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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

642 terms

Page 12 of 22

Tidal Theory
Mean Sea Level (MSL)
The average height of the sea over a long period of records
Computed by averaging observed levels over at least a full 19-year Metonic cycle to remove the nodal variation in lunar orbit inclination. It is the reference for land survey datums such as Ordnance Datum Newlyn and for measuring long-term sea level trends. Tidal predictions are expressed above chart datum, not MSL, so the two must be reconciled when combining chart depths with land heights.
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Chart Projections & Datums
Mean Sea Level (MSL) and Ordnance Datum
Average sea height, used as the zero for land maps
Mean sea level is the average height of the sea over a long period, typically 19 years, at a specified place, and it is the basis of national height systems. In Great Britain, Ordnance Datum Newlyn is defined from mean sea level measured at Newlyn between 1915 and 1921 and is the zero for land contours and spot heights. Land map heights therefore differ from chart elevations, which use MHWS, and from soundings, which use chart datum.
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Tidal Calculations

Mean Tide Level (MTL) or Mean Level (ML)

The halfway point of the tide, midway between average high and low water

Mean tide level is the average of MHWS, MHWN, MLWN and MLWS, or in rough work the mean of a single high and low water, quoted as a height above chart datum. With MHWS 5.4 m, MHWN 4.2 m, MLWN 1.8 m and MLWS 0.6 m, mean level is 3.0 m. Tidal curves are drawn about it, and it approximates mean sea level closely enough for most pilotage estimates.

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Speed, Log & Distance

Measured Mile

A charted course with shore transits marking exactly one nautical mile

Pairs of shore beacons define two transits one nautical mile apart, and the vessel runs at steady revolutions on the course marked on the chart, which is set perpendicular to the transits. Timing between transits gives speed as 3600 divided by the elapsed seconds, so 6 minutes 40 seconds, that is 400 seconds, equals 9.0 knots. Reciprocal runs are averaged, traditionally by the mean of means, to remove the effect of tidal stream.

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Anchoring & Mooring

Mediterranean Moor (Med Moor)

Anchoring off and backing your stern in to a quay

The anchor is let go four to five boat lengths off the quay, square to it, and the vessel then backs down veering cable until stern lines can be passed ashore. The cable is then hove taut to hold the stern clear of the wall. Bows-to is a common alternative that protects the rudder and propeller. Crossing a neighbour's cable is the usual hazard, so note where others have laid theirs.

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Chart Projections & Datums

Mercator Projection

The standard sea chart where a straight line is a constant compass course

A cylindrical conformal projection in which meridians are equally spaced parallel straight lines and parallels are spaced increasingly with latitude in proportion to the secant of latitude. Its defining property is that a rhumb line plots as a straight line, so a course can be laid off with a single protractor angle. Scale is true only along the equator or the chosen standard parallel and grows without limit toward the poles, making it unusable above roughly 70 to 80 degrees.

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Great Circle & Ocean Routing

Mercator Sailing

Calculating a straight-line course and distance using chart stretching

Mercator sailing solves the rhumb line between two positions using meridional parts, the tabulated stretched latitude scale of the projection. The course comes from tan(course) equals difference of longitude in minutes divided by difference of meridional parts, and the distance from difference of latitude divided by cos(course). It is exact for any distance, unlike plane sailing, and remains the standard manual method for ocean rhumb line legs.

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Celestial Navigation Concepts

Meridian Passage (Mer Pass, Local Apparent Noon)

The moment a body crosses your north-south line, giving latitude directly

At the instant of transit the body bears due north or due south, the altitude is at maximum, and latitude follows from zenith distance and declination without any need for exact time. Sun meridian passage is tabulated in the almanac as a GMT value at the Greenwich meridian; correct for longitude at four minutes per degree to get local timing. In practice the observer begins measuring a few minutes early and follows the sun until it stops rising.

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Chart Projections & Datums

Meridional Parts (MP)

The stretched latitude scale used to build a Mercator chart

Meridional parts express the distance of a parallel from the equator on a Mercator projection, measured in units of one minute of longitude at the equator. Tabulated in Norie's and other nautical tables, the difference of meridional parts between two latitudes allows exact Mercator sailing, where tangent of course equals difference of longitude in minutes divided by difference of meridional parts. This replaces plane sailing for long distances where meridian convergence matters.

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Meteorological Tide (Storm Surge)

Extra rise or fall of sea level caused by weather, not astronomy

The difference between observed and predicted sea level, driven by barometric pressure and wind. The inverse barometer effect gives approximately 0.01 m of sea level change per hectopascal, so a 980 hPa depression against a 1013 hPa standard raises level about 0.33 m. Wind set-up in shallow water can add considerably more, and negative surges from strong offshore winds or high pressure can remove a metre or more of expected depth.

