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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.
Loading this page…
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
Page 3 of 22
The classic American textbook and reference on all navigation
First published by Nathaniel Bowditch in 1802 and maintained today by the US National Geospatial-Intelligence Agency, Pub 9 covers piloting, dead reckoning, celestial navigation, electronic systems, oceanography, meteorology and the mathematics behind them, with extensive tables. It is issued free in digital form and is the standard general reference in American practice, roughly filling the role that the Mariner's Handbook and Admiralty Manual of Navigation share in British usage.
Read definition →A sudden violent turn into the wind with the boat lying over
A broach typically happens downwind in strong conditions or under spinnaker, when a wave or gust drives the bow down, the boat heels hard, the rudder ventilates or lifts clear and steering is lost as the vessel slews up towards the wind. Recovery is by dumping the mainsheet and spinnaker or headsail sheet to depower, then steering back onto course once the boat stands up and the rudder bites again.
Read definition →Sailing with the wind coming over the quarter, well aft
The true wind lies roughly between 110 and 160 degrees off the bow. Sheets are well eased, a spinnaker or cruising chute is often carried, and the motion is comfortable because apparent wind speed is reduced by the boat's own progress. For most displacement yachts the broad reach gives the best downwind velocity made good, which is why sailing a series of broad reaches beats running dead before the wind.
Read definition →The precise orbit data each satellite sends about where it is
Each satellite transmits its own detailed orbital parameters and clock corrections within the navigation message, allowing the receiver to compute the satellite's position at the instant of transmission. Ephemeris data is only valid for a few hours and must be refreshed; downloading a fresh set takes roughly thirty seconds of continuous tracking per satellite. Stale or missing ephemeris is the usual reason a receiver switched on after a long lay-up is slow to fix.
Read definition →A sextant with an internal artificial level for use without a horizon
Developed for aircraft, it replaces the sea horizon with a bubble level so altitudes can be measured when no horizon is visible, such as in haze or at night. Accelerations from vessel motion displace the bubble, so accuracy at sea is markedly poorer than a marine sextant against a good horizon, typically several minutes of arc. Averaging over a timed interval reduces but does not eliminate the acceleration error.
Read definition →Rules reserving certain flash patterns for particular buoy types
Rhythms are allocated so that a mark can be identified at night from its light alone. Composite group flashing Fl(2+1) is reserved for preferred channel marks. Group flashing two, Fl(2), in white indicates isolated danger. White Q and VQ rhythms belong to cardinals. Morse A, isophase, occulting and LFl 10s in white denote safe water. Lateral lights use red or green in any other rhythm, and special marks use yellow.
Read definition →Stand back to the wind and low pressure lies to your left
In the Northern Hemisphere, an observer with their back to the true wind has lower pressure on the left hand and higher pressure on the right; in the Southern Hemisphere the sense is reversed. Because surface friction backs the wind relative to the isobars by roughly 10 to 20 degrees over the sea, the centre of the low actually lies a little behind the beam, nearer 100 to 120 degrees from the wind direction.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.