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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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Rigid flag A replica warning that a diver is down
A vessel engaged in diving operations which cannot display the full lights and shapes of a vessel restricted in her ability to manoeuvre exhibits a rigid replica of the International Code flag A, white and blue swallow-tailed, not less than one metre high, with all-round visibility. It means pass well clear at slow speed. The red flag with a white diagonal stripe is a United States and recreational convention, not a signal under the collision regulations.
Read definition →Getting through the calm belt near the equator as quickly as possible
The Intertropical Convergence Zone separates the two trade wind systems and brings light fluky airs, squalls and heavy rain. Its width and latitude vary seasonally and by longitude, and satellite imagery, forecast files and other vessels' reports are used to find the narrowest section. The usual tactic is to cross close to due south or north on the shortest line, motoring if fuel allows, rather than seeking wind by detouring.
Read definition →Log using sound frequency shift to measure speed over ground
Transmits acoustic beams, typically at several hundred kilohertz to a few megahertz, and measures the frequency shift of returns. In bottom track mode, available while the seabed is within acoustic range, often up to a couple of hundred metres, it gives true speed over ground, and dual-axis units also give athwartships speed at bow and stern for berthing. Beyond bottom track range it switches to water track, measuring speed relative to a layer of water.
Read definition →A tide that pauses or peaks twice near the top of the curve
Caused by shallow-water overtide constituents, chiefly the quarter-diurnal M4 and sixth-diurnal M6, distorting the sinusoidal shape. Southampton experiences a prolonged stand or double high water, and Portland a double low water. Because the standard Rule of Twelfths assumes a sinusoid, it must not be applied at such ports; the Admiralty publishes special curves keyed to low water rather than high water for these places.
Read definition →A distance-off trick using two relative bearings, one twice the other
Note the distance run between the moment an object bears at a relative angle and the moment that relative angle has doubled. The resulting triangle is isosceles, so the distance run equals the distance from the object at the second bearing. Example: with the object 30 degrees on the bow, run until it is 60 degrees on the bow; if 2.4 nautical miles were logged, the object is then 2.4 nautical miles off. It assumes a steady course and negligible current.
Read definition →Five short rapid blasts meaning your intentions are unclear
When a vessel in sight of another doubts whether sufficient action is being taken by the other to avoid collision, or fails to understand her intentions, she sounds at least five short and rapid blasts on the whistle. This may be supplemented by five or more short rapid flashes of an all-round white light. It is a question, not an insult, and should prompt both vessels to reassess the risk of collision immediately.
Read definition →A code from 0 to 9 describing how rough the sea is
Codes wind-generated sea by significant wave height: 0 calm glassy with no waves, 1 calm rippled up to 0.1 metres, 2 smooth 0.1 to 0.5, 3 slight 0.5 to 1.25, 4 moderate 1.25 to 2.5, 5 rough 2.5 to 4, 6 very rough 4 to 6, 7 high 6 to 9, 8 very high 9 to 14, and 9 phenomenal over 14 metres. A separate Douglas code describes swell independently of the wind sea.
Read definition →The anchor slipping across the bottom instead of holding
Detected by transits opening, GPS track drifting downwind in a smooth line, unexpected depth change, or a chain that jerks and rumbles rather than lying steady. Remedies in order are veering more cable, reducing windage, running the engine ahead to ease the load, and finally recovering and re-laying, ideally in better holding. Dragging most often follows a tide turn, a wind shift, or a rushed initial set.
Read definition →How fast the tidal stream is running, in knots
British practice calls the speed of the tidal stream the rate, given in knots and often in tenths, while drift more strictly means the total distance the stream carries the vessel over a stated interval. US practice uses drift for the speed itself. Chart diamonds tabulate rate in separate spring and neap columns for each hour before and after high water at the reference port, and intermediate values are interpolated by the computation of rates diagram.
Read definition →How far a bank or rock uncovers above the water at low tide
Charted as an underlined figure, in metres and decimetres, measured upward from chart datum. A drying height of 1 underlined 8 means the feature stands 1.8 m above chart datum, so with a height of tide of 2.5 m there is 0.7 m of water over it. The associated drying area is usually tinted green and bounded by the low water line. American charts use the same underlining convention.
Read definition →The time from low water to the following high water
Used in secondary port work where only high water differences are tabulated, and as a check on tabulated times. Around north-west Europe duration is commonly close to six hours but often asymmetric, with a shorter flood and longer ebb in rivers carrying freshwater discharge. Where duration departs markedly from six hours, the Rule of Twelfths should be scaled to that duration or abandoned in favour of the printed curve.
Read definition →The constantly updated position and motion information in AIS reports
Comprises latitude and longitude with a position accuracy flag, course over ground, speed over ground, true heading from a gyro or satellite compass, rate of turn in degrees per minute, UTC second of the report and navigational status. Speed over ground resolution is 0.1 knots and course over ground 0.1 degrees. Note that heading and course over ground differ whenever tide or leeway acts, so a target's aspect may not match its track.
Read definition →Treats tides as waves shaped by ocean basins and Earth's rotation
Laplace's approach models the tide as a forced long wave responding to the tide-generating potential but constrained by basin geometry, depth, friction and Coriolis deflection. It explains why real tides propagate as progressive and standing waves, why the range varies enormously between coasts, and why high water rarely coincides with the Moon's transit. Resonance occurs where a basin's natural period approaches the forcing period, as in the Bay of Fundy.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.