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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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Speed, Log & Distance
Cable
A tenth of a nautical mile, used for short distances inshore
One cable equals 0.1 nautical mile, that is 185.2 metres, and is the natural unit for pilotage instructions such as passing two cables off a headland. It derives historically from the length of a ship's anchor cable, and older British practice sometimes took it as 100 fathoms, about 183 metres. On charts it corresponds to one minute of latitude divided into ten parts, the subdivisions normally printed on the latitude border.
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Compass & Magnetism
CADET / Correcting and Uncorrecting Mnemonics
Memory aids for converting between compass, magnetic and true
CADET stands for Compass Add East to get True, meaning that going from compass towards true one adds easterly errors and subtracts westerly ones; reversing the process reverses the signs. The older rhyme Error East, Compass Least, Error West, Compass Best expresses the same relationship. Another common chain is True Virgins Make Dull Companions, ordering True, Variation, Magnetic, Deviation, Compass, with Add Whisky reminding that westerly errors are added going down the chain.
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IALA Buoyage

Cardinal Mark

Buoy telling you which side of it holds safe water

A cardinal mark is named for the quadrant of safe water relative to the danger, so a north cardinal is passed on its north side. Bodies are pillar or spar, painted black and yellow, and always carry two black cones as a topmark. The cones point toward the black band or bands: both up for north, both down for south, base to base for east, point to point for west. Lights are always white and quick or very quick.

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Anchoring & Mooring

Catenary

The natural sag in a heavy chain between boat and anchor

The curve assumed by a suspended cable under its own weight. Catenary keeps the pull at the anchor shank close to horizontal, which is what allows most anchor designs to develop full holding power, and it absorbs surge as the bow rises. Catenary is progressively lost as wind load increases; above roughly force 7 the chain may straighten, so a nylon snubber or extra scope becomes essential.

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Celestial Navigation Concepts

Celestial Sphere and the First Point of Aries

The imaginary star globe and its agreed zero reference point

Navigators treat the sky as an infinite sphere concentric with the Earth on which bodies are located by declination and hour angle. The origin for star coordinates is the First Point of Aries, the ascending node where the sun crosses the celestial equator going north at the March equinox. Precession moves this point slowly westward, roughly 50 arcseconds a year, which is why star SHA and declination values must be taken from a current almanac.

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Electronic Charts & ECDIS

Cell Permit

The licence key that lets your equipment open a chart cell

Under the data protection scheme each encrypted cell requires a permit tied to the specific installation's unique identifier and carrying a subscription expiry date. Permits are supplied as a file that is installed before or with the data. If a permit expires the cell stops receiving updates and the system flags the chart as out of date, so permit renewal dates belong in the vessel's planned maintenance routine, not in the pre-departure rush.

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Chart Symbols & Abbreviations

Chart 5011 (INT 1)

The booklet that decodes every symbol used on Admiralty charts

Admiralty Chart 5011 is published as a book rather than a chart and reproduces the international INT 1 symbol set agreed through the IHO. Sections are lettered: A covers chart layout, B positions and distances, C to G topography and seabed, I tides and currents, J to L depths and rocks, M to Q services, buoys and lights, and R to T are supplementary. Germany, the USA and others publish equivalents; the symbols are largely common.

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Nautical Publications

Chart Catalogue

The index showing which charts exist for a given area

NP 131, the Admiralty Catalogue of Charts and Publications, contains index diagrams with numbered chart outlines overlaid on small scale base maps, allowing the navigator to identify the passage, coastal, approach and harbour charts needed for a voyage. It also lists publications, digital products and folio contents. Equivalent online catalogues are maintained by NOAA and other national hydrographic offices, and are usually more current than a printed edition.

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Chart Projections & Datums

Chart Datum (CD)

The zero level that charted depths are measured down from

Chart datum is the vertical reference for soundings and drying heights, chosen so that the tide will seldom fall below it. In the United Kingdom and most IHO member states it is set at or near Lowest Astronomical Tide, so predicted heights of tide are added to charted depths to obtain actual depth. It is a local datum specific to each port or region, not a continuous surface, and it differs from the datums used for land heights.

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Chart Symbols & Abbreviations

Chart Legend / Title Block

The block of text on a chart explaining how to read it

The title block carries the chart's name, scale, projection, the horizontal datum (typically WGS84 on modern charts), the vertical datum for soundings, the units of depth and height, and the source of survey data. Notes alongside warn of hazards, restricted areas and radio reporting requirements. Always read it before plotting: a chart quoting soundings in fathoms or feet, or positions on a local datum, will mislead a navigator who assumes metres and WGS84.

