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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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A Mercator turned on its side, good for narrow north-south areas
The cylinder is rotated so it touches a chosen central meridian rather than the equator, giving minimal distortion in a north-south strip a few degrees wide. It underpins the Universal Transverse Mercator grid and national systems such as the Ordnance Survey National Grid, and it is used for large scale harbour and coastal charts in higher latitudes where standard Mercator scale distortion would be objectionable. Rhumb lines are no longer straight, but over a single sheet the difference is negligible.
Read definition →Adding up several short legs mathematically to find one net movement
When a vessel sails a series of short courses, as when tacking, each leg is resolved into northerly and easterly components using departure equals distance times sine of course and difference of latitude equals distance times cosine of course. Summing the signed components gives the net difference of latitude and total departure, from which a single equivalent course and distance follow. Departure is converted to difference of longitude by dividing by the cosine of the mean latitude.
Read definition →Single masthead lantern combining sidelights and sternlight on small yachts
A sailing vessel of less than 20 metres may combine her red, green and white lights in one lantern carried at or near the top of the mast, keeping the standard arcs of 112.5, 112.5 and 135 degrees. Height gives excellent range offshore for very little current draw. It must never be shown together with deck-level sidelights, nor when the engine is driving the boat, and it is less useful inshore against shore lighting.
Read definition →An intense tropical low with violent winds circling a calm eye
A warm-core system forming over sea temperatures of about 26 degrees Celsius or more, usually between 5 and 20 degrees latitude. It is classed as a tropical storm at sustained winds of 34 to 63 knots and as a hurricane, typhoon or cyclone at 64 knots and above. Warning signs at sea include a long low swell from an unusual direction, a pressure fall exceeding about 3 hPa below the diurnal normal, and increasing cirrus.
Read definition →The actual wind over the water, unaffected by the boat's motion
Wind speed and direction relative to the surface of the sea, which is what forecasts, the Beaufort scale and synoptic charts describe. It is what an anchored or stationary vessel feels. Modern instruments compute it from apparent wind combined with boat speed and heading, so its accuracy depends on correct calibration of the log, the masthead unit alignment and, for full accuracy over the ground, the tidal stream.
Read definition →The angle between the true wind and the boat's heading
Measured in degrees from 0 dead ahead to 180 dead astern, port or starboard. It is the independent variable in a polar diagram, which tabulates predicted boat speed against true wind angle and true wind speed, and is therefore the basis of performance sailing and routeing software. Unlike apparent wind angle, true wind angle does not change as the boat accelerates, which makes it the meaningful measure for tactical decisions.
Read definition →Using tugs to push, pull or steer a ship alongside
Tugs are engaged for berthing, unberthing and turning where wind, stream or confined water exceed the vessel's own control, particularly on high-sided ships at low speed when the rudder has little effect. They work made fast forward and aft, or pushing on the shoulder and quarter, and their capability is quoted as bollard pull in tonnes. Escort tugs work astern to steer or retard a laden tanker. Speed must be reduced before a tug makes fast.
Read definition →Widened harbour area where large ships can swing round
An area at the head of a channel or off a berth, dredged and widened so that a vessel can be turned, usually with tug assistance and often against a stemmed tidal stream. Its diameter is designed around the longest vessel expected, commonly around twice her length, and it is charted with the maintained depth. Small craft must keep well clear while a large vessel swings, since she may occupy the whole basin and cannot give way.
Read definition →Two measured distances from two landmarks cross to give position
A fix obtained by measuring the distance to two charted objects and plotting each as an arc centred on the object, the vessel lying at the intersection. Ranges usually come from radar, and radar ranges are more trustworthy than radar bearings because horizontal beam width smears bearings by roughly half the beam width while range accuracy is limited mainly by pulse length. Choose objects roughly 60 to 120 degrees apart in relative direction for a good cut, and pick steep, isolated, positively identified targets rather than low sloping shores.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.