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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.
Loading this page…
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.