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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.
Page 15 of 22
A fix from a radar target's measured distance and direction
Radar measures range far more accurately than bearing, typically to within about one percent of range scale, while bearing error can be one or two degrees because of horizontal beamwidth. The preferred practice is therefore to fix from two or three radar ranges to well-defined isolated features, plotting arcs, rather than relying on a radar bearing. A single range plus a visual bearing of the same object is a common and sound compromise near the coast.
Read definition →Overnight land fog that can drift out over harbours at dawn
Forms over land on clear, calm nights when the ground radiates heat, chilling the air near the surface below its dew point. It does not form over the open sea, but it drifts down rivers and estuaries on a light land breeze and can blanket harbour approaches around sunrise. Moderate wind or daytime heating disperses it, so it usually clears by mid-morning in summer but may persist all day in winter.
Read definition →Crossing bearings taken by radio to shore transmitters or beacons
A fix from bearings of radio transmitters obtained with a direction finding receiver and loop or crossed loop aerial, plotted like visual bearings after applying quadrantal error correction and, over long distances, half convergency. Accuracy is modest, commonly a few degrees, and degrades badly at dawn and dusk through sky wave effects and near coastlines through refraction, so it is treated as an approximate position only. Largely superseded by satellite systems but retained conceptually for homing on distress transmissions and emergency beacons.
Read definition →Tying alongside another moored boat when berths are full
Boats lie side by side with fenders between, bow and stern lines and springs made between hulls, and each vessel additionally running its own head and stern lines ashore so loads are not carried through the inner boat. Masts should be staggered fore and aft to prevent rigging clashing when rolling. Crew crossing another yacht should go forward of the mast, and the inside boat should be consulted about departure times.
Read definition →How close two targets can be in distance yet show separately
Range discrimination is governed by pulse length: two targets on the same bearing merge unless separated by more than half the pulse length in space. A 0.1 microsecond pulse occupies about thirty metres, giving roughly fifteen metres of separation capability, while a 1 microsecond pulse occupies three hundred metres and can only resolve targets about one hundred and fifty metres apart. Display resolution and spot size add a further practical limit.
Read definition →A scanned picture of a paper chart, official but not intelligent
An RNC is a georeferenced digital facsimile of the official paper chart, identical in appearance and content, and carrying the same datum and units as the printed original. The computer cannot interrogate individual features, so no automatic anti-grounding checking is possible. Where ENC coverage does not exist, an ECDIS may operate in raster mode, but only when supported by an appropriate folio of up-to-date paper charts for the area.
Read definition →Judging depth and dangers from the colour and pattern of water
Interpreting water colour, surface texture and breaking patterns to identify shoals, coral heads and channels where charting is poor, standard practice in tropical waters. Deep water appears dark blue, sand shelves pale green or turquoise, coral heads brown or black, and overfalls or tide rips mark shallow patches and strong streams. It requires high sun behind the observer, good polarised glasses and an elevated viewpoint such as the spreaders. It fails with the sun ahead, in cloud or chop, and near sunset, so timing the approach matters.
Read definition →A survey-grade technique giving centimetre positions using a nearby base station
RTK resolves the whole-cycle ambiguity of the satellite carrier wave rather than relying only on code measurements, using a data link from a base station or network correction service. Once ambiguities are fixed, horizontal accuracy of one to two centimetres is achievable, degrading as the baseline from the base extends beyond a few tens of kilometres. It is used for hydrographic survey, dredging and berthing systems rather than routine yacht navigation.
Read definition →The receiver checks its own satellites for a faulty measurement
RAIM uses redundant measurements to test the consistency of the solution: with five usable satellites a receiver can detect that one range is faulty, and with six it can usually identify and exclude the offender. The result is an integrity warning rather than an accuracy improvement, alerting the navigator that the displayed position may not meet the required alert limit. Not all basic yacht receivers implement it.
Read definition →A stream that runs one way then reverses along the same line
Characteristic of narrow channels, estuaries and constrictions where topography confines the flow to a single axis. The stream builds, peaks near mid-flow, decays to slack and then reverses through approximately 180 degrees. Because rate follows an approximately sinusoidal pattern, the Rule of Thirds gives useful hourly estimates: about one third of maximum in the first and last hours, two thirds in the second and fifth, and full rate in the middle two.