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Chart Symbols & Abbreviations

Chart Number, Edition Date and Print Code

Details identifying which version of a chart you are holding

The chart number appears in each corner, with the edition date and any new edition date printed in the bottom margin. Small corrections applied through Notices to Mariners are added to the bottom left margin as year and notice number, for example 2024-1136. A chart is only as safe as its corrections, so compare the last entry with the current cumulative list before departure. Withdrawn or superseded charts should be marked and removed from the working folio.

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Chart Symbols & Abbreviations

Chart Scale and Natural Scale

The ratio showing how much the chart shrinks the real world

Expressed as a representative fraction such as 1:150000 for a passage chart, 1:25000 for coastal work and 1:5000 or larger for a harbour plan. On a Mercator chart the scale varies with latitude, so distances must always be measured on the latitude scale at the side of the chart, level with the area of interest, never on the longitude scale along the top or bottom. One minute of latitude equals one nautical mile.

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Depth Sounding & Bathymetry

Charted Depth and Chart Datum (CD)

The number on the chart, measured from the lowest tide level

Charted depths are soundings reduced to chart datum, the level below which the tide seldom falls, taken on modern British and international charts as Lowest Astronomical Tide. Actual depth equals charted depth plus height of tide, so a 3.2 m sounding with 1.8 m of tide gives 5.0 m. Drying heights above chart datum are shown underlined. Low barometric pressure and strong offshore winds can depress the real level below prediction, so a margin should always be retained above the calculated figure.

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Celestial Instruments & Sight Reduction

Chronometer and Watch Error

The accurate timepiece and the known amount by which it is wrong

Traditionally a gimballed spring chronometer kept on GMT and never reset; its error and daily rate were logged against a time signal. Modern practice uses a quartz watch checked against GNSS time, radio time signals or an internet source. Watch error is applied to the recorded time of each sight, and the rate allows interpolation between checks. A four-second timing error moves a sun position line about one nautical mile in equatorial regions.

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Position Fixing

Circle of Position

A curved position line drawn at a fixed distance around a known object

Any range measurement, whether by radar, vertical sextant angle, rising or dipping light, or a distance-measuring beacon, constrains the vessel to a circle centred on the object. Two such circles intersect at two points, one of which is usually rejected by an approximate bearing or by dead reckoning. Circles of position combine well with straight bearing lines and are the underlying geometry of satellite ranging, where the loci become spheres.

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Celestial Navigation Concepts

Circle of Position (Circle of Equal Altitude)

The circle around a body's ground position where its height looks the same

Observers anywhere on a circle centred on the geographical position see the body at the same altitude. The circle's radius, the zenith distance, equals 90 degrees minus the observed altitude, and one minute of arc equals one nautical mile. A body observed at 40 degrees altitude thus gives a circle of radius 3000 nautical miles. Because such circles are enormous, only a short arc near the assumed position is plotted, and it is treated as a straight line.

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AIS & Vessel Tracking

Class A AIS Transponder

The high-power shipborne unit fitted to larger commercial vessels

Mandated by SOLAS Chapter V for vessels of 300 gross tonnage and upwards on international voyages, all cargo vessels of 500 gross tonnage and upwards, and all passenger ships. It transmits at 12.5 watts using SOTDMA, giving it priority access to slots. Dynamic reporting intervals vary with vessel behaviour, from about every 2 to 10 seconds when underway to every 3 minutes at anchor, with faster rates when turning.

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AIS & Vessel Tracking

Class B AIS Transponder

Lower-power unit for small craft and leisure yachts

Designed for non-SOLAS vessels, Class B transmits at 2 watts and carries lower slot priority than Class A. The original Class B (CSTDMA) listens for a free slot before transmitting and reports position typically every 30 seconds under 2 knots. Class B SO (SOTDMA) transmits at 5 watts with reserved slots and faster update rates. Class B omits some data fields such as rate of turn, navigational status and voyage destination.

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Tidal Calculations

Clearance Calculation (Under-keel)

Checking there is enough water beneath the boat over a shallow patch

Required height of tide equals draught plus desired under-keel clearance minus charted depth. A yacht drawing 1.8 m wanting 0.5 m clearance over a 1.2 m sounding needs a tide of at least 1.1 m. Add allowance for squat, for negative surge in high pressure or offshore wind, for swell reducing instantaneous depth by roughly half the wave height, and for survey age in poorly charted waters.

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Coastal Piloting Techniques

Clearing Bearing

A safe bearing limit that keeps you clear of a known danger

A single position line, drawn from a conspicuous charted object so that it just passes clear of a hazard, defines a safe side. The navigator notes, for example, NLT 095 degrees True on the church tower, meaning the bearing must not be less than that value while passing. Expressed as NMT (not more than) or NLT (not less than), it requires no fix and no plotting at the time, only a single hand bearing compass observation compared with the pre-computed limit.

