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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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IALA Buoyage
East Cardinal Mark
Pass to the east of it to stay in safe water
Topmark is two black cones base to base. The body is black with a single broad horizontal yellow band, echoing the cones pointing away from the middle. The light is white VQ(3) every 5 seconds or Q(3) every 10 seconds: three flashes, remembered as three o'clock for east. Deep water lies to the eastward, so the mark is left to the west when passing. Cardinal marks are identical in Region A and Region B.
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Tidal Streams & Currents
Ebb Stream
The stream running while the tide is generally falling
Typically sets seaward and is often stronger and longer-lasting than the flood in rivers, because freshwater discharge reinforces it and the channel cross-section is smaller at low water. An ebb running against a fresh onshore wind creates steep, breaking seas over bars and in tidal gates, so bar entries are normally timed for the last of the flood or slack rather than the ebb.
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Electronic Charts & ECDIS
ECDIS Backup Arrangement

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What you navigate with if the chart computer dies

A paperless ship must carry an accepted fallback that allows navigation to be taken over without delay and the voyage to be completed safely. In practice this means either a second independent type-approved ECDIS with its own power supply and independent position input, or a full folio of corrected paper charts. The arrangement must also cover route planning, chart updating and record keeping, and the changeover procedure should be practised, not merely documented.

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Depth Sounding & Bathymetry

Echo Sounder

Instrument that measures depth by timing a sound pulse to the seabed

A transducer emits a pulse, commonly in the 50 to 200 kHz band for small craft, and the receiver times the return echo. Depth equals half the two-way travel time multiplied by the speed of sound, taken as about 1500 metres per second in seawater, so a 0.1 second round trip indicates roughly 75 metres. Higher frequencies give better resolution in shallow water, lower frequencies penetrate deeper and cope better with aeration and soft mud.

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Tidal Streams & Currents

Eddy and Back Eddy

A local swirl of water running counter to the main stream

Forms in the lee of a headland, breakwater or island where flow separation creates a recirculating zone. A back eddy can be exploited to make ground against a foul main stream by hugging the shore inside the separation line, a standard inshore tactic at places such as Portland Bill. The shear line between eddy and main stream is often marked by a visible tide line and can affect steering sharply.

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Speed, Log & Distance

Electromagnetic Log (EM Log)

Speed sensor using a magnetic field and moving seawater

Applies Faraday's law of induction: seawater moving through the sensor's magnetic field acts as a conductor and generates a small voltage proportional to flow velocity, detected by two electrodes. Having no moving parts it resists fouling and works down to very low speeds, making it common on commercial ships and increasingly on yachts. It measures speed through the water only, within the boundary layer, so calibration and correct flush mounting still matter.

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Radar Navigation

Electronic Bearing Line (EBL)

A rotating line on screen used to take a bearing of a target

The EBL is a line from own ship, or from an offset origin, whose direction is shown numerically as relative or, with a heading input, as true bearing. Accuracy is limited by beamwidth and by heading sensor error, so bearings should be taken to the centre of an echo and treated as less reliable than ranges. An offset EBL combined with a VRM is a quick way to measure the range and bearing between two other vessels.

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Electronic Charts & ECDIS

Electronic Chart Display and Information System (ECDIS)

The type-approved chart computer that can legally replace paper charts

ECDIS is a navigation system that displays official vector chart data with own-ship position from GNSS, combined with heading, log, radar and AIS inputs, and provides route planning, route checking and grounding alarms. Only a type-approved installation running official data, with an accepted backup arrangement, satisfies SOLAS chapter V chart carriage requirements. Carriage became mandatory for most cargo and passenger ships through a phased schedule completed in the mid-2010s.

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Electronic Charts & ECDIS

Electronic Chart System (ECS)

A chart plotter that is useful but not legally chart-equivalent

ECS covers any electronic charting equipment that is not type approved as ECDIS, including yacht plotters, tablet applications and many pilot systems. It may display official or privately produced vector or raster data, and can be extremely capable, but it does not meet SOLAS chart carriage requirements and its alarms and data quality are not governed by the performance standard. Aboard a yacht it is best treated as an aid alongside paper charts.

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Electronic Charts & ECDIS

Electronic Navigational Chart (ENC)

Official vector chart data made of objects rather than a picture

An ENC is an official database issued by or on the authority of a government hydrographic office, in which every feature is an object carrying attributes such as depth, colour, period or name. Data is supplied in cells with a fixed naming convention whose third character indicates the usage band, from overview through to berthing. Because it is a database, the ECDIS can interrogate it, generate alarms from it and redraw it at any scale.

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Celestial Instruments & Sight Reduction

Electronic Sight Reduction (Calculator and App Methods)

Software that works the sight arithmetic from the raw sextant reading

Programmable calculators, dedicated navigation computers and phone applications embed the almanac ephemeris and the cosine formula, taking sextant altitude, time, height of eye and index error and returning intercept and azimuth from the dead reckoning position directly. This removes assumed position rounding and table interpolation. Since the usual purpose of celestial work is independence from electronics, prudent practice keeps a paper almanac, tables and worked forms aboard as the true backup.

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GNSS & Satellite Navigation

Elevation Mask Angle

The lowest satellite angle a receiver will accept above the horizon

Signals from satellites low on the horizon travel obliquely through the ionosphere and troposphere, suffer greater delay uncertainty, and are far more prone to reflection off the sea surface. Receivers therefore ignore satellites below a set elevation, commonly between five and ten degrees. Raising the mask improves individual measurement quality but reduces the number of satellites available and can worsen geometry, so most marine sets leave the default alone.

