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The YouSail sailing glossary

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Celestial Instruments & Sight Reduction
Shades and Filters
Coloured glasses that protect the eye when observing the sun
Swing-in index shades sit between the index mirror and the horizon mirror to attenuate the body, while horizon shades reduce glare from a bright sea surface. Never observe the sun without adequate index shading, since the concentrated reflected image can cause permanent retinal damage. Polarising filters offer variable attenuation. Choose the lightest combination that still gives a crisp limb, because excessive filtering blurs the edge and degrades the sight.
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Depth Sounding & Bathymetry
Shallow and Deep Depth Alarm
An audible warning set on the sounder for chosen depth limits
A settable threshold on the echo sounder that sounds when displayed depth falls below or rises above preset values. The shallow alarm is normally set to the planned minimum safe depth, that is intended under keel clearance plus draught, referenced to whatever datum the display is calibrated to, and should be reset for each leg as the plan's margins change. A deep alarm is useful when following a contour or when anchored, warning that the vessel has dragged into deeper water off the selected bank.
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AIS & Vessel Tracking

Ship Reporting System (SRS)

Scheme requiring ships to report position and details in a sea area

Adopted by the IMO under SOLAS Chapter V Regulation 11 and detailed in the relevant Admiralty Sailing Directions and List of Radio Signals. Reports use standard formats with lettered fields, for example A for ship name and call sign, B for time, C or D for position and E and F for course and speed. Examples include mandatory reporting in the Dover Strait and Torres Strait. Voluntary schemes such as AMVER support search and rescue coordination.

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Nautical Publications

Ship's Log Book

The running written record of the vessel's navigation and events

Entries are made at regular intervals and at every significant event, recording time, position with its source, course steered, log reading, speed, wind, barometric pressure, weather, sea state, engine hours and any change of watch or unusual occurrence. It is the primary evidence for reconstructing a dead reckoning position if electronics fail, and is a legal document in the event of an incident or insurance claim. Entries should be made in ink and never erased.

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Meteorology for Navigators

Shipping Forecast

The regular broadcast forecast for named sea areas around Britain

Issued by the Met Office for 31 sea areas around the British Isles and broadcast on BBC Radio 4 long wave four times daily, with gale warnings, the general synopsis, then area forecasts in a fixed order and format. Each area gives wind direction and force, weather, and visibility, using defined terms: good exceeds 5 nautical miles, moderate is 2 to 5, poor is 1000 metres to 2, and fog is below 1000 metres.

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Depth Sounding & Bathymetry

Shoal Draught Handrail Technique

Following a chosen depth contour to stay clear of danger

Rather than aiming at a mark, the navigator steers to keep the echo sounder reading within a band, for instance holding between 8 and 12 metres to parallel a coast clear of an offshore shoal. Because the sounder gives a continuous independent measurement, it cross-checks GNSS and remains valid if position fixing fails. It works best where contours run roughly parallel to the track and the seabed slopes steadily rather than in steps.

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COLREGs - Lights, Shapes & Sound Signals

Short Blast and Prolonged Blast

The two standard whistle lengths used in every sound signal

A short blast lasts about one second; a prolonged blast lasts from four to six seconds. Every manoeuvring, warning, narrow channel and restricted visibility signal is built from these two units, so timing matters, because a hesitant prolonged blast is easily read as a short one. Vessels of 12 metres or more are required to carry a whistle, and vessels of 20 metres or more must carry a bell in addition.

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Depth Sounding & Bathymetry

Side Scan Sonar

Towed sonar that produces a picture of the seabed's texture

A towfish emits fan-shaped beams to each side and records echo strength against time, building an image where hard, rough or raised features return strongly and shadows reveal height. It maps objects such as wrecks, boulders and cable routes far better than it measures depth, since it gives limited depth information directly. It is commonly run alongside multibeam so that contacts detected in the imagery can be assigned an accurate least depth.

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COLREGs - Lights, Shapes & Sound Signals

Sidelights

Red port and green starboard lights covering the forward sectors

Green to starboard and red to port, each showing an unbroken arc of 112.5 degrees from right ahead to 22.5 degrees abaft the beam. Screened inboard so neither is seen across the bow, they combine with the 135 degree sternlight to cover the full circle. A vessel under 20 metres may combine them in one lantern on the centreline. Ranges are 3 miles at 50 metres or more, 2 miles from 12 metres, and 1 mile below.

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Celestial Navigation Concepts

Sidereal Hour Angle (SHA)

A star's fixed angular position measured westward from the First Point of Aries

Since the stars keep essentially constant positions relative to each other, each navigational star is given an SHA and a declination that change only slowly through the year. The almanac lists both for 57 selected stars. Star GHA equals SHA plus GHA of Aries. Arcturus, for instance, has an SHA near 146 degrees; the exact value drifts by a fraction of a minute of arc over a season, so the current almanac must be used.

