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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 7 of 22

IALA Buoyage
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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Pilotage & Harbour Approach

Fairway

Main navigable channel used by vessels entering a port

The principal navigable route into or through a harbour, generally the deepest water, marked with lateral buoyage and often with a safe water mark or fairway buoy at its seaward end. It may carry one-way working, a speed limit and a requirement to keep to its starboard side. Small craft are commonly asked to keep outside the buoyed channel where depth allows, and must not impede vessels which can navigate only within it.

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Fairway Buoy

Safe water buoy marking the middle of an approach channel

A fairway or mid-channel buoy is a safe water mark, red and white vertically striped, showing a white isophase, occulting, long flash 10s or Morse A light. Deep water exists on both sides, but the ordinary rule of the road still applies: in a narrow channel a vessel keeps to the starboard side of the fairway, so the buoy is normally left to port in Region A and Region B alike when inbound.

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Radar Navigation

False Echoes

Marks on the screen that are not really targets where they appear

Side-lobe echoes from a close strong target produce a series of arcs at the same range on either side of it. Indirect echoes appear when the beam reflects off a mast or funnel, placing a phantom target on a false bearing at the correct range. Multiple echoes show as evenly spaced repeats along the bearing of a large nearby ship, and second-trace echoes from super-refraction place distant land at a wrong short range.

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Wind, Waves & Sea State

Fetch

The open water distance the wind has blown across

The uninterrupted distance over which a wind of roughly constant speed and direction acts on the sea. Together with wind strength and duration it determines wave height; a fully developed sea requires both sufficient fetch and sufficient time. This is why a force 6 gives short steep chop in a narrow sound but a large ocean swell offshore, and why waters close under a weather shore stay comparatively smooth.

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COLREGs - Lights, Shapes & Sound Signals

Fishing Vessel Lights and Shapes

Signals worn by vessels working nets, lines or trawls

A vessel trawling shows two all-round lights in a vertical line, green over white; a vessel fishing other than trawling shows red over white. By day both display two cones with their apices together, and vessels of less than 20 metres may substitute a basket. Sidelights and a sternlight are added only when making way. Where gear extends more than 150 metres horizontally, an all-round white light or a cone apex upwards marks that direction.

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Position Fixing

Fix

A confirmed boat position from two or more independent observations at one time

A fix is a position established without reference to any previous position, obtained from the intersection of two or more simultaneous position lines. Three position lines are preferred because the resulting cocked hat reveals observational error; two lines give no check. The fix is plotted as a dot inside a small circle, labelled with the time in four-figure ship's time. Angles of cut between position lines should ideally approach 90 degrees, and cuts below about 30 degrees produce badly elongated error ellipses.

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Position Fixing

Fix Error Ellipse

The oval area around a plotted fix where the boat probably really is

Because each position line carries an uncertainty band rather than being a perfect line, the intersection defines an area whose shape depends on the angle of cut and the accuracy of each observation. A bearing accurate to plus or minus 3 degrees produces a band about 0.16 nautical miles wide at 3 nautical miles range, since one degree subtends roughly one mile at 60 miles. Navigators use the ellipse to judge safe clearance rather than treating the plotted dot as exact.

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Position Fixing

Fix Interval and Frequency

How often you take a position, based on how close the dangers are

Fix frequency should be short enough that the vessel cannot reach a charted danger between fixes, allowing for the largest plausible positional error. Offshore in open water, hourly or two-hourly fixes suffice. In coastal waters ten to twenty minutes is usual, and in pilotage waters continuous visual monitoring by transits, clearing bearings and sectored lights replaces discrete plotting altogether. The interval is a passage-planning decision recorded before departure, not an improvisation at the time.

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Lights & Light Characteristics

Fixed Light (F)

Steady light that does not flash at all

A fixed light shines continuously without interruption and is written F, or F R for fixed red. Pure fixed lights are now uncommon as principal seamarks because they are easily lost among shore lighting, but they persist on leading lines, bridge and pier-head marks, and obstruction lighting. Combined characteristics exist, such as F Fl, a fixed light with a periodic brighter flash, and Al, an alternating light changing colour without going dark.

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Lights & Light Characteristics

Flashing (Fl)

Light where the dark period is longer than the flash

In a flashing light the duration of light is clearly shorter than the duration of darkness, and flashes appear at regular intervals at a rate of fewer than 50 per minute. Written Fl, with period appended, as Fl 5s. Group flashing repeats a fixed number of flashes each period, written Fl(3)15s. Composite group flashing combines two different group counts, Fl(2+1)10s, and in the IALA system is reserved for preferred channel marks.