Read definition →Reducing sail area as the wind increases
Slab or single-line reefing pulls the mainsail down to a reef cringle at luff and leech, while headsails are reduced by roller furling or by changing to a smaller sail. The seamanlike rule is to reef when the thought first occurs, before the boat becomes overpressed, since excessive heel produces weather helm, leeway and loss of speed. Reefing early also keeps the manoeuvre easy for a short-handed crew.
Read definition →The smooth mathematical Earth shape used instead of the bumpy real one
An oblate spheroid defined by a semi-major axis and a flattening value, approximating mean sea level well enough for coordinate computation. Different ellipsoids fit different regions best, which is why datums proliferated historically: Airy 1830 suits the British Isles, Clarke 1866 suited North America, and WGS84 is a best global fit. Latitude and longitude are meaningless without stating which ellipsoid and datum they refer to.
Read definition →Whether the screen shows movement relative to you or over the ground
In relative motion own ship stays at a fixed point and all echoes move by the vector sum of their motion and the reverse of yours, which shows collision risk directly since a target on a steady bearing tracks straight towards the centre. In true motion own ship travels across the display and land stays still, which is far easier for pilotage and parallel indexing but requires accurate speed and heading inputs to remain correct.
Read definition →Dashed lines fencing off water you may not freely use
Boundaries are drawn as dashed magenta lines with an explanatory abbreviation such as Entry Prohibited, Anchoring Prohibited, Fishing Prohibited or Explosives Dumping Ground. Military practice and firing areas are marked with a similar limit and a reference to a chart note giving broadcast times. Marine nature reserves and no-take zones use comparable outlines. The precise legal status is set out in the chart note or in the relevant volume of the Sailing Directions.
Read definition →Vessel whose work prevents her from manoeuvring normally
A vessel whose nature of work, such as dredging, cable or pipeline work, replenishment at sea or aircraft operations, restricts her ability to keep out of the way. She shows three all-round lights in a vertical line, red over white over red, and by day ball over diamond over ball. Where an obstruction exists she adds two red lights or two balls on the foul side and two green lights or two diamonds on the safe side.
Read definition →A track that crosses every meridian at the same angle
A rhumb line holds a constant true course and therefore spirals toward the pole rather than following the shortest path. It plots as a straight line on a Mercator chart, which is its practical value, and it is the line steered on almost all coastal and moderate-length passages. Over short distances the difference from a great circle is negligible, but on an ocean crossing in high latitudes it can add a significant number of nautical miles.
Read definition →Extensions of high or low pressure between weather systems
A ridge is an elongated extension of an anticyclone, with isobars curving anticyclonically; it brings a temporary lull, backing wind and clearing skies between two depressions. A trough is the opposite, an extension of low pressure with cyclonically curved isobars, often marked as a thick line on charts. Trough passage brings a wind veer, a squall line, and a burst of heavy showers even without a full frontal system.
Read definition →Using a light appearing over the horizon to fix distance off
When a light first appears or disappears at the horizon, the observer is at its geographical range, so a single compass bearing at that instant gives a full fix. Note the eye height, look up the dipping distance table or apply 2.08 times the sum of the square roots of the heights in metres, and lay off the resulting distance along the bearing. Rise and dip the light several times by standing up and crouching to confirm the moment.
Read definition →Fresh river water flowing seaward over denser salt water
In a partially mixed or salt-wedge estuary, buoyant fresh water runs out near the surface while a saline wedge intrudes along the bottom on the flood, giving a two-layer flow that can set in opposite directions at different depths. After heavy rainfall the surface outflow may add a knot or more to the ebb and delay the start of the flood, which matters for deep-keeled yachts and for anchoring in the stream.
Read definition →Marks distinguishing rocks that show, cover, or lurk below
A rock that never covers is shown as an asterisk-like symbol with its height above MHWS; a rock awash at chart datum is a cross-like symbol; an underwater rock with a known depth is a sounding enclosed by dots. A rock dangerous to surface navigation is ringed by a dotted danger line. The abbreviation Co for coral heads and the term isolated danger apply similarly. Give all of them a generous berth, since survey resolution is often poor around rocky ground.