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Celestial Instruments & Sight Reduction

Sight Form / Sight Reduction Worksheet

A printed form guiding you through each step of working a sight

A structured layout that sequences the extraction of GHA and declination, the increments, the corrections to altitude, the LHA computation, the table entry and the final intercept and azimuth. Its value is error prevention: each quantity has one box, and blank boxes reveal omitted steps. Different forms exist for sun, star, planet and moon sights, since the moon uses a second altitude correction from the additional table and both v and d corrections.

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Nautical Publications

Sight Reduction Tables (NP 401 / Pub 229, NP 303 / Pub 249)

Precomputed tables turning a sextant sight into a position line

NP 401, the American Pub 229, tabulates calculated altitude and azimuth for whole degrees of latitude, declination and local hour angle, and suits any body to high precision. NP 303, the American Pub 249, is the air series: Volume 1 gives selected stars for a given latitude and hour angle of Aries, while Volumes 2 and 3 handle bodies with declinations up to 29 degrees. Both replace the spherical trigonometry with lookups.

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Celestial Instruments & Sight Reduction

Sight Reduction Tables NP303 / HO249 (AP3270)

Faster, less precise tables originally designed for air navigation

Volume 1 tabulates altitude and true azimuth directly for seven selected stars per LHA of Aries, requiring no declination entry and giving a fix with minimal work, but it needs replacing every five years or so as precession accumulates. Volumes 2 and 3 cover bodies with declinations up to 29 degrees and are perpetual. Precision is one minute of arc rather than the tenth achievable with NP401, which is ample for ocean passage-making.

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Celestial Instruments & Sight Reduction

Sight Reduction Tables NP401 / HO229

Volumes of pre-computed altitudes and azimuths for any observation

Sight Reduction Tables for Marine Navigation cover latitudes 0 to 90 degrees in six volumes of 15 degrees each. Entered with whole degrees of latitude, declination and LHA, they give computed altitude, an altitude difference d for interpolating the declination minutes, and azimuth angle Z. They are usable for any body and provide high precision, at the cost of bulk and an interpolation step. The US designation is HO229 and the British is NP401.

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Wind, Waves & Sea State

Significant Wave Height (Hs)

The average height of the biggest third of the waves

Defined as the mean height, trough to crest, of the highest one third of waves in a record, chosen because it approximates what an experienced observer reports by eye. It is the quantity given in forecasts, GRIB files and buoy data. In a given sea state the highest individual wave in a three hour period is commonly around 1.8 to 2 times Hs, so a 3 metre forecast implies occasional waves near 6 metres.

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AIS & Vessel Tracking

Silent Mode

Switching off AIS transmission while continuing to receive others

Many transponders offer a receive-only setting. SOLAS Chapter V Regulation 19 requires AIS to be in continuous operation except where an international agreement or standard provides for protection of navigational information, and the master may switch it off where its operation would compromise the ship's security, recording the reason in the log. Leisure crews sometimes use silent mode for privacy, but doing so removes a valuable indication of your presence to commercial watchkeepers.

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Pilotage & Harbour Approach

Sill and Lock Gate

Barrier or gate keeping water in a marina at low tide

Many marinas retain water behind a sill, a wall whose crest height is charted relative to chart datum, or behind lock gates worked at set times. Access depends on tidal height: with a sill crest 2.0 metres above datum, a vessel drawing 1.5 metres and wanting 0.5 metres beneath the keel needs a height of tide of at least 4.0 metres. Tide boards, depth gauges and traffic lights at the entrance show the state of access.

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Depth Sounding & Bathymetry

Single Beam Echo Sounder (SBES)

Sounder that measures one depth directly beneath the vessel

Radiates a conical beam, often between about 3 and 30 degrees wide depending on transducer and frequency, and returns the shoalest depth within that cone. The seabed footprint therefore widens with depth, roughly two times depth times the tangent of half the beam angle. Coverage is a single line along track, so survey by single beam leaves unsurveyed gaps between lines, which is why older chart soundings can miss isolated pinnacles.

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Speed, Log & Distance

Six Minute Rule

Quick mental method converting speed into distance covered

Six minutes is one tenth of an hour, so distance run in six minutes equals speed in knots divided by ten. At 12 knots that is 1.2 nautical miles, and in three minutes half of that, 0.6 nautical miles. The rule is invaluable for rapid pilotage checks, timing between marks and estimating closing ranges without instruments, and it inverts equally well: covering 0.8 nautical miles in six minutes indicates 8 knots.

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Tidal Streams & Currents

Slack Water

The short period when the tidal stream is barely running

The moment of stream reversal, which in a standing-wave regime occurs near high and low water but in a progressive-wave regime occurs near mid-tide. In narrow passages slack may last only a few minutes; in wide bays it may be a soft transition of half an hour or more. Slack is the preferred time for transiting tidal races, entering narrow locks and for diving or anchoring work.

