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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.

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Wind, Waves & Sea State
Named Local Winds (Mistral, Bora, Meltemi)
Regional winds with local names and predictable behaviour
Regionally recognised winds arising from consistent terrain and pressure patterns. The Mistral is a cold, dry, often gale force northwesterly funnelling down the Rhone valley into the Gulf of Lion; the Bora is a squally northeasterly falling from the Dinaric mountains onto the eastern Adriatic; the Meltemi is the persistent northerly of the Aegean in summer. Sailing directions and pilot books describe onset signs, duration and safe anchorages for each.
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COLREGs - Lights, Shapes & Sound Signals
Narrow Channel and Bend Signals
Whistle signals for overtaking in channels and rounding blind bends
A vessel intending to overtake in a narrow channel or fairway sounds two prolonged blasts followed by one short blast to pass on the other vessel's starboard side, or two prolonged followed by two short to pass on her port side. The vessel to be overtaken indicates agreement with one prolonged, one short, one prolonged, one short. Nearing a bend where vision is obscured, sound one prolonged blast, answered by one prolonged from any vessel beyond.
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Chart Projections & Datums

Natural Scale and Scale Bar

How much the chart shrinks reality, expressed as a ratio

Natural scale is stated as a representative fraction such as 1:75000, meaning one unit on the chart equals 75000 on the ground, quoted for a stated latitude because Mercator scale varies. Small scale charts cover large areas with little detail; large scale charts show small areas in detail. On a Mercator chart distance is always measured on the latitude scale at the side, at the same latitude as the leg being measured, never on the longitude scale.

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Depth Sounding & Bathymetry

Nature of the Seabed

Chart abbreviations describing what the bottom is made of

Encoded with standard abbreviations such as S for sand, M for mud, Cy for clay, G for gravel, P for pebbles, St for stones, R for rock, Co for coral, Sh for shells and Wd for weed, sometimes qualified by f for fine, c for coarse, so for soft, h for hard and by colour. It determines holding for an anchor, mud and firm sand generally holding well while rock, weed and shingle hold poorly, and it also affects sonar and radar returns.

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Chart Symbols & Abbreviations

Nature of the Seabed Abbreviations

Short letter codes telling you what the bottom is made of

Standard abbreviations include S sand, M mud, Cy or Cl clay, G gravel, Sh shells, St stones, R rock, Co coral, Wd weed and Oz ooze. Qualifiers precede them, so f fine, m medium, c coarse, so and h soft and hard, and colour codes such as bk black or gy grey. These entries matter chiefly for anchoring, since mud and sand hold well while weed, rock and coral give poor holding or may foul the anchor.

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Nautical Publications

Nautical Almanac

The yearly book of star and sun positions used for sight reduction

The official Nautical Almanac, published jointly in the United Kingdom and United States, tabulates the Greenwich Hour Angle and declination of the Sun, Moon, planets and Aries for every hour of Universal Time, with increments and corrections for minutes and seconds, plus star data, twilight and sunrise tables and altitude correction tables. Yacht almanacs such as Reeds combine reduced astronomical data with tidal, port and radio information for a single region.

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Speed, Log & Distance

Nautical Mile (M or NM)

The standard distance unit at sea, close to one minute of latitude

Defined internationally as exactly 1852 metres. It originates from one minute of arc of a great circle on the earth, and because the earth is an oblate spheroid the length of a latitude minute varies from about 1843 metres at the equator to about 1862 metres at the poles. On a Mercator chart distance is therefore always measured on the latitude scale at the side of the chart, level with the area of interest, never on the longitude scale.

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Celestial Navigation Concepts

Nautical Twilight

The window when both stars and the horizon are visible for sights

Defined by the sun's centre being between 6 and 12 degrees below the horizon. During this period the brighter navigational stars and planets are visible while the sea horizon remains sharp enough to measure against, which is the only practical time for star sights. The almanac tabulates the beginning of morning and end of evening nautical twilight by latitude and date; the usable window shrinks toward the tropics and lengthens in high latitudes.

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AIS & Vessel Tracking

Navigational Status

A manually set code saying what the vessel is currently doing

A Class A field selected by the watchkeeper from a defined list including under way using engine, at anchor, not under command, restricted in ability to manoeuvre, constrained by draught, moored, aground, engaged in fishing and under way sailing. It should agree with the lights and shapes displayed and drives some ECDIS symbology and reporting rates. Because it is set by hand it is often left unchanged after berthing or anchoring, so treat it as advisory.

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Nautical Publications

Navigational Warnings (NAVAREA, Coastal and Local)

Urgent broadcast messages about new hazards or changed marks

The World-Wide Navigational Warning Service divides the oceans into NAVAREAs, each with a coordinator issuing long range warnings, supplemented by coastal warnings for national waters and local warnings for harbours. Subjects include failed lights, drifting objects, new wrecks, buoys off station and military exercises. Delivery is by NAVTEX on 518 kHz in English, by SafetyNET over satellite, and by VHF broadcast. Warnings supersede printed publications until formally cancelled or incorporated.

