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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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IALA Buoyage
IALA Maritime Buoyage System
Worldwide agreed system of buoys and beacons marking safe water
Administered by IALA, the system standardises the shape, colour, topmark, retroreflector and light rhythm of floating and fixed marks. It comprises six mark types: lateral, cardinal, isolated danger, safe water, special, and emergency wreck marking. Only the lateral convention differs geographically, splitting the world into Region A and Region B. All other mark types carry identical meaning everywhere, so a cardinal or isolated danger mark can be read without first knowing which region you are in.
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Nautical Publications
IALA Maritime Buoyage System Publication
The official document defining the international buoyage rules
The IALA Maritime Buoyage System document sets out the lateral, cardinal, isolated danger, safe water and special marks, their shapes, colours, topmarks and light rhythms, and defines the boundary between Region A and Region B. It also covers the emergency wreck marking buoy introduced after the 2006 Tricolor incident. National authorities implement it through their own regulations, and the essentials are summarised in Chart 5011 and in every yachting almanac.
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IALA Buoyage

IALA Region A

Area where red marks are left to port entering harbour

Region A covers Europe, Africa, Australasia, most of Asia and the Gulf states. Entering a harbour or proceeding in the conventional direction of buoyage, port-hand marks are red with can (cylindrical) shape and any red can topmark; starboard-hand marks are green with conical shape and a green cone topmark, point upward. Lights, where fitted, match the body colour: red to port, green to starboard. Preferred channel marks use the opposite colour as a horizontal band.

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IALA Region B

Area where red marks are left to starboard entering harbour

Region B covers North, Central and South America, Japan, South Korea and the Philippines. The lateral colours are reversed relative to Region A: starboard-hand marks are red, conical, with red lights, giving rise to the North American mnemonic red right returning; port-hand marks are green cans with green lights. Shapes and topmark geometry follow the same rules as Region A, so only the colour assignment changes. Cardinal, isolated danger, safe water and special marks are unchanged.

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Great Circle & Ocean Routing

Ice Limit

Charted lines showing how far south icebergs and pack ice reach

Routeing charts, ocean passage publications and ice service bulletins show mean and extreme limits of pack ice and of icebergs for each month. In the North Atlantic the International Ice Patrol broadcasts the iceberg limit and recommended track shifts during the spring and summer season. Prudent practice is to plan south of the extreme limit, since bergs and growlers drift well beyond the mean line and small ice gives a poor radar return.

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Nautical Publications

IMO COLREG Publication

The official printed text of the collision regulations

The International Regulations for Preventing Collisions at Sea 1972, as amended, are published by the IMO as the COLREGs consolidated edition, with the annexes on light positioning, sound signal appliances, additional signals for fishing vessels and distress signals. National maritime administrations publish equivalent statutory editions. Carriage of an up-to-date copy is a legal requirement for most commercial vessels, and prudent for any yacht making international passages, since amendments occur periodically.

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Points of Sail & Sailing Manoeuvres

In Irons

Stuck head to wind with the sails flapping and no steerage

A boat ends up in irons when it fails to carry enough way through a tack, stalls head to wind and loses steerage, after which the rudder is useless and the vessel may start making sternway. Recovery is by backing the jib to windward to push the bow onto the desired side, sometimes with the tiller reversed while making sternway, then bearing away to rebuild speed before trying again.

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Celestial Instruments & Sight Reduction

Increments and Corrections Tables

Almanac pages that scale hourly data down to the exact second

Because the almanac tabulates GHA only each hour, the increment for the elapsed minutes and seconds must be added. Separate columns handle the sun and planets, Aries, and the moon, since each moves at a different rate. Additional v and d corrections account for the small departure of planets and the moon from a uniform hourly rate and for the change of declination. A single wrong page here typically produces a position line displaced by a whole number of miles.

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Celestial Instruments & Sight Reduction

Index Error (IE)

The sextant's built-in zero offset that must be removed from every reading

Determined by setting the instrument to zero and viewing the horizon or a star; any step in the horizon line, or separation of the two star images, is the index error. If the reading is on the arc it is subtracted, and if off the arc it is added, remembered as if it is on, take it off. Sun measurement of on-and-off arc diameters provides a check, since the two values should sum to about four times the semi-diameter.

