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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.
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Keeping check that the anchor is still holding
Maintained by taking beam and ahead transits or compass bearings of fixed marks and rechecking them regularly, backed by a GPS anchor alarm set to the swinging circle radius plus a margin for position error. Depth alarms are also useful, since dragging usually changes depth. Watchkeeping intervals should shorten as wind rises, tide turns, or when holding ground is known to be marginal.
Read definition →Marks showing where you may or must not drop the hook
A small anchor symbol indicates a recommended or charted anchorage, often within a dashed magenta limit line and sometimes lettered or numbered for berth allocation. An anchor crossed through by a diagonal line marks an area where anchoring is prohibited, typically over submarine cables, pipelines or in a fairway. Related symbols mark cable areas with a wavy line and the word Cable, and spoil grounds and dumping areas with dotted limits.
Read definition →How squarely two position lines cross each other
The reliability of a two-line fix depends on the angle between the lines. A cut near 90 degrees minimises the error ellipse, while a shallow cut converts a small bearing error into a large positional displacement along the bisector. For three objects, aim for roughly 60 degrees between adjacent bearings, spanning about 120 degrees of horizon. As a rule of thumb reject fixes where the cut falls below about 30 degrees unless nothing better is available.
Read definition →The control that thins out the solid blob of a rain shower
Precipitation returns as a large diffuse mass that can conceal targets within it. This control responds only to the leading edge of an echo, so extended returns such as rain are reduced to an outline while discrete targets remain. It also thins genuine land echoes and small targets, so it should be applied sparingly and backed off as soon as the shower passes; it is useful for separating targets that have merged.
Read definition →The control that suppresses the ring of wave echoes around you
Wave returns are strongest close to the vessel and fall away rapidly with range, so this control applies suppression that is heaviest at short range and tapers off with distance. It must be adjusted only far enough that clutter breaks up into a speckled pattern with occasional returns still visible; winding it up until the screen centre is clean will delete small craft, buoys and liferafts. Set it upwind, where clutter is worst.
Read definition →An area of high pressure with light winds and settled weather
Air subsides and diverges outwards, circulating clockwise in the Northern Hemisphere and anticlockwise in the Southern. Subsidence creates an inversion that suppresses shower cloud but traps moisture and haze beneath it, so summer highs give light winds, good sailing and poor visibility, while winter highs bring frost, low stratus or anticyclonic gloom. Highs move slowly and can block the eastward progress of depressions for days.
Read definition →The wind you actually feel on a moving boat
The vector sum of true wind and the wind of the vessel's own motion. Sailing upwind it is stronger than the true wind and drawn further forward; sailing downwind it is weaker and drawn aft, which is why running feels deceptively gentle. Example: 15 knots true on the beam with 6 knots boat speed gives roughly 16 knots apparent about 22 degrees forward of the beam. Sails are trimmed to apparent wind.
Read definition →Gathering and weighing all information before choosing a route
The first stage of passage planning, in which every relevant source is collected and assessed: charts of suitable scale and known datum, pilots and sailing directions, tide tables and tidal stream atlases, light lists, notices to mariners, routeing measures and reporting schemes, climatology and current forecasts, together with the vessel's draught, endurance, equipment and crew experience. Its output is a decision on feasibility and broad route options, not yet a detailed track.
Read definition →Maintained deep channel leading from open water to berths
A channel maintained by dredging to a stated depth below chart datum, shown on the chart with its dredged depth and the date of dredging, and usually edged by lateral marks or covered by a leading line. Available water equals the dredged depth plus the height of tide, but siltation between surveys can make the charted figure optimistic, so a deep-draught vessel should confirm the port's latest declared depth and any notices before entry.
Read definition →A reflecting liquid surface used to take sights ashore without a sea horizon
A shallow tray of oil, or a mercury bath in older equipment, provides a truly horizontal reflecting surface. The observer brings the body's direct image into coincidence with its reflection, and the measured angle is twice the apparent altitude. Halve the reading, then apply index error and the normal corrections, but apply no dip correction because no sea horizon is used. Bubble horizons built into aircraft sextants serve the same purpose but are less precise at sea.
