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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 1 of 2
A sextant angle limit that warns you are approaching a hazard
A pre-computed vertical or horizontal sextant angle sets a boundary. For a vertical danger angle, the angle to a lighthouse of known height increases as the vessel closes; keeping the observed angle less than the pre-set value keeps the vessel outside a circle around the danger. Horizontal danger angles between two marks work identically, the angle growing as the vessel approaches. Both give continuous warning without plotting and remain valid regardless of compass error.
Read definition →The worse half of a tropical storm, where wind pushes you inward
The half of a revolving storm on the side where the storm's forward motion adds to its circulating wind, and where the wind tends to blow a vessel into the track ahead. It is the right-hand semicircle relative to the direction of travel in the Northern Hemisphere and the left-hand in the Southern. The opposite half is the navigable semicircle, in which running with the wind on the appropriate quarter takes a vessel clear.
Read definition →The correction printed on a chart to move satellite positions onto it
Charts not compiled on WGS84 carry a caution giving the adjustment in seconds or fractions of a minute of latitude and longitude to be applied to satellite-derived positions before plotting. The shift is stated with its direction, for example positions should be moved 0.05 minutes south and 0.09 minutes east. Where the survey is old or the shift is unquantified, the note may instead warn that the shift cannot be determined, and the chart must then be used with extra caution.
Read definition →Working out where you are from course steered and distance run
Dead reckoning advances a known position using only the course steered through the water and the distance logged, deliberately ignoring tidal stream, current and leeway. It is plotted as a cross on the course line with a single arrowhead, labelled with time. Because it excludes the water's own movement it is a systematic rather than probabilistic estimate, but it remains the essential fallback when electronic positioning fails and the basis from which the estimated position is derived.
Read definition →Falling back to manual position keeping when the plotter stops working
On losing satellite positioning the navigator immediately notes the last known good position, the time, the log reading and the heading, and begins a manual plot from that point. Thereafter course, log, leeway and tidal data are recorded at fixed intervals, commonly every 15 to 30 minutes coastal, and an EP is carried forward. Cross-checking against soundings, radar ranges and any visual position lines rebuilds confidence, and the plot continues until an independent fix restores certainty.
Read definition →A body's angular distance north or south of the celestial equator
The celestial equivalent of latitude, tabulated in the Nautical Almanac for each hour of GMT with a d correction for the hourly rate of change. The sun's declination swings between about 23.4 degrees north at the June solstice and 23.4 degrees south at the December solstice, crossing zero at the equinoxes. Stars have effectively fixed declinations over a year, so they are listed once per almanac page rather than hourly.
Read definition →East-west distance in nautical miles, before converting to longitude
Departure is the east-west component of a run measured in nautical miles along a parallel of latitude, and it must be converted into difference of longitude because a minute of longitude shrinks with the cosine of latitude. The relationship is departure equals difference of longitude in minutes times cosine of mean latitude. At 60 degrees north, 10 nautical miles of departure corresponds to 20 minutes of longitude. Departure also names the act of taking a final fix on leaving the coast.
Read definition →A line joining points of equal charted depth
Standard metric contours are drawn at 2, 5, 10, 20, 30, 50, 100 and 200 metres and beyond, with the shallower ones often given blue tint bands. Contours are interpolated from soundings and are therefore smoothed representations of the real seabed. In pilotage a chosen contour makes an excellent handrail, and a depth contour used as a limit not to be crossed is a clearing line that the echo sounder alone can monitor.
Read definition →Using seabed depth to help place the boat when nothing is visible
A single reduced sounding matching a charted contour is a position line, weak on a gently shelving bottom but strong along a steep shelf edge or a narrow channel. A more powerful technique records a series of soundings against distance run, plots them to chart scale on a strip of paper or tracing overlay, and slides that profile along the intended track until the pattern matches. Corrections for height of tide and transducer offset must be applied first.
Read definition →Colour bands that show at a glance how deep the water is
The display divides depth into bands using the shallow contour, safety contour and deep contour. In four-shade mode, deep water is white, water between the safety and deep contours takes a pale blue, water between shallow and safety contours a stronger blue, and shallow water the deepest blue; drying areas take their own tint. Two-shade mode simplifies this to safe and unsafe about the safety contour, which many navigators prefer for clarity.