Read definition →Gently tilting the sextant so the body just kisses the horizon
Rotating the instrument slightly about the line of sight makes the body's image swing through an arc; the lowest point of that arc is the moment the sextant is truly vertical. Reading at any other tilt gives an altitude that is too large. The observer adjusts the micrometer until the limb just grazes the horizon at the bottom of the swing, then calls the time to the second for the timekeeper to record.
Read definition →An unexpectedly huge wave far bigger than those around it
Conventionally defined as an individual wave whose height exceeds about twice the significant wave height of the surrounding sea. They arise from constructive interference between wave trains, from focusing by currents such as the Agulhas, and from nonlinear effects within steep seas. Because they are usually preceded by a deep trough, they present a near vertical face; they are rare but well documented and are a design case for offshore vessels.
Read definition →A stream whose direction turns steadily through the compass
Typical of open sea and wide shelf areas away from constraining coastlines, where Coriolis deflection causes the current vector to rotate, generally clockwise in the northern hemisphere, completing a full circuit each tidal cycle. There is no true slack water; instead the rate passes through a minimum on the minor axis. Tidal diamonds offshore therefore show the direction changing progressively hour by hour rather than reversing abruptly.
Read definition →The automatic scan of a planned route for dangers along it
Route check tests every leg of the planned route against the chart database within a user-set corridor half-width, listing every crossing of the safety contour and every prohibited or cautionary area encountered. It is only as good as the corridor width, the safety contour setting and the underlying data quality, and it does not assess tidal height, so a leg that passes the check at high water may still be unusable. Every reported item must be examined individually.
Read definition →A monthly ocean chart of typical winds, currents, ice and storms
Admiralty routeing charts, and the equivalent United States pilot charts, present one sheet per ocean per month showing wind roses with percentage frequency and force, mean surface currents in knots, ice limits, tropical storm tracks, fog and air and sea temperatures, plus recommended routes and great circle distances between main ports. They express climatological averages, not forecasts, and are used for seasonal planning rather than day-to-day decisions.
Read definition →How to behave in and around organised shipping lanes
A vessel using a scheme must proceed in the appropriate lane in the general direction of traffic flow, keep clear of the separation line or zone, and normally join or leave at the termination. A vessel crossing must do so on a heading as nearly as practicable at right angles to the general direction of flow, which means heading, not course made good, so tidal set is not compensated. Vessels under 20 metres and sailing vessels must not impede power-driven vessels following a lane.
Read definition →Which of two sailing boats gives way to the other
When two sailing vessels approach so as to involve risk of collision, the one with the wind on the port side keeps out of the way of the one with the wind on the starboard side. When both have the wind on the same side, the vessel to windward keeps out of the way of the vessel to leeward. If a port-tack vessel cannot determine whether the other is on port or starboard, she must keep out of the way.
Read definition →The boat coming up from behind must keep clear
Any vessel overtaking any other must keep out of the way of the vessel being overtaken, and this rule overrides the other steering and sailing rules, so a sailing vessel overtaking a power-driven vessel gives way. A vessel is overtaking when coming up from more than 22.5 degrees abaft the beam, that is from a position where at night only the sternlight and neither sidelight would be seen. Any subsequent change of bearing does not make her a crossing vessel.
Read definition →Two power vessels meeting bow to bow both turn right
When two power-driven vessels meet on reciprocal or nearly reciprocal courses so as to involve risk of collision, each must alter course to starboard so they pass port to port. The situation exists when a vessel sees the other ahead or nearly ahead, at night seeing both sidelights and the masthead lights in line or nearly in line. If there is any doubt whether the situation exists, the vessel must assume it does and act accordingly.
Read definition →Power vessel with another on its right gives way
When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side keeps out of the way and must, if the circumstances of the case admit, avoid crossing ahead of the other vessel. At night the give-way vessel typically sees the red sidelight of the stand-on vessel, giving rise to the mnemonic that seeing red means stop. The customary avoiding action is a bold alteration to starboard.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.