Loading this page…
Explore YouSailClose menu
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.
Page 1 of 2
A small rotating wheel in the hull that counts water flow
A bladed wheel protrudes a few millimetres through the hull, usually forward and below the turn of the bilge where flow is least disturbed, and magnets in the blades trigger a sensor once per revolution. Pulses are counted for speed and totalised for distance. It is cheap and reasonably linear over normal yacht speeds but stalls at very low speed, is sensitive to hull position, heel and aeration, and requires regular cleaning through its seacock valve.
Read definition →The urgency call for a serious problem that is not yet life-threatening
Pan-Pan, spoken three times, precedes an urgency message concerning the safety of a vessel or a person where immediate assistance is not required, such as engine failure in open water, a broken rudder under tow, or a crew member needing medical advice. It ranks below distress but above all other traffic. A medical advice request may be qualified as Pan-Pan Medico, and DSC urgency alerting is available on channel 70.
Read definition →A line set at your planned passing distance from a fixed object
A parallel index is drawn parallel to the intended ground track at the planned passing distance from a selected charted radar-conspicuous object. Keeping the object's echo sliding exactly along the line confirms the vessel is on track; the echo drifting inside or outside the line reveals set immediately and continuously, without plotting. It requires an accurate heading input and a correctly identified object, and works equally well in relative or true motion.
Read definition →Written plan covering the whole voyage from berth to berth
A documented plan for the entire intended voyage, prepared before departure and built in four stages: appraisal, planning, execution and monitoring. It records courses and distances, waypoints, hazards, tidal windows, contingency ports, communications and reporting points. SOLAS chapter V requires voyage planning by every vessel, including pleasure craft in many states, and the plan should exist in a form that can be checked afterwards, whether a chart plot, a plan sheet or a spreadsheet.
Read definition →The written record of how a voyage was planned before departure
SOLAS Chapter V requires every vessel, including pleasure craft in principle, to plan its intended voyage. The documented plan covers the appraisal of charts and publications, the planned tracks with courses and distances, no-go areas, clearing bearings, tidal windows, contingency ports, waypoint lists and abort points. Retaining it in writing demonstrates due diligence and lets any crew member take over navigation. It should be reviewed and amended if conditions change during the passage.
Read definition →Time for one whole flashing cycle before the pattern repeats
The period is measured in seconds from the start of one flash or group to the start of the next identical event, and is the single most reliable discriminator between similar lights. Time at least three cycles and average, since a stopwatch started on the first flash is prone to reaction error. In Fl(2)10s the 10 seconds includes both flashes, the short eclipse between them and the long eclipse before the next group.
Read definition →A pocket beacon registered to a person rather than a boat
A PLB uses the same 406 MHz Cospas-Sarsat alerting as an EPIRB but is smaller, registered to an individual, manually activated and typically specified for a shorter minimum operating period, commonly twenty-four hours. It is not float-free and many models must be held clear of the water with the antenna upright, so a lifejacket pocket mounting is usual. It is a personal supplement to, not a replacement for, the vessel's EPIRB.
Read definition →Charted position where the pilot boards or leaves ship
A charted area, often shown by a magenta circle labelled Pilots or by a dedicated buoy, where the pilot launch or helicopter meets arriving vessels. Ports publish the boarding position, the notice required, the VHF channel and any alternative bad-weather position. The vessel is expected to arrive at the agreed time, make a lee, reduce to a manageable transfer speed of typically five to eight knots, and hold a steady course while the pilot transfers.
Read definition →Monthly climate charts of wind, current and weather for ocean planning
One sheet per ocean per month, showing wind roses giving the percentage frequency and force of wind from each of eight or twelve directions, prevailing surface currents with mean rates, percentage frequency of gales and fog, mean air and sea temperatures, ice limits and tropical storm tracks. Admiralty issues them as Routeing Charts, the US NGA as Pilot Charts. They present long term climatology, not a forecast, so they inform departure timing rather than daily decisions.
Read definition →Rope ladder rigged over the ship's side for the pilot
A side ladder rigged, secured and tended under the ship's responsibility, with a responsible officer in attendance and direct communication with the bridge. The climb from the water should not exceed 9 metres; above that a combination arrangement with an accommodation ladder is used. Steps must be horizontal and equally spaced, typically 310 to 350 millimetres apart. A lifebuoy with self-igniting light, a heaving line and adequate lighting are required at the point of access.
