Sailing glossary: weather, tides and passage planning from A to Z

Anyone planning a sailing trip reads marine forecasts, tide curves and weather models and comes across technical terms from seamanship, meteorology and navigation. This glossary explains the most important of them in a few sentences: from AIFS, the ECMWF's AI weather model, through the turn of the tide and chart datum to wind sea and wind setup.

Where Sailseer shows a term or uses it in its calculations, the entry says how, with a link to the relevant page or guide. The explanations follow common usage in German recreational sailing. For navigation, official charts, pilot books, marine forecasts and warnings remain authoritative.

Terms from A to Z

AIFS

The Artificial Intelligence Forecasting System of the European Centre for Medium-Range Weather Forecasts (ECMWF), a weather model based on machine learning instead of physical equations. Sailseer counts AIFS as one of nine models in the consensus, but only for wind speed and direction, because it does not output gusts (Weather models).

AIS

The Automatic Identification System: ships transmit their identifier (MMSI), name, position, course and speed over VHF, and other ships and shore stations receive it. It is mandatory for commercial shipping above a certain size, and many yachts have a receiver or transponder. Sailseer shows traffic density and live ships as layers on the map, not for collision avoidance.

Anchor watch

Keeping an eye on whether the anchor is holding, once done by a watch on deck, today often by a GPS alarm that goes off when the boat leaves a set radius. In the Sail section Sailseer has an anchor watch in the browser with an alarm if the boat drags or the GPS fails, and it checks many anchorages against the wind of the next 14 hours (Map).

Bar

A shoal of sand or mud off a river mouth, a harbour entrance or in a tidal inlet, over which the sea can break in onshore wind and with wind against tide. At the day's destination of a trip, Sailseer checks the shallowest bar in the last 10 nm of the day's route and works out the tidal window from it (Tides and tidal streams).

Bascule bridge

A movable bridge whose deck is raised to let boats through, usually at fixed times or on request; boats with a mast have to wait outside these times. Sailseer builds opening times from official sources and from the operators into the passage time, for example at the Lindaunis bridge over the Schlei (Bridges and locks).

Beating to windward

Sailing against the wind in a zigzag: because a sailing boat cannot sail straight into the wind, it alternates between starboard and port tack in boards, tacking in between. At 45 degrees to the wind the distance sailed is about 40 percent longer than the direct line. Sailseer lays out the tacks for each pair of waypoints in open water, with tacking loss, current and wind for each place and hour (Beating to windward).

Beaufort (Bft)

The scale of wind force from 0 (calm) to 12 (hurricane), named after the British admiral Francis Beaufort. Force 6 is 22 to 27 kn (strong breeze), force 8 is 34 to 40 kn (gale), and from 64 kn force 12 is reached. Sailseer shows the wind in knots, Beaufort, km/h or m/s, switchable in the settings of the map.

Bora

A cold, very gusty fall wind from the north-east on the eastern coast of the Adriatic, which plunges from the Dinaric Alps onto the coast, particularly violently around Senj and in the Velebit Channel. Its gusts are far above the mean wind, and it often sets in suddenly. Sailseer recognises Bora situations in the model consensus and explains them on the Bora page.

Broad reach

A course on which the wind comes in at an angle from astern, between a beam reach and running downwind. On this course most cruising boats sail fast and smoothly. In its sailing recommendation Sailseer counts 110 to 150 degrees to the true wind as a broad reach (Map).

CAPE

Convective Available Potential Energy, the energy in joules per kilogram that rising air can release to build thunderclouds. High values show that thunderstorms are possible, not that they will come; that takes a trigger such as a front or strong heating. Sailseer shows the thunderstorm potential from CAPE as a map layer and treats a situation with 800 J/kg or more and at least 40 percent probability of rain as a thunderstorm situation that counts against sailing (Gusts and thunderstorms).

Chart datum (CD)

The reference level to which the depths on a chart refer; in tidal waters usually the lowest astronomical tide (LAT), in the German Baltic, for example, mean water level. The actual depth of water is the charted depth plus the current water level above chart datum. Sailseer shows the water level above a chart datum estimated from model series, which is not official (Tides and tidal streams).

Clearing in

The formalities when entering another country by boat: customs, immigration and, depending on the country, the port authority, health authority or fees, usually in a harbour designated for this (port of entry). When leaving a country outside shared customs and Schengen areas, you clear out accordingly. Sailseer lists clearance, customs, Schengen border, papers and fees for each country and at every border crossing on the route (Clearing in).

Close-hauled

The course on which a sailing boat sails as close to the wind as it can; cruising yachts usually manage about 45 degrees to the true wind. A destination even closer to the wind can only be reached by beating. Sailseer calls courses under 45 degrees beating and 45 to 70 degrees close-hauled, and your own boat's pointing angle can be set in the boat profile (Beating to windward).

Consensus

Combining several weather models into one joint forecast, so that a single model with an outlier does not decide the plan. Sailseer forms the consensus for each hour from the median of the models, together with their spread and how closely they agree; if the models are far apart, the rating deducts points (Weather models).

Cross sea

A sea in which two wave systems from different directions run over each other, for example wind sea and swell after a wind shift, or off headlands. It is irregular, steep and uncomfortable even when the individual waves are not high. Sailseer detects cross seas from the wave models and factors them into comfort and the rating (Weather window).