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Emergency Navigation & Distress

NAVTEX

An automatic printer or screen receiving coastal safety broadcasts

NAVTEX delivers maritime safety information by narrow-band direct printing on 518 kHz for international English language broadcasts, with 490 kHz used for national language services and 4209.5 kHz in some tropical areas. Messages carry a station letter and a subject character, so a receiver can be set to reject categories such as pilot or Loran messages, though navigational, meteorological warnings and search and rescue information cannot be rejected. Coverage is typically several hundred miles.

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Neap Tides

The smallest tidal ranges, near first and last quarter Moon

At quadrature the solar and lunar contributions oppose, so M2 and S2 partly cancel and range falls to a minimum, often around a third to a half of the spring range in European ports. High water is lower and low water higher than at springs, and the times of high water shift relative to springs. Neaps present the greatest risk of a vessel that grounds remaining stranded through subsequent tides.

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Tidal Calculations

Negative Surge Allowance

Reducing your expected depth when weather is pushing the sea down

High barometric pressure, typically above 1013 hPa, and persistent offshore winds can lower observed level below prediction, occasionally by more than a metre in shallow shelf seas such as the southern North Sea. Apply roughly 0.01 m per hectopascal above 1013 as a first approximation, then add a further allowance for wind set-down. National authorities issue negative surge warnings; deep-draught vessels and shoal-draught passages should treat these as operational limits.

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GNSS & Satellite Navigation

NMEA Position Sentence (GGA and RMC)

The standard data messages that carry your fix to other instruments

NMEA 0183 sends comma-separated sentences over a serial link, commonly at 4800 baud. GGA carries time, latitude, longitude, fix quality, satellites used, HDOP and antenna height, while RMC gives time, date, position, speed and course over ground, and magnetic variation. NMEA 2000 replaces this with a 250 kbit/s CAN bus carrying binary parameter groups, allowing multiple talkers and listeners on a single backbone with far greater throughput.

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Passage Planning

No-Go Area

Shaded chart area the vessel must never enter

An area highlighted on the chart during planning that the vessel cannot enter at any state of tide, either because charted depth plus the least expected height of tide is less than draught plus the required under-keel clearance, or because of wrecks, rocks, obstructions or prohibited zones. Shading them makes the safe water obvious at a glance. Areas passable only above a certain tidal height are marked separately as conditional rather than no-go.

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Points of Sail & Sailing Manoeuvres

No-Go Zone

The arc either side of the wind where sails will not drive

Within roughly 40 to 45 degrees either side of the true wind direction for a typical cruising yacht, the sails cannot generate lift and simply flap, so the boat stops. The total blocked sector is therefore about 80 to 90 degrees wide, narrower for a performance keelboat and wider for a heavy long-keeled vessel. Ground to windward is gained only by tacking around the edges of this sector.

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Nodal Cycle (18.6-Year Cycle)

A slow 18.6-year wobble in the Moon's orbit that alters tidal range

The regression of the lunar nodes causes the Moon's maximum declination to vary between roughly 18.3 and 28.6 degrees over 18.61 years. This modulates constituent amplitudes by a few per cent, increasing M2 slightly when declination range is small and strengthening the diurnal constituents when it is large. Prediction software applies nodal factors f and u to each constituent for the year concerned, which is why very old tide tables degrade.

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Lights & Light Characteristics

Nominal Range

How far a light can be seen in average clear weather

Nominal range is the luminous range of a light in a homogeneous atmosphere of meteorological visibility 10 nautical miles, and is the figure printed on British Admiralty charts, for example 18M. It depends only on intensity, not on height or on the observer's eye. In visibility better than 10 miles the actual luminous range is greater; in haze or rain it can be far less, and the luminous range diagram in the light list converts between them.

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North Cardinal Mark

Pass to the north of it to stay in safe water

Topmark is two black cones, both points upward. The body is black above yellow, matching the upward-pointing cones. The light is white continuous quick flashing, Q, or very quick flashing, VQ, with no group and no interruption; typical rates are around 50 to 60 flashes per minute for Q and 100 to 120 for VQ. Safe water lies to the north, so a vessel should pass with the mark to the south of it.

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COLREGs - Lights, Shapes & Sound Signals

Not Under Command (NUC)

Vessel unable to manoeuvre because of some exceptional breakdown

A vessel which through exceptional circumstance, such as steering gear or main engine failure, is unable to manoeuvre as the rules require and so cannot keep out of the way. She exhibits two all-round red lights in a vertical line, or two black balls by day, adding sidelights and a sternlight only when making way through the water. In restricted visibility she sounds one prolonged followed by two short blasts every two minutes.

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Nautical Publications

Notices to Mariners (NtM)

Regular official updates that keep charts and books current

The UKHO issues a weekly edition containing chart corrections, amendments to Sailing Directions and Lists of Lights, temporary and preliminary notices, and navigational warnings. Corrections are applied in ink to paper charts and recorded in the bottom left margin. An annual summary edition reissues standing notices. National authorities issue their own series, and the US Coast Guard publishes Local Notices to Mariners for inshore waters not covered by the national edition.

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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.