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Anchoring & Mooring

Snubber (Anchor Bridle)

A stretchy rope taking the chain load off the windlass

A length of nylon, typically 8 to 12 metres, attached to the chain by a hook or soft splice and made fast to a cleat or, as a bridle, to both bows of a catamaran. Chain is then slacked so the nylon carries the load. It absorbs snatch loads, protects the windlass gypsy, which is not designed to hold anchor loads, and greatly reduces noise transmitted through the hull.

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Passage Planning

SOLAS Voyage Planning Requirement

International rule that every vessel must plan its voyage

SOLAS chapter V extends voyage planning to all ships and, in many national implementations, to pleasure vessels. The intended voyage must be planned in advance using appropriate charts and nautical publications, taking account of ships' routeing systems and any relevant guidance, and the whole track must be checked for adequate depth and sufficient margins of safety. For small craft, compliance normally means a simple recorded plan covering weather, tides, limitations, crew and contingencies.

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Radar Navigation

Solid-State FMCW Radar

Low-power modern radar using a continuous swept signal, not big pulses

Instead of a high-power magnetron pulse, these sets transmit a low-power frequency-modulated continuous wave and derive range from the frequency difference between transmitted and returned signals. Advantages are excellent short-range performance with almost no minimum range, instant start-up with no warm-up, very low radiated power and long service life. Long-range performance against small targets in a seaway can be weaker than an equivalent magnetron set, although pulse-compression designs have narrowed the gap.

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COLREGs - Lights, Shapes & Sound Signals

Sound Signals in Restricted Visibility

Fog signals used when vessels cannot see each other

In or near an area of restricted visibility a power-driven vessel making way sounds one prolonged blast at intervals of not more than two minutes; under way but stopped and making no way, two prolonged blasts about two seconds apart. Sailing, fishing, towing, not under command, restricted and constrained vessels sound one prolonged followed by two short. A vessel at anchor rings the bell rapidly for about five seconds every minute. Vessels under 12 metres may use any other efficient signal.

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Depth Sounding & Bathymetry

Sound Velocity Profile (SVP)

Measurement of how fast sound travels at each depth in the water

Sound speed in seawater rises with temperature, salinity and pressure, ranging roughly from 1450 to 1550 metres per second. Survey grade sounders apply a measured profile because refraction bends beams and an incorrect assumed velocity biases depths, an error of about 15 metres per second producing roughly one percent depth error. Small craft sounders simply assume a fixed value, which is adequate for pilotage but not for charting quality data.

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Depth Sounding & Bathymetry

Sounding

A single charted depth figure at one position on the chart

Printed in metres and decimetres on metric charts, so 27 with a small subscript 5 means 2.75 metres below chart datum, and depths over about 21 metres are shown in whole metres. The pattern of soundings is selected to show the shoalest depths in an area rather than an average, and the density reflects survey coverage. Older fathoms charts require conversion, one fathom equalling 1.8288 metres.

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Chart Projections & Datums

Source Data Diagram (Zone of Confidence, ZOC)

A panel showing how trustworthy each part of the chart's survey is

Charts carry a source data diagram or reliability diagram identifying the age, method and scale of the surveys used in each area, from modern multibeam coverage to nineteenth century leadline work. In electronic charts this is formalised as zones of confidence, categories A1, A2, B, C and D, each with stated position and depth accuracy and seafloor coverage. Navigating on a chart whose source is a sparse lead line survey demands much greater clearance than the printed soundings alone suggest.

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Depth Sounding & Bathymetry

Source Data Diagram and Survey Reliability

A chart panel showing how old and how good each survey was

A diagram or note on the paper chart, corresponding to the zone of confidence attribution carried on electronic charts, dividing the sheet into areas and stating the date, scale and method of the underlying survey. Areas surveyed by lead line in the nineteenth century may have wide unsounded gaps between sounding lines and poorly fixed positions, so apparently clear water proves nothing. Passage plans should widen margins, avoid least depth areas and increase under keel clearance wherever survey quality is stated as poor.

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Chart Symbols & Abbreviations

Source Diagram / Zone of Confidence (ZOC)

A panel showing how good the survey data behind the chart is

Paper charts carry a source data diagram giving the year, surveying authority and method for each block of the chart. Electronic charts use ZOC categories A1, A2, B, C, D and U, each defining a position and depth accuracy and the degree of seafloor coverage; A1 is the best, U means unassessed. A nineteenth century lead line survey in category C or D may miss isolated pinnacles entirely, which is why prudent navigators avoid relying on soundings alone in such areas.

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South Cardinal Mark

Pass to the south of it to stay in safe water

Topmark is two black cones, both points downward. The body is yellow above black, following the downward cones. The light is white VQ(6) plus long flash every 10 seconds, or Q(6) plus long flash every 15 seconds. The long flash, of not less than two seconds, is essential: it distinguishes six quick flashes from the nine of a west cardinal when counting is uncertain in a seaway. Safe water lies to the southward.

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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.