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The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.

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Passage Planning
Landfall Waypoint
Safe offshore position for arriving at an unfamiliar coast
A position placed well offshore in deep water and clear of dangers, from which the approach to the coast begins. It is chosen so that the first expected sighting, a powerful light, a radar-conspicuous headland or a landfall buoy, can be identified and confirmed before committing to the entry. Approaching down a charted leading line, or deliberately offset to one side so the direction of error is known, is safer than aiming at the entrance itself.
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Chart Projections & Datums
Latitude Scale as Distance Scale
Measuring miles using the numbers up the side of the chart
One minute of latitude equals one nautical mile by definition, so the vertical border of a Mercator chart is the distance scale. Because the projection stretches latitude with the secant of latitude, dividers must be applied at the mid-latitude of the leg being measured, and a long leg is measured in short steps. The longitude scale along the top and bottom is never used for distance, since a minute of longitude shortens with cosine of latitude.
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Points of Sail & Sailing Manoeuvres

Layline

The line from which you can just fetch the mark in one board

A layline is the track, on either tack, that leads directly to a windward mark or harbour entrance without needing another tack. It is drawn on the chart or plotter using the boat's close hauled angle corrected for tidal stream and leeway. Overstanding the layline wastes distance sailed, while tacking short means an extra pair of tacks, so laylines are approached with a margin in shifty or tide-swept water.

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Depth Sounding & Bathymetry

Lead Line

A weighted marked line lowered by hand to measure depth

The traditional hand lead of about 3 to 4 kilograms on a marked line, still valuable as an independent check and for sounding from a tender. Traditional marks lie at 2, 3, 5, 7, 10, 13, 15, 17 and 20 fathoms, intervening depths being deeps. A hollow in the base can be armed with tallow to bring up a sample of the bottom, giving the nature of the ground for anchoring or for a line of soundings fix.

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Lights & Light Characteristics

Leading Lights

Two lights in line giving a precise track to steer

Also called range lights in United States practice, they are two lights at different heights and distances; when the higher rear light appears directly above the lower front light the vessel is on the leading line, whose true bearing is printed on the chart. Sensitivity is high because a small lateral error swings the lights apart visibly. The nearer front light swings further, so if the rear light appears to the right of the front you are right of track and must alter to port: steer towards the lower light.

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Coastal Piloting Techniques

Leading Line (Leading Marks, Range Line)

Two marks kept in line ahead guide you safely down a channel

A charted pair of fixed marks, one low and near and one higher and further back, that when kept in line define a safe approach track through a channel or over a bar. The chart states the leading line's true bearing, for example 047 degrees, and the marks are frequently lit for night use. Sensitivity to lateral error increases with the separation of the marks and decreases with distance off, so a leading line is most precise close in and should be abandoned beyond its charted limit.

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Pilotage & Harbour Approach

Leading Line (Leading Marks)

Charted pair of marks kept in line to stay safe

A pair of marks or lights, the rear one higher and further inland, deliberately established so that keeping them in line holds a vessel on a safe charted approach bearing, printed in degrees true and given from seaward. If the rear mark appears to open to the right of the front mark, the vessel has moved left of the line, so she alters towards the rear mark to regain it. Lit pairs allow the same approach at night.

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Depth Sounding & Bathymetry

Least Depth

The shallowest water found over a wreck, rock or shoal

Charted as a sounding over the obstruction, often with a wreck or rock symbol, and it governs whether a vessel can safely pass. A wreck whose least depth has been determined by sweeping or diving is shown differently from one whose clearance is unknown, the latter being surrounded by a danger line and treated as a hazard at any draught. Always compare least depth plus predicted tide against draught with a working margin.

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Tidal Streams & Currents

Lee-bowing the Tide

Sailing so the stream pushes you up to windward

A tactic when beating: choose the tack on which the stream strikes the leeward bow, so its component acts to windward and improves the ground track made good toward an upwind mark. It also shifts the apparent wind slightly forward, which can be exploited. The benefit is real but often overstated for a single tack; over a whole beat what matters is the total ground made good against the mark.

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Dead Reckoning & Estimated Position

Leeway

The sideways slip of the boat pushed downwind by the wind

Leeway is the angular difference between the heading and the water track, caused by the wind's lateral force on hull, rig and topsides. It is estimated by eye from the angle between the fore and aft line and the wake, or from experience of the individual boat. Typical values for a cruising yacht are near zero when running, 5 degrees close reaching and 10 degrees or more beating in fresh conditions with reduced sail. It is applied downwind of the heading.