Read definition →White over red all-round lights marking a pilot boat on duty
A vessel engaged on pilotage duty exhibits at or near the masthead two all-round lights in a vertical line, white above red, remembered as white over red, pilot ahead. Under way she adds sidelights and a sternlight; at anchor she adds the anchor lights as well. When she is not engaged on pilotage duty she shows only the lights prescribed for a vessel of her length, and by day she flies flag H when a pilot is aboard.
Read definition →Navigating close inshore using visible marks rather than plotting
Navigation in confined water where dangers lie close aboard and there is no time for chartwork, so position is held by continuous reference to visible and radar features: transits, leading marks, clearing bearings, depth contours and the expected sequence of buoys. Confirmation is by eye rather than by three-bearing fixes. The technique depends on preparation done in advance, the largest scale chart available, and a lookout watching the water instead of a screen.
Read definition →Sketch or list of marks and headings for entering harbour
A simple written or sketched plan for the approach, prepared before arrival and designed for use in the cockpit or on the bridge wing without returning to the chart table. It lists each leg with its course, distance, expected depth, the next mark to look for and its light characteristic, plus transits, clearing bearings, wheel-over points, the abort point and the harbour working channel. The aim is to reduce entry to a sequence of confirmations.
Read definition →A simple written list of marks, courses and distances for the approach
A compact working document prepared before entering confined waters, listing each leg in sequence with course to steer, distance, expected time, tidal height and stream, marks and lights expected with their characteristics, clearing bearings and abort points. Presented as a vertical ladder it can be read in the cockpit without returning to the chart table, which matters when the navigator is also helming. It should record the height of tide at the critical shallow point and the least expected under keel clearance.
Read definition →Sailing too close to the wind so the boat slows down
Pinching means holding a heading inside the boat's best close hauled angle, so the luff of the headsail starts to lift, leading edge flow separates and speed falls away sharply. Although the angle to the mark looks better, both boat speed and lift from the keel drop, leeway increases and upwind velocity made good falls. Windward telltales lifting or streaming unevenly are the classic indication to bear away a few degrees.
Read definition →Speed measured from the water pressure created by moving forward
A forward-facing tube senses dynamic pressure, which rises with the square of speed, and this is compared with static pressure to derive speed through the water. The square law makes the instrument insensitive and inaccurate at low speeds but responsive at high speeds, which is why it is more common on fast craft than cruising yachts. The small orifice is prone to blocking by silt or marine growth, giving a sudden under-reading.
Read definition →Short-distance navigation maths that treats the Earth as flat
Plane sailing solves the right-angled triangle formed by difference of latitude, departure and distance, using distance equals difference of latitude divided by cosine of course, and departure equals distance times sine of course. The flat-Earth assumption keeps errors negligible over distances up to roughly 600 nautical miles and away from very high latitudes. For longer runs, Mercator sailing with meridional parts replaces it, because the convergence of meridians can no longer be ignored.
Read definition →Turning the chosen route into detailed courses, waypoints and notes
The second stage, in which the appraised route is laid off on the largest suitable charts as a continuous berth-to-berth track. Each leg receives a course, a distance, an expected speed and a running time, while waypoints, no-go areas, clearing bearings, abort points, tidal gates, reporting points and contingency anchorages are marked. Charted depths are compared against draught plus the required under-keel clearance, and chart and publication references are noted so the work can be checked.
Read definition →A blank gridded sheet used to plot ocean sights at large scale
Printed with a fixed set of parallels and a compass rose, the sheet is customised for the working latitude by scaling the longitude spacing, since one minute of longitude equals one minute of latitude times the cosine of the latitude. The navigator labels the parallels for the area in use, marks the assumed position, lays off the azimuth and intercept, and draws the position line. Latitude minutes serve directly as the distance scale in nautical miles.
Read definition →Position past which there is not enough fuel to return
On an ocean passage, the position beyond which remaining fuel, water or provisions are insufficient to return to the departure port, so the vessel must continue to the destination or to a port of refuge. It is computed from endurance, distance run and expected speed made good, keeping a reserve of at least ten per cent plus a weather allowance. It differs from an abort point, which concerns manoeuvring space rather than endurance.