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Tidal Streams & Currents

Flood Stream

The stream running while the tide is generally rising

Usually sets toward the head of an estuary or up-channel, but the correlation with rising water is not exact: in many channels the stream continues to run for an appreciable time after high water because it is driven by the horizontal pressure gradient of the progressing tidal wave rather than by local water level. Always take the stream from a tidal atlas or diamond, not from an assumption about high water.

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Compass & Magnetism

Fluxgate Compass

An electronic magnetic sensor feeding heading to instruments and autopilots

A fluxgate uses saturable magnetic cores driven by an alternating current; the surrounding field unbalances the cores, and the resulting second harmonic signal is proportional to the field component along each axis. Two or three orthogonal sensors resolve the horizontal field into a heading, output digitally to autopilots, radar overlays and chartplotters. Fluxgates need a calibration turn on installation, are sensitive to heel unless gimballed or tilt compensated, and can be corrupted by nearby direct current cables.

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Lights & Light Characteristics

Fog Signal

Sound made by a light structure to warn ships in fog

Charted alongside the light characteristic and abbreviated by type: Horn, Siren, Bell, Whistle, Gong or Dia for diaphone. A period is often given, such as Horn(2)30s, meaning two blasts every thirty seconds. Sound in fog is unreliable for bearing because refraction, wind shear and the vessel's own noise create silent sectors, so a fog signal should be treated as confirmation that a mark is somewhere near, never as a bearing.

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Depth Sounding & Bathymetry

Forward Looking Sonar (FLS)

Sonar that scans ahead of the boat for shallowing or obstructions

Transmits a beam forward and downward to give warning of rising ground or isolated obstructions before the vessel reaches them. Useful range is strongly depth dependent, commonly around eight to ten times the water depth, so in five metres of water it may show only forty to fifty metres ahead, which at six knots is under thirty seconds. It supplements but does not replace careful pilotage and slow speed in uncharted or coral waters.

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Anchoring & Mooring

Fouled Anchor

An anchor snagged on the bottom or tangled with its cable

Occurs when the anchor hooks a mooring chain, cable, wreck or rock, or when the flukes trap their own cable after the vessel swings with the tide. Clearing methods include motoring over the anchor from the opposite direction, using a tripping line led to the crown, or passing a bight of line under the obstruction. A foul hawse describes two cables twisted together after repeated swinging at a moor.

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Coastal Piloting Techniques

Four-Point Bearing (Bow and Beam)

Special case of doubling the angle using 45 and 90 degrees

A point is 11.25 degrees, so four points is 45 degrees. Log the distance run from when an object lies 45 degrees on the bow until it is abeam at 90 degrees. That distance equals the beam range, giving the distance off at the moment of the beam bearing and hence a fix combined with the beam bearing itself. It is popular because both angles are easy to judge accurately, but like all such methods it degrades badly in a cross tide.

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Passage Planning

Fuel and Endurance Reserve

Checking you carry enough fuel, water and food

The calculation of range against consumption, with a margin. Motoring endurance is usable tank capacity divided by hourly consumption, multiplied by speed: a yacht burning 2 litres per hour at 5 knots from 100 usable litres has about 50 hours and 250 miles. Good practice keeps a reserve of at least 20 per cent for headwinds, foul stream, manoeuvring and diversion, and applies the same thinking to water, gas and battery capacity.

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Radar Navigation

Gain and Tune

The two basic controls that set receiver sensitivity and peak performance

Gain sets receiver amplification and should be adjusted on a long range scale with clutter controls off, until a light speckled background noise is just visible; less than that means weak targets are being lost. Tune matches the receiver to the transmitter frequency in magnetron sets and drifts as the magnetron ages, so a set that has lost range often only needs retuning. Solid-state radars are inherently tuned and offer no such control.

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Meteorology for Navigators

Gale Warning

An official alert that winds of force 8 or more are expected

Issued when mean winds of Beaufort force 8, that is 34 knots or more, or gusts of 43 to 51 knots, are expected. Severe gale is force 9 or gusts 52 to 60 knots, storm force 10 or gusts 61 to 68, violent storm force 11 or gusts 69 knots and above, and hurricane force is force 12. Timing terms are strictly defined: imminent within 6 hours, soon within 6 to 12, later beyond 12.

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Page 7 of 22

Sailing reference

The YouSail sailing glossary

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