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Celestial Instruments & Sight Reduction

Micrometer Drum and Vernier

The parts of the sextant that let you read fractions of a degree

Whole degrees are read where the index arm marker falls on the arc. The micrometer drum, geared to the arm, reads minutes of arc from 0 to 60, and a vernier scale alongside it subdivides to 0.2 or 0.1 minutes. Older instruments use a vernier directly against the arc without a drum. A reading of 42 degrees on the arc, 17 on the drum and 4 on the vernier is recorded as 42 degrees 17.4 minutes.

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Meteorology for Navigators

Mid-latitude Depression (Frontal Low)

A travelling area of low pressure carrying fronts and bad weather

Forms on the polar front where cold and warm air masses meet, then deepens as it tracks generally eastward, typically at 20 to 30 knots in the temperate zone. Its classic structure has a warm front ahead, a warm sector, and a faster cold front behind that eventually catches up to form an occlusion. Winds circulate anticlockwise in the Northern Hemisphere and back then veer sharply as the fronts pass.

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Mixed Tide

Two unequal high waters and two unequal low waters daily

Results from comparable semi-diurnal and diurnal constituent amplitudes, giving a marked diurnal inequality with higher high water, lower high water, higher low water and lower low water each cycle. Typical of the Pacific coasts of North America and much of Australia. Form factors between roughly 0.25 and 3.0 denote mixed regimes. Navigators there must identify which of the day's two low waters is the critical one for clearance calculations.

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Emergency Navigation & Distress

MOB Waypoint

A one-touch button storing the position where someone fell in

Pressing the man overboard key on a GNSS receiver, plotter or DSC radio instantly saves the current position as a waypoint and switches the display to bearing, range and often a return course to it. Because the casualty drifts with tidal stream and wind while the boat manoeuvres, the mark is a datum for the start of the search, not the person's present position, and a dedicated lookout must still be posted.

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Passage Planning

Monitoring

Continuously checking the vessel is where the plan expects

The fourth stage: fixing position at intervals matched to the proximity of danger and comparing it with the planned track by independent means, typically a satellite position cross-checked against visual bearings, transits, radar ranges or the echo sounder. Cross-track error, time to the next waypoint and depth are watched against limits set during planning. Monitoring also means testing whether the plan's assumptions, especially tidal height, stream and wind, still hold.

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Anchoring & Mooring

Mooring Buoy and Pick-up Line

A permanent float attached to heavy tackle on the seabed

A visitor's buoy is normally rated for a maximum vessel length or tonnage stated in the pilot or harbour guide. Where fitted, the pick-up line and small buoy lead to the main riser; take the line to the bow, then secure with your own rope rove through the mooring eye and back aboard so it can be slipped from deck. Never rely on a light pick-up line alone to take the load.

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Lights & Light Characteristics

Morse Code Light (Mo)

Light flashing a letter in Morse code

Written Mo(A)6s, meaning the Morse letter A, a short flash followed by a long flash, repeated every six seconds. Mo(A) in white is a permitted safe water and landfall rhythm and is widely used on offshore platforms and pilot boarding stations. Mo(U), two short and a long, warns of a danger or a shoal ahead in some national systems. The Morse rhythm is identified by ear as much as by counting.

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Position Fixing

Most Probable Position (MPP)

The best estimate of where you are, weighing all available evidence

When observations disagree, the navigator forms a judgement rather than mechanically averaging. A satellite position, an estimated position from dead reckoning, a single strong position line and a sounding are weighed according to their known reliability and age. The MPP is plotted as the working position from which the next course is laid off, and the reasoning is noted in the log. Where a danger lies close by, the MPP is biased deliberately toward the hazard so that the resulting decisions err safely.

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COLREGs - Lights, Shapes & Sound Signals

Motorsailing Cone

Black cone point-down shown by a yacht sailing under engine

A sailing vessel also being propelled by machinery counts as a power-driven vessel under the steering and sailing rules, and must exhibit forward, where it can best be seen, a conical shape with its apex pointing downwards. The international rules give no size exemption; the old under-12-metre proviso was deleted in 2003, and survives only in some national inland rules such as those of the United States. After dark the same vessel shows masthead light, sidelights and sternlight rather than sailing lights.

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Depth Sounding & Bathymetry

Multibeam Echo Sounder (MBES)

Sounder that maps a wide swath of seabed in one pass

Forms many narrow beams athwartships in a fan, typically producing a swath width of about three to five times water depth, giving full bottom coverage when lines are run with overlap. Data must be corrected for vessel motion using an inertial sensor, for sound velocity using measured profiles, and reduced to chart datum. Modern harbour and approach charts derived from multibeam surveys carry far higher confidence than lead line or single beam sources.

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GNSS & Satellite Navigation

Multipath

Satellite signals bouncing off water or rigging before reaching the aerial

Reflected signals arrive slightly later than the direct signal and bias the range measurement, typically by a few metres at sea but potentially much more alongside steel structures or quay walls. Low elevation satellites and reflective surfaces such as a wet coachroof, radar scanner or mast are the usual culprits. Siting the antenna clear of and slightly above metallic structures, with an unobstructed sky view, is the practical remedy.

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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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Page 12 of 22

Sailing reference

The YouSail sailing glossary

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