speed in knots is roughly 3 times the period. A 10 second swell therefore has a wavelength near 156 metres and travels at about 30 knots. Short periods relative to height mean a steep, uncomfortable sea; long periods mean an easy, rolling one.

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Wind, Waves & Sea State

Wave Refraction and Shoaling

Waves slowing, bending and steepening as they reach shallow water

Waves begin to feel the bottom at a depth of about half their wavelength. As depth reduces they slow and shorten, so their height grows, and the crests bend to become more parallel to the depth contours. This concentrates wave energy on headlands and shoals and spreads it in bays, which is why a swell that is barely noticeable offshore can break heavily on a bar or over a shallow patch.

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Wind, Waves & Sea State

Wave Steepness

How tall a wave is compared with its length, showing breaking risk

The ratio of wave height to wavelength. Theory gives a limiting steepness of about 1 in 7, beyond which the crest becomes unstable and the wave breaks. Steepness rises where waves run against a contrary current, where they shoal, or where a new wind sea is building quickly. Dangerous conditions for small craft correlate far better with steepness than with height alone, which is why wind against tide is so significant.

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Passage Planning

Waypoint (WPT)

Marked position where the planned route changes direction

A position defined by latitude and longitude marking the start, end or turning point of a leg. Waypoints must be placed in safe water clear of dangers, entered on the same datum as the chart in use, and never set on a buoy, since buoys drag, are moved and get struck by vessels homing on them. In practice the turn begins at a wheel-over point short of the waypoint, sized to speed and rate of turn.

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Points of Sail & Sailing Manoeuvres

Weather Helm

The boat's tendency to turn into the wind, felt through the tiller

Weather helm arises when the centre of effort of the sail plan lies aft of the hull's centre of lateral resistance, a condition worsened by heel, which pushes the driving force outboard and creates a turning couple. A light weather helm of a few degrees of rudder gives feel and is designed in as a safety feature. Excessive helm means the boat is overpressed and calls for a reef, more mainsheet traveller down, or a flatter sail.

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Great Circle & Ocean Routing

Weather Routeing

Choosing an ocean track using forecast weather rather than distance alone

Weather routeing combines gridded forecast data with the vessel's speed performance to select the fastest, safest or most economical track, which is frequently longer in miles than the great circle. It may be run onboard by software or supplied by a shore-based routeing service that issues initial recommendations and daily updates. The master retains full responsibility for the route actually sailed and for departing from any advice received.

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Passage Planning

Weather Window

Period of settled weather long enough for the whole passage

The interval during which forecast conditions are acceptable for the intended crossing, judged against the vessel, the crew and the realistic passage time with a margin for slower progress; a 30 hour passage wants roughly 36 to 40 hours of usable forecast. Assess wind strength and direction relative to the track, wind against tide, sea state and swell, visibility and the timing of any front or trough, rather than the headline wind speed alone.

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Anchoring & Mooring

Weighing Anchor

Recovering the anchor and getting under way

The vessel motors slowly up towards the anchor while the cable is hove in, keeping the chain leading roughly vertical to avoid overloading the windlass. When the cable is up and down, the anchor is broken out; once clear of the bottom it is aweigh and the vessel is legally under way. The anchor should be washed clean and secured with a chain stopper or lashing before proceeding to sea.

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West Cardinal Mark

Pass to the west of it to stay in safe water

Topmark is two black cones point to point, giving the wineglass shape used as a memory aid. The body is yellow with a single broad black horizontal band. The light is white VQ(9) every 10 seconds or Q(9) every 15 seconds: nine flashes, remembered as nine o'clock for west. Because a west cardinal often marks the western edge of a shoal, the safe water lies westward and the mark is left to the east.

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Chart Projections & Datums

WGS84 (World Geodetic System 1984)

The global reference system that satellite receivers use

WGS84 is an Earth-centred, Earth-fixed geodetic datum with an associated ellipsoid, and it is the native reference frame of GPS. It is effectively coincident with ITRS and, for navigational purposes, with ETRS89 in Europe at the metre level. Modern charts are increasingly published on WGS84 or a compatible datum, and a chart that is so referenced carries a note to that effect, allowing satellite positions to be plotted directly without shift.

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Pilotage & Harbour Approach

Wheel-over Point

Position where the helm goes over to begin a turn

The point at which a turn is started so the vessel settles neatly on the new track, placed short of the waypoint by an allowance derived from her advance and transfer at the planned speed and rudder angle. It is identified by a beam bearing, a radar range on a headland or a distance to run. A large ship may need several cables; a yacht turning at 5 knots needs only a boat length or two.

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Coastal Piloting Techniques

Wheel-Over Point (WOP)

Where you start the turn so the boat settles on the new track

Because a vessel turns along an arc, the helm must be applied before the intersection of two planned tracks. The distance before that point is roughly the tactical turning radius multiplied by the tangent of half the alteration angle, plus an allowance for the time to apply helm. It is marked on the chart with an associated abeam or radar range on a conspicuous feature so the moment can be recognised without a fix.