Read definition →Compass error caused by the boat's own iron and electrics
Deviation arises from permanent magnetism in the hull and engine, induced magnetism in soft iron, and stray fields from loudspeakers, electronics and portable devices. Unlike variation, it changes with the vessel's heading rather than with position, and so must be tabulated on a deviation card against each heading. It is named East when the compass north points clockwise of magnetic north. A well sited and compensated compass on a small yacht should show deviation of only a few degrees.
Read definition →A table listing your compass error on every heading
Produced by swinging the compass, the card lists deviation, named East or West, at intervals of typically 10, 15 or 45 degrees of ship's head, often with a smooth curve fitted through the values. It is dated, signed by the compass adjuster, and posted near the steering position. It must be redetermined after any structural work, engine change, new electronic installation, a lightning strike or a long period in one berth, and periodically as permanent magnetism decays.
Read definition →The temperature at which air becomes saturated and moisture condenses
The temperature to which air must be cooled, at constant pressure and moisture content, for condensation to begin. It is measured with a wet and dry bulb hygrometer, the difference between the readings being the wet bulb depression. When air temperature and dew point converge to within about one degree Celsius, fog or low cloud is likely; comparing dew point with sea surface temperature is the standard test for sea fog risk.
Read definition →Black diamond marking a tow longer than 200 metres
Where the length of the tow, measured from the stern of the towing vessel to the after end of the tow, exceeds 200 metres, both the towing vessel and the vessel or object being towed exhibit a black diamond shape where it can best be seen. By night the towing vessel shows three masthead lights in a vertical line instead of two. The signal warns of a long, low and often almost invisible towline.
Read definition →The north-south and east-west change between two positions
Difference of latitude is the arc between two parallels, expressed in degrees and minutes, where one minute equals one nautical mile everywhere. Difference of longitude is the arc between two meridians, whose linear value diminishes with latitude. Both are signed by direction, north and east conventionally positive. They are the fundamental quantities in plane, parallel and Mercator sailing calculations, and are the output of any dead reckoning arithmetic performed numerically rather than by plotting.
Read definition →Shore stations broadcast corrections that sharpen your satellite position
A reference station at a surveyed position computes the error in each satellite range and broadcasts corrections in RTCM format, historically over marine radiobeacons in the 283.5 to 325 kHz band. A vessel within a few hundred miles applies them and typically achieves accuracy of one to three metres, degrading with distance from the station. Many national authorities have withdrawn or reduced their radiobeacon services as SBAS coverage has matured.
Read definition →Electronic versions of the official books, updated automatically
Admiralty Digital Publications include e-NP versions of Sailing Directions, Lists of Lights and Lists of Radio Signals, plus TotalTide for tidal predictions and digital List of Lights and Radio Signals applications. Updates arrive by download and are applied without hand correction, which reduces error but creates dependence on a working computer. Under SOLAS, approved digital publications can satisfy carriage requirements provided the system meets the flag state's conditions, usually including a backup arrangement.
Read definition →A radio button that sends an automatic digital alert with your details
DSC is the automated alerting layer of the GMDSS. Holding the guarded distress button on a DSC-equipped VHF, MF or HF set transmits a burst containing the vessel's MMSI, the position and time taken from a connected or internal GNSS receiver, and where selected the nature of the distress. Receiving sets alarm and display the data. The alert is only the alarm; it must be followed by a voice Mayday on channel 16.
Read definition →A number showing how good or bad the satellite geometry is
DOP expresses how satellite geometry amplifies ranging errors into position errors: position error approximately equals the user equivalent range error multiplied by the relevant DOP. Widely spread satellites give a low figure, while satellites clustered in one part of the sky give a high one. Horizontal DOP below about 2 indicates good geometry for navigation, values above roughly 5 warrant caution, and the NMEA GGA and GSA sentences report it directly.
Read definition →Range found from the moment a light appears over the horizon
A light first seen as the vessel closes the coast is at its geographical range. The range in nautical miles is approximately 2.08 times the sum of the square roots of the height of the light and the height of eye, both in metres. Rising and dipping distance tables in almanacs tabulate this directly. Combined with a simultaneous compass bearing it yields a fix. Abnormal refraction and haze can shift the observed moment by a mile or more.
