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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 9 of 22

Pilotage & Harbour Approach
Harbour Bar
Shallow bank across a harbour entrance that breaks in swell
A ridge of sand or shingle deposited across an estuary or river mouth where the outgoing stream meets the sea and loses velocity. Depth over it is the least on the approach and it shifts, so charted soundings may be out of date. With swell running against an ebb stream a bar can break heavily and become impassable even when depth is adequate; entry is normally planned for the last of the flood, with the stream fair.
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Tidal Theory
Harmonic Constituents
The individual cosine waves that are added to predict a tide
Each constituent has a fixed astronomical period and a port-specific amplitude and phase lag derived from long records of observation, ideally a year or more. Principal ones are M2 (principal lunar semi-diurnal, 12.42 h), S2 (principal solar semi-diurnal, 12.00 h), N2 (larger lunar elliptic), K1 and O1 (diurnal). Modern predictions sum tens to hundreds of constituents; shallow-water overtides such as M4 and M6 model distorted estuarine curves.
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Celestial Instruments & Sight Reduction

Haversine and Log Tables

The older logarithmic method of solving the navigational triangle

The haversine of an angle equals half of one minus its cosine, a function chosen because it is always positive, which removes sign ambiguity, and because it behaves well for small angles. Traditional reduction used log haversines with Norie's or Burton's tables, adding logarithms rather than multiplying. The method remains a valid backup and appears in some examination syllabuses, but it demands careful bookkeeping and is slower than either modern tables or a calculator.

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Great Circle & Ocean Routing

Haversine Formula

A distance formula that stays accurate for very short legs

The haversine of an angle is (1 minus cosine) divided by two. The formula states hav(d) equals hav(lat2 minus lat1) plus cos(lat1) cos(lat2) hav(difference of longitude). Because it works with half-angle sines rather than cosines near unity, it avoids the rounding loss that spoils the cosine rule over a few miles. Most navigation software and calculator routines use it, or the equivalent Vincenty method on the ellipsoid for higher precision.

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Pilotage & Harbour Approach

Head Mark

Distant object steered towards to hold a straight track

A single conspicuous feature, such as a chimney, tower, church or notch in the skyline, lying on the extension of the intended track and steered directly at during an approach. It is less accurate than a two-object transit because a cross-set produces no visible change until the bearing has shifted appreciably, so a head mark should always be backed up by a depth check or a clearing bearing on a second object.

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Coastal Piloting Techniques

Head-Up Pilotage

Navigating visually by what you see ahead rather than the chart

The navigator prepares a simple ordered list of expected features, headings and depths before entering, then conducts the approach by eye, using the chart only for confirmation. It suits fast craft and busy harbour approaches where plotting is impractical. Each leg carries an escape plan and an abort point. The technique relies on positive identification of marks, since a misidentified buoy in a head-up sequence propagates the error through the rest of the approach.

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Points of Sail & Sailing Manoeuvres

Heave To

Parking the boat almost stationary using backed sail and opposed helm

From close hauled the boat is tacked without releasing the headsail sheet, so the jib ends up aback, then the helm is lashed to leeward and the mainsail eased. The backed headsail tries to push the bow away while the rudder and main try to bring it up, so the boat settles at roughly 50 to 60 degrees off the wind, forereaching slowly and drifting to leeward and leaving a slick to windward.

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Compass & Magnetism

Heeling Error

Extra compass error that appears when the boat leans over

When a vessel heels, the geometry of the vertical component of the Earth's field relative to the ship's own magnetic structure changes, altering the deviation. On a sailing yacht heeled 20 to 30 degrees the effect can amount to several degrees and reverses sign on the opposite tack, which is one reason a course held on one tack may not repeat on the other. It is largest at high latitudes where the vertical field component dominates.

