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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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The colour used on charts to make cautions and lights stand out
Magenta is reserved for information that must catch the eye: light flares at lighthouses and buoys, radio and radar beacon circles, traffic scheme arrows, restricted and prohibited area limits, submarine cable and pipeline warnings, recommended tracks and caution notes. It is also the colour of most Notices to Mariners corrections applied by hand. Because magenta is used for so many different classes of information, the symbol shape rather than its colour identifies the feature.
Read definition →Adjusting the steered heading so the boat tracks as intended
Leeway is the angle between the vessel's heading through the water and its actual water track, caused by the wind pushing the hull sideways; it can reach 5 to 10 degrees or more on a yacht close hauled in a fresh breeze. Since the compass shows heading, not track, the helm must steer that many degrees to windward of the desired water track. Leeway is applied before tidal set when constructing a course to steer.
Read definition →The downward tilt of the Earth's magnetic field lines
The geomagnetic field is horizontal only near the magnetic equator; elsewhere it dips towards the nearer magnetic pole, reaching 90 degrees at the magnetic poles themselves. Around northwest Europe dip exceeds 65 degrees, so the horizontal component that actually drives the card is weak. Magnetic compass cards are therefore balanced for a specific zone, and a card balanced for the northern hemisphere will tilt and stick if taken far into the southern hemisphere.
Read definition →The wandering magnetic poles and the slow change they cause
The north magnetic pole has migrated across the Canadian Arctic towards Siberia at rates that have exceeded 50 km per year in recent decades, and the field as a whole drifts and weakens gradually. This secular variation is why charts state variation for a specific epoch with an annual rate of change, and why global models such as the World Magnetic Model are revised every five years, in 2015, 2020 and 2025, with out of cycle updates when drift outpaces predictions.
Read definition →Whistle signals telling others which way you are turning
When vessels are in sight of one another, a power-driven vessel under way and taking action gives one short blast for altering course to starboard, two short blasts for altering to port, and three short blasts for operating astern propulsion. These may be supplemented by matching flashes of an all-round white light lasting about a second each. In United States inland waters the equivalent signals state intent and call for an answering signal of agreement.
Read definition →Adding your own marks or dangers to the electronic chart
The mariner may insert objects such as a reported danger from a navigational warning, a caution note, a clearing line or a highlighted area, and these are stored separately from official data so they remain identifiable and are not overwritten by cell updates. They must be dated, attributed to their source and deleted once the official correction arrives. Manual entries are no substitute for applying the issued update files.
Read definition →Equipment that measures range and bearing to objects using radio pulses
A radar transmits a pulse from a rotating antenna and measures the time for energy reflected by a target to return, converting it to range, while the antenna's instantaneous pointing gives bearing. Since radio energy travels roughly 300 metres per microsecond, a target one nautical mile away returns its echo in about twelve microseconds. Its value at sea lies in fixing by range in poor visibility and in detecting collision risk objectively.
Read definition →A hand-held instrument measuring the angle between a body and the horizon
A double-reflecting instrument with an arc of about 60 degrees, one sixth of a circle, that reads angles up to roughly 120 degrees through the doubling effect of two mirrors. The index arm carries the index mirror, while the fixed horizon mirror allows the reflected body and the direct horizon to be seen together. A micrometer drum and vernier permit reading to 0.2 minutes of arc, equivalent to about 0.2 nautical miles in the resulting position line.
Read definition →A general reference on charts, ice, weather and navigation practice
NP 100 explains how Admiralty charts and publications are produced and maintained, how to interpret survey quality and datums, and how to correct a folio. Further chapters cover ice and its navigation, meteorology and oceanography, the operation of aids to navigation, environmental protection obligations and general good practice. It is the standard background text underpinning the rest of the Admiralty series and is well worth reading before relying on any of the specialist volumes.
Read definition →A nine-digit number that uniquely identifies a vessel or station
Issued by the national licensing authority and programmed into AIS, DSC VHF and EPIRB equipment. The first three digits form the Maritime Identification Digits indicating flag state, for example 232 to 235 for the United Kingdom and 366 to 369 for the United States. Ship station numbers use the form MIDxxxxxx. Coast stations begin 00MID, group calls 0MID, and handheld or auxiliary craft numbers use distinct prefixes. Reprogramming normally requires dealer intervention.