Read definition →Distance off from the moment a light first appears over the horizon
As a light rises above or dips below the sea horizon, geometry fixes the range for a given charted elevation and height of eye. The standard approximation is range in nautical miles equals 2.08 times the sum of the square roots of the two heights in metres, allowing for standard terrestrial refraction. For a light 40 metres high and an eye height of 3 metres this gives about 16.7 nautical miles. Anomalous refraction can shift this appreciably.
Read definition →Three levels of chart detail, from bare minimum to everything
Display Base is the permanently displayed minimum, including the coastline, safety contour, own-ship safety-related isolated dangers and traffic routeing elements, and cannot be removed. Standard Display is the default power-up set, adding drying areas, cautionary areas, fixed and floating aids, prohibited areas and chart scale boundaries. All Other adds spot soundings, submarine cables, seabed nature, contour labels and similar detail. Reducing below Standard raises a permanent indication that the display is not standard.
Read definition →Choosing whether the top of the screen is the bow or north
Head-up needs no heading input and matches the view from the window, but the picture smears and rotates every time the vessel yaws. North-up requires a heading input, keeps the picture stable and aligns directly with the chart, making it the normal choice for navigation and plotting. Course-up steadies the picture about the planned course while keeping the intended track upwards, and must be reset whenever the course is altered.
Read definition →Instrument recording distance run and speed through the water
Modern yachts use an impeller or paddlewheel through-hull unit whose rotation rate is proportional to water speed, integrated over time to give distance run. Displays usually show a resettable trip distance and a cumulative total. The reading is essential for dead reckoning, since distance run divided into the elapsed time gives average speed. Fouling by weed or barnacles is the commonest fault, typically causing the log to under-read progressively over a season.
Read definition →The straight-line distance you have actually advanced along the ground
Distance made good is measured from the departure position to the current position along the ground track, and it may be considerably less than the distance logged when beating or when the stream is foul. On a windward passage a yacht may log 30 nautical miles of tacking to make good 20 along the rhumb line. Distinguishing logged distance from distance made good is essential when estimating time of arrival.
Read definition →Sextant angle between two landmarks placing you on a circle
The angle measured between two charted objects places the observer on a circle of position passing through both. Two such angles from three objects give a very precise fix, traditionally plotted with a station pointer or three-arm protractor. The fix becomes unreliable when the observer lies near the circle through all three objects, the so-called revolver or swinger, so choose objects where the middle one is nearer or the spread is well conditioned.
Read definition →Using a sextant angle on a tall object to find your range from it
Measure the vertical angle subtended by an object of known charted height, then compute range. For small angles the range in nautical miles is approximately the height in metres divided by 1.854 times the angle in minutes of arc. Charted light heights are given above MHWS on British charts, so at other states of tide the true height is greater and the raw result is conservative unless corrected. The method is accurate to a few percent inside about three miles.
Read definition →The total water distance covered since the log was last reset
The fundamental input to dead reckoning, where the DR position is found by laying off the course steered from the last known position for the distance shown by the log. If no log is available it is computed as speed multiplied by time, for instance 5.5 knots for 40 minutes giving 5.5 times 40 divided by 60, that is 3.67 nautical miles. Recording it in the deck log at every fix and course change is essential practice.
Read definition →Books of pre-computed sea distances between ports
Admiralty Distance Tables, published in three volumes as NP 350, give the shortest practicable navigable distance in nautical miles between principal ports, routed through junction points and around continents and through canals rather than as straight great circles. Volume 1 covers the Atlantic and Mediterranean, Volume 2 the Indian Ocean and adjacent seas, Volume 3 the Pacific. They are used for voyage estimating, fuel and provisioning calculations and charter party distance disputes.
Read definition →One high water and one low water each day
Occurs where the diurnal constituents K1 and O1 dominate over M2 and S2, giving a single cycle of period close to 24 hours 50 minutes. Found in parts of the Gulf of Mexico, the South China Sea and sections of the Antarctic coast. The form factor F equals the ratio (K1+O1)/(M2+S2); values above about 3.0 indicate a diurnal regime, and values below about 0.25 a purely semi-diurnal one.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.