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Great Circle & Ocean Routing

Initial Great Circle Course

The first heading to steer when starting a great circle track

Because a great circle cuts successive meridians at changing angles, only the departure heading is quoted as the initial course; the final course at destination differs from it by the convergency. It is obtained from the spherical cosine or haversine solution, from sight reduction tables, or directly from a chart plotter. In practice the track is re-solved at each waypoint, typically every five degrees of longitude, and the heading updated.

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Celestial Navigation Concepts

Intercept (Marcq St Hilaire Method)

The difference between the height you measured and the height you computed

From an assumed position, compute the altitude and azimuth the body would have. Subtract the computed altitude from the observed altitude; the difference in minutes of arc is the intercept in nautical miles. If observed exceeds computed the vessel is that far toward the body's azimuth, remembered as Ho Mo To, and the position line is drawn perpendicular to the azimuth at the intercept's end. Intercepts over about 30 miles suggest a poor assumed position and warrant recomputation.

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Pilotage & Harbour Approach

International Port Traffic Signals (IPTS)

Standard light signals showing whether vessels may enter port

A vertical column of lights at the harbour entrance. Three flashing red lights mean a serious emergency, all vessels stop or divert. Three fixed red mean vessels shall not proceed. Red over white over red means do not proceed except vessels holding specific orders. Three fixed green mean proceed, one-way traffic; green over white over green means proceed, two-way traffic. A yellow light offset beside the column exempts vessels with specific orders. Local variants persist, so consult the pilot book.

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Tidal Calculations

Interpolation Between Springs and Neaps

Adjusting the printed tidal curve for a day that is neither

On a standard port curve, plot the day's high water height on the upper height scale and low water on the lower, join them with a straight line, then follow the spring curve or neap curve according to which the day's range is nearer, interpolating proportionally between the two dotted and solid lines. For secondary ports, interpolate the tabulated time and height differences between the two reference times and heights given.

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Meteorology for Navigators

Isobar

A line on a weather map joining places of equal pressure

Drawn on synoptic charts at fixed intervals of atmospheric pressure reduced to mean sea level, most commonly every 4 hectopascals, though 2 or 5 hPa spacings are also used. One hectopascal equals one millibar. Isobar spacing is the primary visual cue to wind strength, while their curvature indicates cyclonic or anticyclonic flow, with winds blowing nearly parallel to them rather than across them.

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Great Circle & Ocean Routing

Isochrone Method

Drawing lines showing how far the boat can get each period

The classic routeing algorithm advances the vessel from its position along a fan of headings for a fixed time step, commonly six or twelve hours, using forecast wind and the performance polar. The outer envelope of reachable points forms an isochrone, and the process repeats from points on that curve. The optimum track is recovered by tracing back from the first isochrone that reaches the destination, giving the least-time route.

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Compass & Magnetism

Isogonic Line and Agonic Line

Lines on a map joining places with the same magnetic variation

Isogonic lines connect points of equal variation and are printed on magnetic variation charts and in GIS models; agonic lines are the special case where variation is zero, so magnetic and true north coincide. An agonic line currently runs through parts of the eastern United States and another through Europe and Africa, both moving steadily. Following isogonics on a routeing chart shows how much the variation correction will change over an ocean passage.

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Isolated Danger Mark

Buoy moored on a small hazard with clear water around it

The body is black with one or more broad red horizontal bands, and the topmark is two black spheres mounted vertically, one above the other. The light is white group flashing two, Fl(2), commonly every five or ten seconds. It is stationed on or moored directly above an isolated danger of limited extent, such as a rock or wreck, which has navigable water all around; it does not indicate a preferred side to pass.

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Lights & Light Characteristics

Isophase (Iso)

Light with equal times on and off

In an isophase light the duration of light and darkness are exactly equal, so Iso 4s shows two seconds lit and two seconds dark. The rhythm is distinctive, easily counted and hard to confuse in a busy background of shore lighting. Isophase is one of the permitted white rhythms for safe water marks, alongside occulting, one long flash every ten seconds and Morse A. The former term equal interval is now obsolete.

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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.