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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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An adjustable ring you expand to measure the distance to a target
The VRM is an electronically generated circle centred on own ship whose radius is displayed numerically, and it is the most accurate measurement a radar offers. The correct technique is to touch the ring against the nearest edge of the target echo, not its centre. Most sets provide two independent markers, allowing a radar fix from two or three ranges to well-identified charted features, which is generally superior to a fix by bearings.
Read definition →The angle between true north and magnetic north at your position
Also called magnetic declination ashore, variation is the local difference between the direction of the Earth's magnetic field and the geographic meridian. It is named East when magnetic north lies clockwise of true north and West when anticlockwise. Values range from zero near an agonic line to more than 30 degrees in some regions and towards very large values near the poles. Variation changes slowly with time, typically a few minutes of arc per year, so charted values need updating.
Read definition →The three-sided plot combining boat movement, tide and resulting track
All dead reckoning problems reduce to one triangle whose sides are the water track, the tidal stream vector and the ground track. Any two known sides determine the third. Constructed for the EP, the tide vector is added to the end of the water track; constructed for course to steer, the tide vector is laid first from the start position. All three vectors must cover the same time period, and all directions must be in degrees true.
Read definition →How fast you are really progressing towards the wind or a mark
VMG is the component of boat speed resolved along the direction of interest, so it equals boat speed multiplied by the cosine of the angle between heading and that direction. Sailing at six knots at 45 degrees to the true wind yields about 4.2 knots of upwind VMG. Pinching raises the angle but kills speed, so the best upwind and downwind angles are those which maximise VMG, taken from the boat's polar.
Read definition →The highest latitude point reached on a great circle track
Every great circle has two vertices, in opposite hemispheres and 180 degrees of longitude apart. At the vertex the track runs due east or due west, so the course is exactly 090 or 270 degrees true. Vertex latitude follows from Napier's rules as cos(latitude of vertex) equals cos(departure latitude) multiplied by sin(initial course). Checking the vertex before departure shows immediately whether ice or storm limits will be breached.
Read definition →Figures showing how much room there is under or between structures
Vertical clearance is printed in brackets against a bridge, overhead cable or pipeline, in metres above MHWS, so an entry reading 18m gives the least safe air draught at spring high water. Overhead power cables carry a separate safe clearance allowing for electrical arcing, usually less than the physical clearance. Horizontal clearance between bridge piers is given in metres between the vertical faces. Always add a personal margin for masthead instruments and swell.
Read definition →Distance off found by measuring a lighthouse's height as an angle
Measure the angle subtended between the light or summit and the waterline or base, then compute distance from distance in nautical miles equals 1.854 times height in metres divided by the angle in minutes of arc. Charted elevations are given above MHWS on British charts, so at low water the true height is greater and the computed distance slightly short, which errs safely. Traditional tables such as Norie's give the same result directly. Practical range is roughly 0.5 to 5 nautical miles.
Read definition →Very rapid flash, faster version of the quick light
A very quick light flashes at 80 to 159 flashes per minute, conventionally 100 or 120. It carries exactly the same meanings as Q on cardinal marks but on a shorter overall period: VQ for north, VQ(3)5s east, VQ(6) plus long flash 10s south and VQ(9)10s west. Ultra quick, UQ, at 160 or more flashes per minute, is rarer and appears on some offshore structures and lightfloats where extreme conspicuity is wanted.
Read definition →Shapes and lights shown by a vessel stuck on the bottom
A vessel aground exhibits three black balls in a vertical line by day, and by night the lights prescribed for a vessel at anchor together with two all-round red lights in a vertical line. In restricted visibility she gives three separate and distinct strokes on the bell immediately before and after the rapid ringing of the anchor signal. Vessels of less than 12 metres are not required to exhibit the aground lights and shapes.
Read definition →Deep ship that cannot safely leave the dredged channel
A power-driven vessel which, because of her draught in relation to the available depth and width of navigable water, is severely restricted in her ability to deviate from the course she is following. In addition to the normal lights for a power-driven vessel she may show three all-round red lights in a vertical line, or a black cylinder by day. The status does not exist in United States inland rules.
Read definition →Shore radio service that organises shipping in busy waters
A shore-based service using radar, AIS, cameras and VHF to monitor and organise traffic in a port or its approaches. Levels of service run from information broadcasts, through navigational assistance on request, to traffic organisation with instructions on movement, timing and sequence. Participating vessels report at charted positions with the required particulars and keep watch on the working channel, but the master remains responsible for the safe navigation of his vessel throughout.
Read definition →The international voice distress, urgency and calling channel
Channel 16 operates on 156.800 MHz and is the international frequency for distress, urgency and safety voice traffic, and for initial calling where dedicated calling channels are not in use. A continuous listening watch is maintained by coast stations and is expected of vessels at sea. Routine conversation is shifted to a working channel immediately, and periods of radio silence may be imposed while distress traffic is in progress.
Read definition →The digital-only channel that carries automatic distress alerts
Channel 70 works on 156.525 MHz and is reserved exclusively for digital selective calling; voice transmission on it is prohibited. DSC-capable sets keep an automatic watch on this channel whenever powered, so distress, urgency, safety and routine calls are received even when the operator is monitoring a working channel. Once an alert is exchanged, the subsequent voice communication moves to channel 16 or a designated working channel.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.