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Passage Planning

Leg

Single straight section of the route between two waypoints

The straight track between consecutive waypoints, described by a course and a distance from which a running time follows at the planned speed; a 12 mile leg at 6 knots takes two hours. Each leg carries its own notes: expected tidal stream and the resulting course to steer, least charted depth, the cross-track limit permitted, dangers abeam and their clearing bearings, and the marks or lights that should come into view along it.

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Lights & Light Characteristics

Light Characteristic

The full description of how a light flashes, coloured and timed

The characteristic is the shorthand printed against every lit mark on a chart and in the light list. It comprises rhythm, group number if any, colour, period in seconds, elevation in metres and nominal range in nautical miles. For example Fl(3)WR 15s 21m 18/14M means three white or red flashes every 15 seconds, focal plane 21 metres above MHWS, visible 18 miles white and 14 miles red. Identify by timing the full period with a stopwatch.

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Chart Symbols & Abbreviations

Light Description Abbreviation

The compact code beside a light giving its full characteristics

The sequence is character, colour, period, elevation and range, for example Fl(3)WR.15s24m18/14M. That reads as a group flashing light showing three flashes, with white and red sectors, repeating every 15 seconds, elevation 24 metres above MHWS, nominal range 18 miles white and 14 miles red. Elevation is measured to MHWS on British charts and to mean high water on US charts, which matters when computing rising and dipping distances.

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Lights & Light Characteristics

Light Vessel and Lanby

Floating platforms carrying major lights offshore

A light vessel is a purpose-built moored hull with a lantern on a mast; a Lanby is a large automatic navigational buoy, typically around 12 metres in diameter, deployed where a light vessel is uneconomic. Both are shown on the chart as a floating symbol with a magenta flare, and both may carry a racon, AIS transmitter and fog signal. Because they lie to a mooring, they swing and their position tolerance exceeds that of a fixed structure.

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Lights & Light Characteristics

Lighthouse

Tall shore structure carrying a major light for landfall

A major light structure with a high focal plane, typically 20 to 60 metres, and a nominal range often exceeding 20 miles. Charted with a magenta flare and the full characteristic. Daymark colours and bands are given in the Admiralty List of Lights, which is the authoritative source, more detailed and more frequently corrected than the chart. Many are now unmanned, monitored remotely, and increasingly LED-equipped, which can reduce the loom formerly seen from incandescent lamps.

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Great Circle & Ocean Routing

Limiting Latitude

The highest latitude a passage plan allows the vessel to reach

A limiting parallel is imposed by ice, expected gale frequency, sea state, charterparty or insurance conditions, or company standing orders. If the calculated great circle vertex lies poleward of it, the plan is converted to a composite great circle so the track just touches the parallel and no further. The chosen value is recorded in the passage plan and monitored during the crossing as forecasts change.

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Coastal Piloting Techniques

Line of Soundings

Matching a run of echo sounder depths to the chart to find position

Record depths at regular intervals along a steady course, reduce each to chart datum by subtracting the height of tide, and plot the sequence to the chart scale on tracing paper or the plotter's edge. Slide the strip over the chart along the course line until the pattern matches the charted contours. It works best where the seabed has distinctive relief and gives a poor result on flat or featureless bottoms.

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Great Circle & Ocean Routing

Load Line Zones

Sea areas that set how deeply a ship may be loaded seasonally

The International Convention on Load Lines divides the oceans into tropical, summer, winter and winter North Atlantic zones, some seasonal by date, shown on load line zone charts. A ship's permissible draught changes as it crosses a boundary, so the intended track and dates directly affect cargo intake and bunkering. Ocean route selection must therefore respect zone limits as well as distance and weather.

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Celestial Navigation Concepts

Local Hour Angle (LHA)

How far west of your own meridian the body lies

Computed as GHA minus longitude west, or GHA plus longitude east, adding or subtracting 360 degrees to keep the result in range. LHA is one of the three entering arguments to the sight reduction formula, along with declination and latitude. When using tabular methods such as NP401 or HO229, the assumed longitude is deliberately chosen so that LHA becomes a whole degree, which removes one interpolation from the work.

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Compass & Magnetism

Local Magnetic Anomaly

A patch of seabed geology that pulls the compass off

Iron-rich rock, ore bodies and volcanic formations can distort the local field by several degrees or occasionally tens of degrees over limited areas. Charts annotate such areas as Local Magnetic Anomaly, sometimes with the observed extent, for example in parts of the Baltic, around Iceland and off western Australia. Since the effect changes with position rather than heading, it cannot be corrected by adjusting the compass, and navigators should cross check with GNSS course or transits when passing through.