Read definition →The shore authority that runs a search and rescue operation
Each coastal state maintains rescue coordination centres responsible for a defined search and rescue region, working to the IAMSAR manual. On receiving an alert, the centre appoints a search and rescue mission coordinator, tasks lifeboats, helicopters and other assets, and may designate an on-scene coordinator among the vessels present. Merchant ships have an obligation under SOLAS to respond to distress and to follow the coordinating centre's instructions.
Read definition →Briefing where master and pilot agree the plan together
On boarding, pilot and master exchange information. The pilot card states draught fore and aft, length, beam, manoeuvring characteristics, engine type and response, thrusters and the status of navigational equipment; the pilot gives the intended route, berth and side alongside, tug arrangements, expected traffic, tidal conditions and abort options. The agreed plan is recorded and monitored by the bridge team. The pilot advises, but the master retains command and must challenge anything unsafe.
Read definition →White light shining forward over a wide arc from a power-driven vessel
A white light placed over the fore and aft centreline showing an unbroken arc of 225 degrees, from right ahead to 22.5 degrees abaft the beam on each side. Power-driven vessels of 50 metres or more carry two, the after one higher than the forward one; smaller vessels may carry one. Rule 22 minimum ranges are 6 miles at 50 metres or more, 5 miles from 20 to 50 metres, 3 miles from 12 to 20 metres, and 2 miles below 12 metres.
Read definition →The spoken radio call used when life or vessel is in grave danger
Mayday, from the French venez m'aider, is the distress signal transmitted when a vessel or person is threatened by grave and imminent danger and requires immediate assistance. The word is spoken three times, followed by this is and the vessel name three times, then the call sign or MMSI, position, nature of distress, assistance required and number of persons on board. It is normally sent on VHF channel 16 after a DSC distress alert.
Read definition →Passing on someone else's distress call when they cannot
A vessel transmits Mayday Relay when it learns of another craft in distress that cannot itself transmit, when it receives a distress call that is clearly not being acknowledged by a coast station, or when further help is judged necessary. The relaying station uses its own identity, not the casualty's, and states the casualty's position and situation so the coordinating rescue authority understands who is reporting and who is in danger.
Read definition →The average height of high water at neap tides
Averaged over the high waters of days when the range is least. With MLWN it defines the mean neap range, against which the day's actual range is compared to decide how far to interpolate between the neap and spring curves. Where a passage requires a certain depth, checking the required height against MHWN shows whether the plan works at neaps or only on springs.
Read definition →The average height of high water at spring tides
The average, over a long period, of the heights of two successive high waters during those 24 hours when the range is greatest. On Admiralty charts the elevations of lights and of coastal features such as cliff tops are measured from MHWS, and the coastline itself is drawn at that level. Together with MHWN, MLWN and MLWS it forms the four tabulated levels used in secondary port interpolation.
Read definition →The average height of low water at neap tides
The highest of the four standard low water levels, so a vessel that grounds shortly after high water on a falling spring tide may not refloat until neaps have passed. It completes the four-level framework used for secondary port height differences and provides the neap curve's lower anchor point. In the United States, the equivalent statistical levels are computed over the National Tidal Datum Epoch of 19 years.
Read definition →The average height of low water at spring tides
Averaged in the same way as MHWS but for the low waters of the greatest-range days. On older charts MLWS was itself the chart datum, so predicted heights could go negative; modern charts use LAT, which lies a little below MLWS. The gap between the two, typically a few tenths of a metre, matters when comparing an old chart's soundings with a current prediction.
Read definition →The average height of the sea over a long period of records
Computed by averaging observed levels over at least a full 19-year Metonic cycle to remove the nodal variation in lunar orbit inclination. It is the reference for land survey datums such as Ordnance Datum Newlyn and for measuring long-term sea level trends. Tidal predictions are expressed above chart datum, not MSL, so the two must be reconciled when combining chart depths with land heights.