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Emergency Navigation & Distress

Williamson Turn

A ship turn that brings you back along your own track

Used when a person is lost overboard in poor visibility or the time of the fall is uncertain, the Williamson turn returns the vessel onto the reciprocal of the original heading and back down its own wake. The rudder is put hard over toward the casualty's side, held until the heading has changed about sixty degrees, then shifted hard over the other way, easing to steady on the reciprocal course. It costs distance but guarantees track recovery.

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Wind, Waves & Sea State

Wind Acceleration Zone (Cape Effect)

Wind speeding up as it squeezes around headlands and through gaps

Airflow forced around or between obstructions compresses and accelerates, so wind off a steep headland or through a strait can exceed the gradient wind by 50 percent or more, often a full Beaufort force or two. The effect is strongest with stable air and high ground, and is commonly paired with a corresponding wind shadow and fluky conditions to leeward. Local pilots and coastal forecasts flag the well-known sites.

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Wind, Waves & Sea State

Wind Against Tide

Wind blowing opposite to the current, creating short steep seas

When a tidal stream runs against the wind, the waves are compressed: wavelength shortens while height increases, so steepness rises sharply and crests break. A 3 knot stream opposing a force 5 can produce conditions resembling a gale in still water. Notorious examples occur off headlands and in tidal races. The remedy is timing, planning passage through such areas with the stream running with the wind or at slack water.

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Wind, Waves & Sea State

Wind Sea

The rough, irregular waves being raised by the wind right now

Waves actively generated by the local wind, characterised by short periods, irregular crest lengths, a broad spread of directions and frequent whitecaps. Wind sea grows with wind speed, fetch and duration until the sea is fully developed, after which further growth stops. Forecasts and observation logs report wind sea and swell separately because the combined, or total, significant wave height is not simply the sum of the two components.

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Tidal Streams & Currents

Wind-Driven Surface Current

Water dragged along by sustained wind blowing over the sea

A sustained wind generates a surface drift of roughly 2 to 3 per cent of wind speed, so a 30-knot blow gives close to 0.7 knots. Ekman transport deflects the mean flow of the wind-driven layer about 90 degrees, to the right in the northern hemisphere, with the surface layer itself deflected some 20 to 45 degrees. Over long ocean legs this adds meaningfully to the tidal and current allowance in the plan.

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Anchoring & Mooring

Windlass

The deck winch used to raise chain and anchor

Manual, electric or hydraulic, with a gypsy machined for one specific chain size and pitch, and usually a warping drum for rope. Electric units draw very high current, so the engine should run while operating them. The gypsy and its clutch are not intended to hold anchor loads at rest, which is the job of a chain stopper or snubber. Many have a chain counter to display veered length.

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COLREGs - Steering & Sailing Rules

Windward Side

The side opposite the mainsail, used to define your tack

For the purposes of the sailing vessel rule, the windward side is the side opposite that on which the mainsail is carried, or in a square-rigged vessel the side opposite that on which the largest fore-and-aft sail is carried. This definition matters when running dead downwind, where apparent wind direction is ambiguous: a boat with the boom out to port is on starboard tack regardless of the burgee. Gybing therefore changes right of way instantly.

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Withy

Simple branch or pole stuck in a mudbank to mark it

Withies are informal local marks, traditionally willow saplings, used in shallow creeks and drying harbours where conventional buoyage is uneconomic. Conventions vary by locality and are seldom charted, so a current pilot book or local knowledge is essential. In some East Coast and Dutch waters an upright bushy withy marks one side and an inverted or bound one the other. Positions may shift, and they should never be treated as surveyed marks.

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Compass & Magnetism

World Magnetic Model (WMM)

The global mathematical model that predicts magnetic variation anywhere

Produced jointly by the British Geological Survey and the US NOAA National Centers for Environmental Information, the WMM is a spherical harmonic representation of the main geomagnetic field, released on a five year cycle with annual change coefficients. It is the source of the variation figure computed by chartplotters, autopilots and electronic compasses, and specified for defence use. Its stated accuracy degrades near the poles and it cannot represent local magnetic anomalies caused by geology.

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Chart Symbols & Abbreviations

Wreck Symbols

Chart marks showing sunken vessels and whether they are dangerous

A wreck showing any part of its hull at chart datum is drawn in profile; a wreck with masts or superstructure showing has a different symbol again. A dangerous wreck at a depth of 28 m or less, or of unknown depth, is a hull outline with a dotted danger circle or is shown by the classic hull-and-cross symbol. Non-dangerous wrecks carry the abbreviation Wk with a sounding. Wrecks swept by wire drag show the swept depth with a line under the figure.

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Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.