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Speed, Log & Distance

Log Calibration

Adjusting the log so it reads the correct distance and speed

Run a known distance in both directions to cancel tide, using a measured mile with transit marks or a GNSS-derived leg in calm water, and compare log distance with actual. The correction percentage is the actual distance divided by logged distance, multiplied by one hundred. Average the reciprocal runs rather than trusting one. Repeat in similar loading and trim, since heel and speed affect the sensor's position within the hull's boundary layer.

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Dead Reckoning & Estimated Position

Log Reading and Distance Run

The mileage counter used to measure how far you have sailed

The distance run comes from the ship's log, a paddlewheel, impeller or electromagnetic sensor measuring speed through the water and integrating it to a totalising distance. Log readings should be recorded at every fix, course change and watch change so any interval can be reconstructed. Impeller logs foul quickly with weed and growth and under-read progressively, so the log should be calibrated against a measured distance run in both directions to cancel tidal effect.

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Lights & Light Characteristics

Long Flashing (LFl)

Light with a single flash lasting two seconds or more

A long flash is defined as a flash of not less than two seconds duration. LFl 10s, one long white flash every ten seconds, is the classic landfall and safe water buoy rhythm, chosen because a long flash is easier to pick up at extreme range and against a lit shoreline than a short one. The long flash is also the terminal element in the south cardinal rhythms Q(6)+LFl 15s and VQ(6)+LFl 10s.

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AIS & Vessel Tracking

Long Range Identification and Tracking (LRIT)

Mandatory reporting scheme where ships send position to flag authorities

Established under SOLAS Chapter V Regulation 19-1 and applying to passenger ships, cargo ships of 300 gross tonnage and upwards on international voyages and mobile offshore drilling units. Equipment transmits identity, position and time normally four times a day via satellite communications to a data centre. Unlike AIS the reports are not broadcast openly, access being restricted to flag, port and coastal states within defined distances, which makes LRIT a security and SAR tool rather than a collision avoidance aid.

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Electronic Charts & ECDIS

Look-Ahead Guard Zone

A watched box ahead of the ship that triggers grounding alarms

The look-ahead is a sector or rectangle projected ahead of own ship, set by time or distance and by a width appropriate to the vessel and the waters. The system continuously interrogates the SENC within it and warns of crossing the safety contour or of areas with special conditions such as traffic separation zones or restricted areas. Setting it too short in pilotage waters removes useful warning time; setting it too wide in narrow channels causes alarm fatigue.

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Lights & Light Characteristics

Loom of a Light

Glow of a light seen before the light itself appears

The loom is the diffuse glow scattered by haze, cloud or the atmosphere above a powerful light that is still below the horizon. Sweeping loom from a rotating lantern can be detected many miles beyond the geographical range and is valuable for early landfall recognition, but it must not be used as a rising and dipping observation because the geometry is undefined. It can, however, be timed to confirm the period of the characteristic.

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Celestial Instruments & Sight Reduction

Lower and Upper Limb Sights

Choosing the bottom or top edge of the sun or moon to measure

The sextant cannot measure to the centre of a disc, so an edge is used and the semi-diameter of about 16 minutes is applied. Lower limb sights add the semi-diameter and upper limb sights subtract it, with the almanac's altitude correction tables carrying separate columns for each. Lower limb is normal for the sun; upper limb becomes necessary for a partly illuminated moon or when cloud obscures the lower edge.

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Chart Projections & Datums

Lowest Astronomical Tide (LAT)

The lowest tide level that astronomy alone can ever produce

LAT is the lowest predicted level under any combination of astronomical conditions and average meteorological conditions, derived from analysis of at least 19 years of predictions to cover the lunar nodal cycle. Adopted by the IHO as the standard chart datum, it guarantees that predicted heights of tide are almost always positive. Meteorological effects such as high barometric pressure and strong offshore wind can still take the actual level below LAT, occasionally by half a metre or more.

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Points of Sail & Sailing Manoeuvres

Luff Up

Turning the bow closer to the wind, sheeting in as you go

Luffing brings the heading nearer the wind, and sheets must be trimmed in as the turn progresses or the sails will flog. It is used to work back to windward, to spill wind in a gust as a temporary depowering measure, or to slow the boat when approaching a mooring or a person in the water. Luffing too far takes the boat into the no-go zone and stops it.

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Lights & Light Characteristics

Luminous Range

Actual visible distance of a light in the weather at hand

Luminous range is the nominal range corrected for the prevailing meteorological visibility, taken from the luminous range diagram in the Admiralty List of Lights. A light of nominal range 20M may show only 8M in visibility of 3 miles, or over 25M on an exceptionally clear night. The distance a light is actually seen is the lesser of luminous and geographical range, unless loom or refraction extends detection beyond the horizon.

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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.