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Chart Symbols & Abbreviations

Height Datum for Land Features (MHWS)

The level from which charted land and light heights are measured

Vertical clearances and the elevations of lights, hilltops and other conspicuous objects on Admiralty charts are referenced to Mean High Water Springs, whereas soundings and drying heights use chart datum. US charts commonly use mean high water instead. Because MHWS is well above chart datum, actual clearance under a bridge at a given moment equals the charted clearance plus MHWS minus the present height of tide, and only equals the charted figure at spring high water.

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Celestial Instruments & Sight Reduction

Height of Eye (HE) and Dip

How high your eye is above the water, which shifts the horizon

The visible horizon lies below the true horizontal by the dip angle, approximately 1.76 times the square root of the height of eye in metres, expressed in minutes of arc. At 2.5 metres in a small yacht, dip is about 2.8 minutes; at 9 metres on a ship's bridge wing, about 5.3 minutes. Dip is always subtracted from the sextant altitude. An error of one metre in the assumed height costs a few tenths of a mile.

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Tidal Calculations

Height of Tide

How far the water surface stands above chart datum right now

Always a vertical height above chart datum, never a depth. Add it to the charted sounding to get actual depth, and subtract it from a charted drying height to test whether a bank is covered. For example, a charted 2.4 m sounding with a 3.1 m height of tide gives 5.5 m actual depth; a drying height of 1.8 m with the same tide is covered by 1.3 m.

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Tidal Calculations

Highest Astronomical Tide (HAT)

The highest level tides reach astronomically, used for bridge clearances

The counterpart of LAT, derived over the same 19-year span. Vertical clearances under bridges and overhead power cables on Admiralty charts are measured from HAT, giving the worst-case least clearance; US charts generally use Mean High Water. Elevations of lights are measured from MHWS on Admiralty charts. Using the wrong reference when calculating air draught clearance produces an error of the order of the spring range.

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Anchoring & Mooring

Holding Ground

How well the seabed material grips an anchor

Charts and pilots record the bottom quality using abbreviations such as M for mud, S for sand, Cy for clay, R for rock and Wd for weed. Firm sand and stiff mud give good holding for most modern anchors; soft ooze, shingle, kelp and smooth rock give poor holding. Where holding is doubtful, use more scope, a heavier anchor, or an anchor type suited to the specific bottom.

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Radar Navigation

Horizontal Beamwidth

How wide the radar beam spreads sideways as it sweeps round

Horizontal beamwidth is the angular width of the transmitted beam and depends on antenna length and wavelength: a longer open array gives a narrower beam. A large open array may achieve around one degree, while a small yacht radome is more typically four to six degrees. The beamwidth is added to the true angular width of every target, which is why land and buoys always paint wider on screen than they really are.

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Position Fixing

Horizontal Sextant Angle (HSA)

A precise fix from measuring the angles between three charted objects

Two horizontal angles between three objects define two position circles whose second intersection is the vessel. Plotted with a station pointer or by constructing the circles geometrically, this technique is largely independent of compass error and was the standard for hydrographic survey work. The fix fails when the vessel lies on or near the circle passing through all three objects, the so-called revolver or swinger, so select objects with the middle one nearer the vessel than the outer two.

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Speed, Log & Distance

Hull Speed

The rough speed limit of a displacement hull set by its length

Approximated by 1.34 multiplied by the square root of the waterline length in feet, or 2.43 times the square root of the length in metres, giving speed in knots. A 30 foot waterline yields about 7.3 knots. It marks where the wave the hull generates has a length equal to the hull and drag rises steeply. Light displacement and planing hulls exceed it readily, so treat it as a guide rather than a barrier.

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Tidal Streams & Currents

Hydraulic Current

Water rushing through a channel because sea level differs at each end

In a strait or canal joining two bodies of water whose tides differ in time or range, the difference in head drives a flow largely independent of the local vertical tide. Slack water occurs when the two levels equalise, not at local high or low water, so timings must be taken from a stream table rather than inferred from a tide table. Hell Gate, the Cape Cod Canal and parts of the Pentland Firth behave this way.

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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 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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Page 9 of 22

Sailing reference

The YouSail sailing glossary

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