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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.
642 terms
Page 1 of 22
Abort Point
Last position from which an approach can still be abandoned
The point on an approach beyond which the vessel can no longer turn back or stop safely, so the decision to continue must be taken before reaching it. It is derived from turning circle, stopping distance, channel width and available depth, and is marked by an unambiguous trigger such as a bearing, a range or a transit. If the entry criteria, for example a visual reference, a working engine or a berth confirmation, are unmet, the vessel turns away.
Read definition →Abort Point and Contingency Anchorage
The last place you can safely turn back or stop
The predetermined position beyond which the vessel is committed to the entry, chosen where there is still sea room, adequate depth and a workable escape course. It is fixed during planning by reference to a transit, a clearing bearing, a radar range or a waypoint, and paired with a contingency anchorage or bolt hole offering shelter and holding. Criteria for aborting, such as marks not identified, visibility below a stated limit or engine failure, are agreed before the approach so the decision is not made under pressure.
Read definition →Accumulated Error in DR
How dead reckoning drifts further from the truth as time passes
Errors in heading, log calibration, leeway estimate and tidal prediction each accumulate roughly in proportion to distance run, so a DR position degrades steadily and without warning. A one degree steering bias displaces the vessel about 1 nautical mile in 60 run. Good practice therefore treats the DR as a growing circle of uncertainty rather than a point, reduces the assumed accuracy the longer it runs unfixed, and resets it at every reliable observation.
Read definition →Admiralty List of Lights and Fog Signals (ALL)
Volumes listing every significant light and its full characteristics
Issued as lettered volumes A to V covering the world, each entry gives the international number, name, position, character, period, elevation in metres, nominal range in miles, a structure description and any sector or fog signal detail. Entries carry more information than the chart abbreviation, including sector bearings quoted as seen from seaward. Volumes are reissued annually and corrected weekly by Notices to Mariners. The US counterpart is the NGA List of Lights, with the Coast Guard Light List covering US waters.
Read definition →Admiralty List of Radio Signals (ALRS)
Volumes covering radio services useful to navigators
Published in six numbered volumes with several parts. Volume 1 covers coast radio stations, Volume 2 radio aids to navigation and differential GNSS, Volume 3 maritime safety information services including NAVTEX schedules, Volume 4 meteorological observation stations, Volume 5 GMDSS, and Volume 6 pilot services, vessel traffic services and port operations. Entries give frequencies, broadcast times, working channels and reporting procedures, all corrected weekly through Notices to Mariners.
Read definition →Admiralty Sailing Directions (Pilots)
Books describing coasts, harbours and approaches in narrative detail
The United Kingdom Hydrographic Office publishes a worldwide series of numbered volumes, each covering a defined region, giving descriptions of the coastline, landmarks, tidal streams, port facilities, pilotage requirements and local regulations, with photographs and views. They amplify what a chart cannot show. Volumes are reissued periodically and updated by annual supplements or by Notices to Mariners. The United States equivalents are the NGA Sailing Directions, split into Enroute and Planning Guide volumes.
Read definition →Admiralty Tidal Stream Atlas
Booklets of hourly chartlets showing which way the tide flows
Each page shows a sea area for one hour relative to high water at a reference port, with arrows indicating the direction of the stream and figures giving the neap and spring rates in tenths of a knot, so 15,28 means 1.5 knots at neaps and 2.8 knots at springs. Interpolate for the day's range using the computation of rates diagram. Atlases give a far better regional picture than tidal diamonds, which are point observations.
Read definition →Admiralty Tide Tables (ATT)
The annual books of predicted high and low water times and heights
Published in regional volumes, Part I gives daily predictions for standard ports and Part II gives time and height differences for secondary ports, together with harmonic constants and information on tidal levels such as MHWS, MHWN, MLWN and MLWS. Volume 1 covers the United Kingdom and Ireland. Predictions are astronomical only and are given in a stated time zone, so seasonal clock changes and meteorological effects must be applied by the navigator.
Read definition →Advection Fog (Sea Fog)
Fog made when warm damp air blows over a colder sea
Formed when air is cooled below its dew point by contact with a cooler sea surface, so it depends on the air-sea temperature difference rather than on nightfall. Unlike land fog it persists in wind, often surviving force 5 or 6, and can last for days over cold currents or spring seas. Comparing forecast air temperature with sea temperature, and watching the wet bulb depression fall towards zero, gives useful warning.
Read definition →Age of the Tide
The delay between new or full Moon and the largest tides
Arises because the ocean's response to the tide-generating potential is not instantaneous; friction and basin inertia impose a phase lag. Quantitatively it is proportional to the difference between the S2 and M2 phase lags at the port. Values of roughly one to two days are common in north-west Europe, so the highest spring range often falls a day or two after the syzygy date shown in an almanac.
