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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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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the least depth on the approach and its position and depth can shift after gales or floods, so charted figures may be outdated.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 →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 →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.