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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.

All A B C D E F G H I J K L

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51 terms

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Radar Navigation
Racon (Radar Beacon)
A shore station that paints a Morse letter on your radar screen
A racon detects an incoming radar pulse and transmits a coded reply, which appears on the display as a dashed Morse character radiating outward along the bearing of the beacon, starting just beyond the beacon's true position. Racons respond mainly to X-band, so they will not appear on an S-band-only picture unless dual band. They mark landfall buoys, bridge spans and offshore structures, and the code is printed against the symbol on the chart.
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IALA Buoyage
Racon Fitted to a Buoy
Radar beacon that paints a coded mark on your radar screen
A racon responds to an incident marine radar pulse by transmitting a Morse-coded signal that appears on the display as a dashed line running radially outward from just beyond the buoy's echo. It typically operates on both 3 cm and 10 cm bands and is annotated on the chart with a magenta circle and the word Racon, sometimes with the Morse letter. Racons are commonly fitted to landfall buoys, offshore structures and important cardinal marks.
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Radar Navigation

Radar Horizon

The distance to the horizon for radar waves, slightly beyond the visual one

Radar energy is refracted slightly more than light in the standard atmosphere, so the radar horizon extends about six per cent further than the optical one. A useful approximation with heights in metres is 2.2 times the sum of the square roots of scanner height and target height, giving nautical miles. A scanner at 9 m detecting a 25 m cliff top gives roughly 2.2 times 8, about 17.6 nautical miles.

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Position Fixing

Radar Range and Bearing Fix

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.

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Meteorology for Navigators

Radiation Fog

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.

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Position Fixing

Radio Direction Finding Fix (RDF Fix)

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.

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Anchoring & Mooring

Rafting Up

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.

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Radar Navigation

Range Discrimination

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.

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Electronic Charts & ECDIS

Raster Navigational Chart (RNC)

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.

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Coastal Piloting Techniques

Reading the Water (Eyeball Navigation)

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.

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GNSS & Satellite Navigation

Real-Time Kinematic (RTK)

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.

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GNSS & Satellite Navigation

Receiver Autonomous Integrity Monitoring (RAIM)

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.

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Tidal Streams & Currents

Rectilinear Stream

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.

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Points of Sail & Sailing Manoeuvres

Reefing

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.

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Chart Projections & Datums

Reference Ellipsoid

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.

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Radar Navigation

Relative and True Motion Display

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.

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Chart Symbols & Abbreviations

Restricted and Prohibited Area Limits

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.

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COLREGs - Lights, Shapes & Sound Signals

Restricted in Ability to Manoeuvre (RAM)

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.

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Chart Projections & Datums

Rhumb Line (Loxodrome)

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.

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Meteorology for Navigators

Ridge and Trough

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.

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Lights & Light Characteristics

Rising and Dipping Distance

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.

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Tidal Streams & Currents

River Discharge and Density Current

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.

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Chart Symbols & Abbreviations

Rock Awash and Underwater Rock Symbols

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.

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Celestial Instruments & Sight Reduction

Rocking the Sextant (Swinging the Arc)

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.

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Wind, Waves & Sea State

Rogue Wave (Freak Wave)

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.

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Tidal Streams & Currents

Rotary Stream

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.

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Electronic Charts & ECDIS

Route Check

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.

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Great Circle & Ocean Routing

Routeing Chart

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.

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COLREGs - Steering & Sailing Rules

Rule 10 - Traffic Separation Schemes

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.

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COLREGs - Steering & Sailing Rules

Rule 12 - Sailing Vessels

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.

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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.