Read definition →A speed chart showing how fast the boat sails at each wind angle
A polar plots predicted or measured boat speed against true wind angle for a series of true wind speeds, usually from six to thirty knots. It is generated by a velocity prediction program or built from logged data, and identifies optimum upwind and downwind angles for best velocity made good. Routeing software needs an accurate polar; an optimistic one produces routes the vessel cannot actually keep up with.
Read definition →Getting latitude in the north from the height of the Pole Star
Because Polaris lies close to the north celestial pole, its true altitude is approximately the observer's latitude. The Nautical Almanac provides three small corrections, a0, a1 and a2, entered with LHA of Aries, latitude and month, which together account for the star's roughly one degree offset from the pole. Their sum, minus 1 degree, is applied to Ho. The method is unavailable south of the equator and degrades in low northern latitudes.
Read definition →An older projection once used for coastal survey charts
Each parallel is developed on its own tangent cone, so parallels appear as non-concentric arcs and only the central meridian is straight and true to scale. It is neither conformal nor equal-area, but distortion is small within a narrow band either side of the central meridian, which suited long survey sheets. Historically standard for United States Coast and Geodetic Survey work, it has largely been superseded by Mercator and transverse Mercator for marine charting.
Read definition →Alternative harbour that can be reached if things change
A port, anchorage or sheltered bay identified in advance as reachable if the weather deteriorates, a crew member is injured or gear fails. For each one the plan should note the approach and its dangers, the depth and any tidal access restriction, whether entry is feasible at night or with an onshore wind and swell, and the contact channel. On a coastal passage bolt-holes should be spaced so one is always within reach.
Read definition →Lateral buoy marking the left edge of a channel inbound
In Region A a port-hand mark is red, can-shaped if it is a buoy, and carries a single red can topmark on pillar or spar bodies. Its light, if any, is red and may be any rhythm other than the composite group flashing (2+1) reserved for preferred channel marks; Fl R, Oc R, Q R and Fl(2)R are all common. Region B port-hand marks are identical in shape but green with a green light and green can topmark.
Read definition →Bits in an AIS report showing how trustworthy the sent position is
The position accuracy flag is set high when the internal fix is better than 10 metres, generally indicating a differentially corrected GNSS solution, and low otherwise. A separate RAIM flag indicates whether receiver autonomous integrity monitoring is active. Reports also carry the electronic position fixing device type. These fields let a receiving system judge whether a target's plotted position deserves confidence, though many installations use an antenna offset that is entered incorrectly.
Read definition →Warnings that a charted feature may not be exactly where shown
PA means the position is approximate, PD that it is doubtful, ED that the feature may not exist at all, and SD that the sounding is doubtful. Rep with a year means the danger was reported in that year but not surveyed. These qualifiers usually attach to isolated dangers, wrecks and shoals in poorly surveyed waters. Treat any of them as a reason to increase clearance substantially rather than to disregard the mark.
Read definition →A drawn line somewhere along which the vessel must lie
The fundamental building block of piloting, obtained from a compass bearing, a transit, a radar range, a depth contour, a sector light boundary or a celestial sight. A single line constrains position to that locus; two crossing lines give a fix and three give a cocked hat that reveals error. Lines are annotated with the time and, where advanced or retarded for a run, with both times separated by a dash.
Read definition →A line drawn on the chart somewhere along which the boat must lie
A single observation of a bearing, transit, range, depth contour or celestial altitude constrains the vessel to a locus rather than a point. Compass bearings give straight lines through the charted object; distance measurements by radar or vertical sextant angle give circular arcs centred on the object; celestial sights give arcs of position drawn as short straight lines near the estimated position. It is drawn as a single arrowhead line labelled with time and, where relevant, the observed value in degrees true.
Read definition →Buoy at a channel fork showing which branch is main
Also called a modified lateral or bifurcation mark, it takes the colour and shape of the side it should be treated as, with a single broad horizontal band of the opposite colour. In Region A, a red can with a green band means preferred channel to starboard, so leave it to port to follow the main channel. The light is composite group flashing, Fl(2+1), in the body colour, a rhythm reserved exclusively for these marks.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.