Read definition →A convenient nearby position used as the starting point for calculation
For tabular sight reduction the latitude is rounded to a whole degree and the longitude chosen so LHA becomes whole, placing the AP within about 30 nautical miles of the estimated position. Calculator or app methods can use the dead reckoning position directly with no rounding. The resulting position line is independent of the AP chosen; only the length and direction of the intercept change, which is a useful arithmetic check.
Read definition →Radio system where ships broadcast their identity, position and movement automatically
A VHF data link operating on two dedicated channels near 162 MHz, AIS 1 (161.975 MHz) and AIS 2 (162.025 MHz), using self-organising time division multiple access to share time slots without a central controller. Transponders broadcast GNSS-derived position, course and speed over ground, heading and identity, and receive the same from others. Range is roughly line-of-sight, typically 20 to 40 nautical miles ship to ship, considerably more to shore stations or satellites.
Read definition →The radar's computer that tracks targets and works out their courses
ARPA acquires targets manually or automatically, tracks them over successive scans and computes each target's true course and speed, closest point of approach and time to it, presenting the result as a vector and a data box. Vectors may be shown relative, for assessing risk of collision, or true, for identifying what a target is actually doing. A tracked solution needs about three minutes to become reliable and degrades after any manoeuvre.
Read definition →Taking several observations in a row and smoothing the results
Random errors from swell, an indistinct horizon and hand tremor typically dominate a single sight. Taking four or five observations over a few minutes and plotting altitude against time should produce a nearly straight line; points far off the line are discarded and the mean of the rest is used with the mean time. This graphical screening is more reliable than averaging blindly, because it exposes a misread drum or a mistimed observation.
Read definition →The compass direction of the body from you, in degrees true
The bearing of the geographical position, needed to lay off the intercept and draw the position line at right angles to it. Sight reduction tables give an azimuth angle Z measured from the elevated pole, which is converted to Zn using rules depending on hemisphere and whether LHA is greater or less than 180 degrees. Comparing a computed azimuth of the sun with a simultaneous compass bearing yields compass error, the classic azimuth or amplitude check.
Read definition →Checking compass error against the Sun's calculated true bearing
The true azimuth of the Sun is computed for the observed time and estimated position, using the almanac and sight reduction tables or a calculator, then compared with its bearing taken over the compass with an azimuth mirror or pelorus. The difference is the total compass error, from which deviation follows once variation is applied. An amplitude observation, taken when the Sun's centre is on the visible horizon, is a simplified version of the same check.
Read definition →Looking astern at a mark you have passed to check your track
A bearing taken on an object behind the vessel, ideally one just left astern, gives an immediate check that the intended ground track is being made good. Because the eye can detect small angular changes on a distant object, a back bearing on a departure mark is a sensitive indicator of cross-tide set. It is the reciprocal of the course made good over the ground from that object, not of the course steered.
Read definition →Wind shifting anticlockwise or clockwise around the compass
Backing is a change in wind direction anticlockwise, for example from west to southwest; veering is clockwise, for example from south to southwest. The terms are absolute and identical in both hemispheres. In the Northern Hemisphere an approaching warm front typically backs the wind, while frontal passage veers it, so the sequence of shifts is a reliable clue to where a depression's centre lies relative to the vessel.
Read definition →Suction and steering pull felt near banks or passing ships
Close to a channel bank or another vessel, asymmetric flow creates a pressure field: the bow is pushed away from the near bank while the stern is drawn towards it, so the vessel sheers and needs continuous helm. Between vessels passing or overtaking at close range the low pressure between the hulls draws them bodily together. The effects increase sharply with speed and with reduced depth, and the remedies are early moderate helm and less speed.
Read definition →A shallow ridge of sediment across a harbour or river entrance
Formed where a river's outflow meets the sea and loses velocity, or by longshore drift across an estuary mouth. It usually carries the least depth on the approach and its position and depth can shift after gales or floods, so charted figures may be outdated. In an ebb tide against an onshore swell a bar becomes dangerously steep and breaking, and the safe practice is to cross near high water slack with the swell behind.