Read definition →Average sea height, used as the zero for land maps
Mean sea level is the average height of the sea over a long period, typically 19 years, at a specified place, and it is the basis of national height systems. In Great Britain, Ordnance Datum Newlyn is defined from mean sea level measured at Newlyn between 1915 and 1921 and is the zero for land contours and spot heights. Land map heights therefore differ from chart elevations, which use MHWS, and from soundings, which use chart datum.
Read definition →The halfway point of the tide, midway between average high and low water
Mean tide level is the average of MHWS, MHWN, MLWN and MLWS, or in rough work the mean of a single high and low water, quoted as a height above chart datum. With MHWS 5.4 m, MHWN 4.2 m, MLWN 1.8 m and MLWS 0.6 m, mean level is 3.0 m. Tidal curves are drawn about it, and it approximates mean sea level closely enough for most pilotage estimates.
Read definition →A charted course with shore transits marking exactly one nautical mile
Pairs of shore beacons define two transits one nautical mile apart, and the vessel runs at steady revolutions on the course marked on the chart, which is set perpendicular to the transits. Timing between transits gives speed as 3600 divided by the elapsed seconds, so 6 minutes 40 seconds, that is 400 seconds, equals 9.0 knots. Reciprocal runs are averaged, traditionally by the mean of means, to remove the effect of tidal stream.
Read definition →Anchoring off and backing your stern in to a quay
The anchor is let go four to five boat lengths off the quay, square to it, and the vessel then backs down veering cable until stern lines can be passed ashore. The cable is then hove taut to hold the stern clear of the wall. Bows-to is a common alternative that protects the rudder and propeller. Crossing a neighbour's cable is the usual hazard, so note where others have laid theirs.
Read definition →The standard sea chart where a straight line is a constant compass course
A cylindrical conformal projection in which meridians are equally spaced parallel straight lines and parallels are spaced increasingly with latitude in proportion to the secant of latitude. Its defining property is that a rhumb line plots as a straight line, so a course can be laid off with a single protractor angle. Scale is true only along the equator or the chosen standard parallel and grows without limit toward the poles, making it unusable above roughly 70 to 80 degrees.
Read definition →Calculating a straight-line course and distance using chart stretching
Mercator sailing solves the rhumb line between two positions using meridional parts, the tabulated stretched latitude scale of the projection. The course comes from tan(course) equals difference of longitude in minutes divided by difference of meridional parts, and the distance from difference of latitude divided by cos(course). It is exact for any distance, unlike plane sailing, and remains the standard manual method for ocean rhumb line legs.
Read definition →The moment a body crosses your north-south line, giving latitude directly
At the instant of transit the body bears due north or due south, the altitude is at maximum, and latitude follows from zenith distance and declination without any need for exact time. Sun meridian passage is tabulated in the almanac as a GMT value at the Greenwich meridian; correct for longitude at four minutes per degree to get local timing. In practice the observer begins measuring a few minutes early and follows the sun until it stops rising.
Read definition →The stretched latitude scale used to build a Mercator chart
Meridional parts express the distance of a parallel from the equator on a Mercator projection, measured in units of one minute of longitude at the equator. Tabulated in Norie's and other nautical tables, the difference of meridional parts between two latitudes allows exact Mercator sailing, where tangent of course equals difference of longitude in minutes divided by difference of meridional parts. This replaces plane sailing for long distances where meridian convergence matters.
Read definition →Extra rise or fall of sea level caused by weather, not astronomy
The difference between observed and predicted sea level, driven by barometric pressure and wind. The inverse barometer effect gives approximately 0.01 m of sea level change per hectopascal, so a 980 hPa depression against a 1013 hPa standard raises level about 0.33 m. Wind set-up in shallow water can add considerably more, and negative surges from strong offshore winds or high pressure can remove a metre or more of expected depth.
Read definition →The parts of the sextant that let you read fractions of a degree
Whole degrees are read where the index arm marker falls on the arc. The micrometer drum, geared to the arm, reads minutes of arc from 0 to 60, and a vernier scale alongside it subdivides to 0.2 or 0.1 minutes. Older instruments use a vernier directly against the arc without a drum. A reading of 42 degrees on the arc, 17 on the drum and 4 on the vernier is recorded as 42 degrees 17.4 minutes.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.