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Lights & Light Characteristics

Elevation of a Light

Height of the light above the sea, used for range sums

Charted elevation is measured from the focal plane of the lantern to Mean High Water Springs on British Admiralty charts, or to Mean Higher High Water in United States practice, and is quoted in metres, as in 21m. It is deliberately referenced to a high tidal level so the stated geographical range is conservative. Elevation must not be confused with the structure height printed on some charts, which is measured from the ground or base.

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Emergency Navigation & Distress

Emergency Position Indicating Radio Beacon (EPIRB)

A floating beacon that alerts rescue satellites with your identity and position

An EPIRB transmits a registered digital distress message on 406 MHz to the Cospas-Sarsat satellite constellation, together with a 121.5 MHz homing signal for final approach by rescue units. Most models include an internal GNSS receiver for immediate position accuracy, and modern units may also carry an AIS locating signal and return link indicator. Float-free versions release hydrostatically and operate for at least forty-eight hours.

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Emergency Wreck Marking Buoy (EWMB)

Temporary buoy marking a brand-new wreck before charts update

Introduced after the 2002 Tricolor collisions, this mark has equal vertical blue and yellow stripes, a standing or upright yellow cross topmark, and an alternating blue and yellow light with a one second flash and half second eclipse, written Al Bu Y 3s. It is deployed as soon as practicable after a new danger appears and is normally removed once the wreck has been surveyed, promulgated by Notice to Mariners and marked conventionally.

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Speed, Log & Distance

Engine Revolution Table (RPM to Speed Curve)

A crib sheet linking engine revs to boat speed when the log fails

Built by running a measured mile or a timed leg at several throttle settings and tabulating revolutions per minute against speed through the water, ideally in both directions so any stream cancels. A typical cruising yacht might show 1800 rpm for 5.2 kn and 2400 rpm for 6.4 kn. It supplies a dead-reckoning speed when the log is fouled or dead, but needs allowance for hull fouling, loading and adverse sea state.

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Celestial Navigation Concepts

Equation of Time (EoT)

The gap between clock time and sundial time through the year

The difference between apparent solar time, governed by the real sun, and mean solar time kept by clocks. It arises from the Earth's elliptical orbit and the tilt of its axis, and ranges from about 14 minutes slow in February to roughly 16 minutes fast in early November, passing through zero four times a year. The almanac tabulates it daily, and it explains why the sun's meridian passage rarely occurs exactly at 1200 local mean time.

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Equilibrium Tide

A simplified model tide on an imaginary ocean covering the whole Earth

Newton's static theory assumes a water-covered globe where the sea surface instantaneously adopts the shape of an equipotential under combined gravitational and rotational forcing. It predicts two bulges and hence a semi-diurnal rise of only about 0.5 m, far less than observed ranges. It cannot explain amphidromic systems, resonance or the phase lag of real basins, but it remains the basis for defining the harmonic constituents used in modern prediction.

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Equinoctial Springs

The extra-large spring tides near the March and September equinoxes

Around 21 March and 23 September the Sun's declination is zero, aligning the solar tidal bulge with the equator and maximising the solar contribution at moderate latitudes; when this coincides with syzygy the resulting springs exceed those of other seasons. In practice the highest astronomical tides of the year at many European ports occur within a few weeks of the equinoxes, a factor worth allowing for when planning shallow-water passages.

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Dead Reckoning & Estimated Position

Estimated Position (EP)

A dead reckoning position improved by allowing for tide and leeway

The EP is constructed by plotting the water track from the last fix using course steered and distance through the water, then applying the tidal stream vector for the same interval as set and drift, and any leeway offset. It is plotted as a dot inside a triangle and labelled with the time. The EP is the navigator's best position in the absence of an observed fix and should be updated whenever a new tidal hour begins.

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Passage Planning

Estimated Time of Arrival (ETA)

Predicted clock time of reaching a waypoint or destination

The time at which the vessel expects to reach a given position, found by dividing the distance remaining by the expected speed made good and adding the result to the time now: 24 miles to run at 6 knots gives four hours. Honest ETAs matter because they drive tidal gate timing, lock and berth bookings, traffic service reporting and shore contacts, and they must be revised whenever speed or the route changes materially.

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Celestial Navigation Concepts

Ex-Meridian Sight

A near-noon sight worked for latitude when exact transit was missed

If cloud hides the sun at the moment of meridian passage, an observation taken shortly before or after can still yield latitude. A reduction to the meridian correction, tabulated against the hour angle from transit and the approximate zenith distance, converts the observed altitude to what it would have been at transit. The method holds only within a limited hour angle either side, since the correction assumes the altitude curve near the maximum is parabolic.

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Electronic Charts & ECDIS

Exchange Set and Update Files

The package of chart cells and corrections you load into the system

Chart data is distributed as an exchange set containing a catalogue file listing the cells, the base cell files, and sequential update files that must be applied in unbroken order. A missed update leaves the cell out of sequence and the system will refuse further updates until the base is reloaded. New editions and re-issues replace the base cell entirely. Weekly delivery by media or download keeps the folio corrected, and a status report should be checked.

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Passage Planning

Execution

Putting the plan into effect and adapting it to conditions

The third stage, beginning with the decision on when to sail and continuing as the plan is carried out in the light of the actual forecast, tidal information, traffic density, visibility and machinery status. Execution covers choosing a departure time to meet a tidal gate, adjusting speed to arrive in daylight or at slack water, and formally amending the plan when a leg becomes unsafe. Amendments should be recorded rather than improvised silently on watch.

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Emergency Navigation & Distress

Expanding Square Search (SS)

A search pattern spiralling outward in ever-larger squares

The expanding square, one of the standard IAMSAR patterns, starts at the datum position and runs successive legs at right angles, each pair of legs longer than the last by one track spacing. It gives dense, uniform coverage close to the datum and suits a small search area where the position is reasonably well known, such as a person overboard. Alternative patterns include the sector search and the parallel track search for larger areas.

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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.