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Coastal Piloting Techniques

Clearing Line

Any charted line that separates safe water from danger

A generic term embracing clearing bearings, clearing transits, clearing depth contours and clearing ranges from radar or a distance-off circle. The line is pre-drawn during passage planning and annotated with the safe side. A depth-based clearing line, for example staying outside the 10 metre contour, is often the most robust in poor visibility because the echo sounder works when no marks are visible. Several clearing lines can be combined to box in a safe corridor.

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Points of Sail & Sailing Manoeuvres

Close Hauled (Beating)

Sailing as near to the wind as the boat will usefully go

Sheets are trimmed hard and the boat sails at the smallest apparent wind angle at which the sails still drive efficiently, typically about 30 to 35 degrees apparent for a modern cruiser, corresponding to roughly 40 to 50 degrees off the true wind. Progress directly upwind is made by beating, sailing alternate close hauled boards and tacking between them, so the distance sailed exceeds the distance made good.

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Points of Sail & Sailing Manoeuvres

Close Reach

Sailing with the wind forward of the beam but not hard on

A close reach lies between close hauled and a beam reach, with the true wind roughly 60 to 80 degrees off the bow. Sheets are eased a little from the close hauled setting and the sails carry a fuller shape. It is often the fastest and most comfortable point of sail for a cruising yacht, since the rig is well loaded without the leeway, heel and slamming associated with beating.

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COLREGs - Steering & Sailing Rules

Close-quarters Situation

Getting near enough that a mistake means collision

Not defined numerically in the rules, deliberately, because an acceptable passing distance depends on the vessels, speeds, sea room and visibility. In open water with large ships, one to two nautical miles is often cited; in a crowded estuary a few hundred metres may be normal. In restricted visibility the obligation is to take action in ample time to prevent the situation developing at all, rather than to manage it once it exists.

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Radar Navigation

Closest Point of Approach (CPA) and Time to CPA (TCPA)

How near a vessel will pass and how long until it does

CPA is the minimum separation that will occur if both vessels hold course and speed, and TCPA is the time remaining until that moment; a negative TCPA means the point has passed. Alarm limits are set to suit the waters, and a small CPA with a short TCPA demands early and substantial action. Both figures assume no manoeuvre, and are invalidated the instant either vessel alters, so they must be re-evaluated.

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AIS & Vessel Tracking

Closest Point of Approach from AIS (CPA)

The nearest distance a target will pass if both vessels hold course and speed

Computed from the relative velocity vector between own ship and the target using their reported positions, courses and speeds over ground. Displays present CPA in nautical miles alongside time to CPA in minutes, with user-set alarm thresholds, for instance 1.0 nautical mile and 12 minutes offshore. Because AIS CPA uses ground-referenced data it can differ from radar-derived CPA in a strong tidal stream, and it assumes both vessels maintain their present motion.

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Co-tidal and Co-range Lines

Chart lines joining places with the same tide time or same range

Co-tidal lines connect points where high water of a given constituent occurs at the same time, normally labelled in hours relative to the Moon's transit of the Greenwich meridian. Co-range lines connect points of equal amplitude for that constituent. They appear on Admiralty co-tidal charts and are used offshore, away from standard ports, to interpolate tidal height for oil-field, survey and deep-draught passage work.

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Position Fixing

Cocked Hat

The small triangle formed when three position lines fail to meet at one point

The triangle results from small random and systematic errors in bearing-taking, plotting or timing. A tight cocked hat with sides under about 0.2 nautical miles suggests a reliable fix, and the position is normally taken at the centre. Prudence requires taking the corner nearest danger when the triangle is large or when a shoal lies close by. A consistently large cocked hat usually points to a systematic cause such as an unapplied deviation or misidentified landmark rather than random error.

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Meteorology for Navigators

Col

The slack area between two highs and two lows

A saddle-shaped region of nearly uniform pressure lying between two anticyclones on one axis and two depressions on the other. Winds are light and variable, making progress under sail difficult and leaving a vessel vulnerable to whatever system arrives next. Cols favour fog and low stratus in cool conditions and heavy thunderstorms in warm, humid air, and they typically break down within a day or two.

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Meteorology for Navigators

Cold Front

Colder air undercutting warm air behind a depression

Shown by blue triangles pointing the way it moves. Its steep slope, near 1 in 50, forces rapid uplift, giving cumulonimbus, heavy showers or thunderstorms and a narrow belt of severe weather. Passage brings a sharp veer of wind in the Northern Hemisphere, often 40 to 90 degrees, a squally increase in strength, a pressure rise and a temperature drop, followed by clearing skies and good visibility.

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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.