Read definition →How much the barometer has risen or fallen recently
Conventionally the change in mean sea level pressure over the past three hours, coded in synoptic reports. A fall of 1 to 2 hPa in three hours suggests a deepening or approaching system; falls of 3 to 5 hPa in three hours indicate a rapidly developing situation and likely gales. A drop of 24 hPa in 24 hours, adjusted for latitude, defines explosive cyclogenesis, often called a weather bomb.
Read definition →Fixed navigation mark standing on the seabed or shore
A beacon is rigidly attached to the ground rather than moored, so its position is exact and it does not swing with tide or wind. Unlit beacons are termed daymarks or perches. Beacons carry the same colours and topmarks as the equivalent buoys, and chart symbols distinguish them by a solid base rather than the sloped buoy symbol. Being fixed, a lit beacon makes a more reliable object for a compass bearing or clearing line than a buoy.
Read definition →A quick fix taken as a landmark passes abeam
When a charted object bears exactly 90 degrees from the heading, the position line runs at right angles to the course, so distance off measured by radar range, vertical sextant angle or an earlier doubling calculation completes the fix at once. It is a strong cut with minimum plotting effort and a natural moment to log position on a coastal passage. The special case of 22.5 degrees then 45 degrees on the bow gives distance run equal to distance off when abeam.
Read definition →Sailing with the true wind coming straight over the side
With the true wind at about 90 degrees to the fore and aft line, sheets are eased so the sails are around halfway out, and the apparent wind still comes noticeably forward of the beam because of the boat's own speed. Heeling force is largely sideways but drive is high, so a beam reach usually produces close to maximum boat speed with an easy motion.
Read definition →Turning the boat away from the wind onto a freer course
Bearing away moves the heading further from the wind, for example from close hauled to a reach, and the sheets must be eased in step with the turn or the sails will stall and the boat will heel excessively. In strong winds the mainsheet is eased first to reduce the turning couple that resists the manoeuvre. Bearing away is often used to build speed after a tack or to open the angle for a gybe.
Read definition →How close two targets can be in bearing yet show separately
Two targets at the same range appear as one echo unless their angular separation exceeds roughly the horizontal beamwidth. With a six degree beam at two nautical miles, targets must be about 0.2 nautical miles apart in the cross-range direction to resolve, since angular separation in radians multiplied by range gives distance. This is the fundamental reason radar bearings are far less accurate than radar ranges and why fixing should favour ranges.
Read definition →A numbered scale describing wind strength by its effect at sea
Runs from force 0 to force 12, with wind speeds specified as ten minute means at 10 metres height. Force 0 is under 1 knot, 1 is 1 to 3, 2 is 4 to 6, 3 is 7 to 10, 4 is 11 to 16, 5 is 17 to 21, 6 is 22 to 27, 7 is 28 to 33, 8 is 34 to 40, 9 is 41 to 47, 10 is 48 to 55, 11 is 56 to 63, and force 12 is 64 knots or more.
Read definition →China's global satellite navigation system, now supported by many chart plotters
BeiDou reached global service with a mixed constellation of medium-Earth-orbit satellites plus inclined geosynchronous and geostationary satellites, the latter giving enhanced coverage and extra services over the Asia-Pacific region. It broadcasts interoperable signals near the GPS and Galileo L1 band, and references the CGCS2000 frame, which is practically equivalent to WGS84 at chart scale. Many current marine receivers track BeiDou alongside GPS to improve fix availability.
Read definition →Planning the route continuously from departure berth to arrival berth
The principle that a passage plan runs from the departure berth to the arrival berth, including pilotage waters at both ends, rather than starting and finishing at the sea buoy. It ensures the harbour approaches, which contain most of the hazards and most of the traffic, are planned on large-scale charts with the same rigour as the ocean legs. Taking a pilot does not remove the master's duty to hold a plan for pilotage waters.
Read definition →A line stopping the boom from crashing across in an accidental gybe
A preventer runs from the boom, ideally from its outer end, forward to a strong point at the bow or a foredeck cleat and back to the cockpit so it can be released under control. It is rigged whenever the boat is running or broad reaching, especially at night or in a seaway. A vang-to-toerail arrangement is weaker and risks bending the boom, so a full-length lead is preferred.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.