Read definition →Aiming Off (Deliberate Offset)
Steering knowingly to one side so you know which way to turn
When making landfall on a low-lying or featureless coast with an unmistakable single target such as a harbour entrance, the navigator deliberately aims a mile or two to one side. On sighting the coast the vessel then turns confidently in the known direction to run down the shore to the entrance. It converts an ambiguous error into a certain one and is especially useful when position uncertainty exceeds the target's visual width.
Read definition →Air Draught and Vertical Clearance
Checking the mast fits under a bridge or cable
Available clearance equals charted vertical clearance plus HAT minus the current height of tide on Admiralty charts, because charted clearance is referenced to HAT. A bridge charted at 14 m with HAT of 5.6 m and a present tide of 2.1 m offers about 17.5 m. Add a margin for VHF aerials, wind indicators, heel and swell, and remember overhead power cables require extra safe distance for electrical arcing.
Read definition →AIS Aid to Navigation (AtoN)
A buoy, beacon or virtual mark that broadcasts its own AIS position
Transmitted in message 21, giving the mark's name, type, position, dimensions and an off-position indicator. A real AtoN carries a physical transponder. A synthetic AtoN is transmitted on behalf of an existing mark from elsewhere, while a virtual AtoN has no physical object at all and is used to mark new wrecks or temporary hazards rapidly. Displays show them with a distinctive diamond-based symbol rather than a ship triangle.
Read definition →AIS and Radar Target Association
Merging the radar echo and AIS report of the same ship into one target
ECDIS and radar apply gating criteria on position, course and speed to decide whether a tracked radar target and an AIS report represent one vessel. When associated, a single symbol is shown, with the operator able to choose which source has priority for display and alarms. Association can fail or attach to the wrong contact where ships are close together, so verify by watching aspect and behaviour rather than trusting the merged label.
Read definition →AIS Base Station
Shore transmitter that receives ship reports and can send data back
Operated by a competent authority, base stations receive the local AIS picture and can transmit messages such as tidal or meteorological data, binary broadcasts, virtual aids to navigation and assignment commands that alter a ship's reporting rate or channel within a defined region. Their coverage sets the practical inshore range of terrestrial AIS, typically 40 nautical miles or more depending on mast height, and feeds VTS and coastguard displays.
Read definition →AIS Limitations and Spoofing
The reasons AIS data can be missing, wrong or deliberately falsified
Not all vessels carry AIS, notably small craft, many fishing boats and warships, so an empty screen never means clear water. Data entry errors affect name, dimensions and status. Because the protocol carries no authentication, identities and positions can be falsified, and vessels may disable transmission to conceal movements. AIS therefore supplements but never replaces radar, a proper lookout by sight and hearing, and the Collision Regulations.
Read definition →AIS Man Overboard Device (AIS-MOB)
A personal beacon worn by crew that reports a casualty's position
Typically integrated into a lifejacket and activated automatically on inflation or manually. It transmits GNSS position using identity numbers beginning 972, allowing the parent vessel and nearby craft to steer directly to the casualty rather than searching visually. Many units also trigger a DSC alert to the parent vessel's VHF. Range is limited by the low antenna height of a swimmer, often only one to four nautical miles in any sea.
Read definition →AIS Safety Related Message
Short text sent over AIS to one vessel or to everyone nearby
Message 12 is addressed to a specific MMSI and acknowledged, while message 14 is broadcast to all receivers. Content is limited to a short free text string and is intended for navigational safety information such as reporting a hazard or confirming a passing arrangement. It is not a substitute for VHF voice communication and should never be used to negotiate collision avoidance, since delivery, display and reading by the other watchkeeper cannot be assumed.
Read definition →AIS Search and Rescue Transmitter (AIS-SART)
A survival craft beacon that shows up as a rescue target on AIS screens
An approved alternative to the 9 GHz radar SART under SOLAS. Once activated it transmits bursts of position reports, normally eight messages per minute, using its own GNSS receiver and an identity beginning with 970. Receivers display it with a distinct SART symbol and raise an alarm. Antenna height governs range, so mounting it as high as practicable in the liferaft matters. Battery endurance is at least 96 hours.
Read definition →AIS Target Symbol
The triangle or shape used to draw an AIS contact on a display
Sleeping targets appear as a small isosceles triangle oriented along heading or course. Selecting one activates it, adding a course over ground vector, a heading line and, where rate of turn is available, a turn indicator. Dangerous targets flash and are shown in bold, and lost targets are crossed through. Presentation is standardised so that AIS symbols remain visually distinct from radar tracked targets on the same display.
Read definition →AIS-SART
A survival beacon that shows up as a target on AIS displays
An AIS-SART transmits its GNSS position on the two AIS frequencies, 161.975 and 162.025 MHz, sending a burst of position reports each minute so it appears on nearby AIS receivers and ECDIS as a distinctively marked active SART target. Range is essentially VHF line of sight, commonly some five to ten miles from a small vessel. It gives numeric position rather than the radar SART's relative bearing and range pattern.
Read definition →All-round Light
Light visible through a full circle of 360 degrees
A light showing an unbroken arc of the horizon of 360 degrees. All-round lights serve as anchor lights, as the status lights of vessels not under command, restricted in ability to manoeuvre, constrained by draught, fishing or engaged on pilotage duty, and as the combined masthead and sternlight permitted to a power-driven vessel of less than 12 metres. Minimum range is generally 2 miles, rising to 3 miles for vessels of 50 metres or more.
Read definition →Almanac
Coarse orbit data for the whole constellation, used to find satellites
The almanac is low precision orbital and health information for every satellite in the constellation, broadcast by each satellite in turn. A receiver uses it to predict which satellites should be visible and with what Doppler shift, so it can search efficiently rather than sweeping blindly. Almanac data stays usable for weeks to months, whereas ephemeris expires within hours; a full almanac download takes several minutes of uninterrupted reception.
Read definition →Alternating Light (Al)
Light that changes colour without ever going dark
An alternating light shows different colours in succession from the same direction, with no eclipse between them, written for example Al WR or Al Bu Y. The IALA emergency wreck marking buoy uses Al Bu Y 3s, alternating blue and yellow with a one second flash of each and a half second eclipse. Alternating lights are visually distinctive but blue is poorly perceived by the dark-adapted eye at long range, limiting practical detection distance.
Read definition →Altitude Corrections (Dip, Refraction, Semi-Diameter, Parallax)
The set of fixes turning a sextant reading into a true altitude
Dip removes the depression of the visible horizon and is about 1.76 times the square root of height of eye in metres, in minutes of arc. Refraction, always subtractive, is around 1 minute at 45 degrees altitude but exceeds 30 minutes at the horizon. Semi-diameter, roughly 16 minutes, adjusts a sun or moon limb to its centre. Parallax in altitude matters chiefly for the moon, where it can reach nearly a degree, and slightly for Venus and Mars.
Read definition →Amphidromic Point
A place in the sea where a tidal constituent has almost no range
A nodal point of a rotary standing wave in a basin, about which co-tidal lines radiate and co-range lines form roughly concentric rings of increasing amplitude. The tidal wave rotates anticlockwise in the northern hemisphere in most basins because of Coriolis deflection. Examples exist in the southern North Sea. Each constituent has its own amphidromes, so a point of zero M2 range may still show a measurable diurnal tide.
Read definition →Amplitude
The sun's bearing at rising or setting, used to check compass
When a body's centre is on the celestial horizon, its bearing north or south of due east or west can be computed from the relation that the sine of the amplitude equals the sine of the declination divided by the cosine of the latitude. The observed compass bearing at that moment, compared with the true value, yields total compass error. In practice the sun's lower limb is observed sitting about half a diameter above the visible horizon.
Read definition →Anchor Buoy and Tripping Line
A float and line letting you pull the anchor out backwards
The tripping line is made fast to the crown or dedicated trip eye and buoyed at the surface, its length slightly exceeding the maximum expected depth so the buoy does not pull the anchor out at high water. It marks the anchor's position for other craft and allows recovery in the reverse direction if the flukes are hooked. In strong streams it can foul propellers, so it is often streamed only where the bottom is known to be foul.
Read definition →Anchor Light
All-round white light shown by a vessel lying at anchor
A vessel at anchor exhibits an all-round white light where it can best be seen. Vessels of 50 metres or more show one forward and a second, lower, near the stern, and those of 100 metres or more also use working lights to illuminate their decks. By day a single black ball is shown forward. Vessels of less than 7 metres are exempt unless anchored in or near a narrow channel, fairway or anchorage where others navigate.
Read definition →Anchor Rode (Cable)
The chain or rope joining the boat to its anchor
All-chain rode is standard on cruising yachts because its weight forms a catenary and it resists chafe on coral or rock. Rope rodes use nylon three-strand for elasticity, always with at least a boat length of chain next to the anchor to keep the pull horizontal and absorb abrasion. Rode should be marked at regular intervals, commonly every five or ten metres, so veered length can be read accurately.
Read definition →