Downdraught gust

A gust that strikes the water from above: from the downdraught of a thunderstorm or shower, or as a fall wind off mountains, cliffs and high islands. It comes suddenly, often from a different direction than the gradient wind, and weather models rarely get its place and time right. Sailseer mentions downdraught gusts with the local winds and warns of thunderstorms nearby for the next hour (Gusts and thunderstorms).

Draught

The vertical distance from the waterline to the lowest point of the boat, usually the keel. Harbours, channels and anchorages also need a hand's breadth of water under the keel as a safety margin. Sailseer checks the departure and destination harbours against the draught in the boat profile, blocks a destination harbour that is shallower than the draught plus 0.3 m, and lays no tacks over water that is too shallow (Harbours and cruising areas).

Ensemble

A forecast made of many runs of the same model with slightly altered starting values; the spread of the runs shows how certain the forecast is. Sailseer uses the ECMWF ensemble with 51 runs and the DWD's ICON-D2-EPS with 20 runs for the probability of strong gusts and the fan around the wind curve (Weather models).

ETA

Estimated Time of Arrival. Sailseer calculates the ETA for every waypoint from the polar, wind and current and reads the weather for exactly that time; in the float plan, an ETA that has passed triggers the email to the shore contacts after a grace period.

GPX

The GPS Exchange Format, an open file format for waypoints, routes and tracks that chart plotters and navigation programs can read. Sailseer exports routes as GPX 1.1 for Garmin, Raymarine and B&G, one route per day of the trip, and imports routes and tracks from OpenCPN, Navionics or the plotter (GRIB and GPX for your plotter).

GRIB

GRIdded Binary, the World Meteorological Organization's file format for weather data on a grid, such as wind, gusts and air pressure per time step. Onboard programs such as OpenCPN and qtVlm and many chart plotters display GRIB files. Sailseer generates a GRIB2 file for the current map view with selectable grid spacing and time step (GRIB and GPX for your plotter).

Gust

A brief peak in wind speed above the mean wind; it is usually measured as the highest three-second average, the mean wind over ten minutes. You reef for the gusts, not for the mean wind. As its reference gust Sailseer takes the value that only a tenth of the model gusts exceed and compares it with the boat's gust limit (Gusts and thunderstorms).

High water (HW)

The highest water level within a tide; in the North Sea, two high waters are on average about 12 hours and 25 minutes apart. It does not mean flooding from a storm surge or rain. Sailseer shows high and low water in the tide curve for each harbour and at the BSH gauges (Tides and tidal streams).

Isobar

A line of equal air pressure on the weather chart. The closer together the isobars, the stronger the wind; it blows roughly parallel to them, over the sea slightly deflected towards the low, and in the northern hemisphere the low lies to your left when you have the wind at your back. Sailseer shows air pressure with isobars as a map layer and the DWD's surface charts with fronts on the map.

Knot (kn)

The unit of speed at sea: one knot is one nautical mile per hour, i.e. 1.852 km/h or about 0.51 m/s. In recreational boating, boat speed, current and wind are usually given in knots. Sailseer calculates in knots and can also show the wind in Beaufort, km/h or m/s (Map).

Leeway

The angle between the direction the bow is pointing and the actual course through the water, because the wind pushes the boat sideways; the offset caused by a current is called set. Leeway is greatest close-hauled, and small, light boats make more leeway than large yachts. Sailseer lays out tacks at the pointing angle plus 5 to 10 degrees depending on boat size, adjustable in the form (Beating to windward).

Light characteristic

The pattern of a lighthouse or lit buoy, made up of colour, rhythm and period, such as Fl(3) R 10s for three red flashes every ten seconds. At night it tells you which light you are looking at. Sailseer shows around 28,100 lights with characteristic and sectors as a layer on the map, in Finland and Portugal from official data.

Local wind

A wind that occurs regularly in a particular area because of terrain, coastline and weather pattern and has its own name, such as the Mistral, Bora or Meltemi. Local winds can be much stronger than the wind in the surrounding area and often bring downdraught gusts or funnelling. Sailseer recognises twelve local winds of the Mediterranean in the model data and names them in the rating (Mediterranean local winds).

Lock

A structure that raises or lowers vessels between two water levels in a chamber with gates at both ends; on the coast it often separates harbour basins or canals from tidal water. Locks have operating hours, and recreational boats are often locked through together with commercial shipping. Sailseer factors the waiting time and the time for locking through into the route, for example at the Kiel-Holtenau lock on the Kiel Canal (Bridges and locks).

Low water (LW)

The lowest water level within a tide. At low water, tidal flats, small creeks and some harbour approaches dry out, which is why trips in the Wadden Sea and to small drying harbours behind the dyke sluices depend on the tide. Sailseer shows low water in the tide curve and, from the water level and creek depths, works out the time windows in which a tidal inlet or sluice harbour is navigable (Tides and tidal streams).

Marine forecast

The official weather forecast for sea areas with wind, sea state, visibility and warnings, broadcast on marine radio, NAVTEX and the internet. It is what counts for the passage, not any single weather model. For every route Sailseer shows the marine forecasts for the areas it touches, for example from the DWD, Met Office, Météo-France or MET Norway, word for word (Weather window).

Meltemi

The persistent northerly wind of the Aegean in summer, strongest in July and August, often force 5 to 7 Beaufort and stronger by day than at night; in the northern Aegean it tends to blow from the north-east, in the south from north to north-west. It can last for days and accelerate sharply off headlands and between the islands. Sailseer recognises the Meltemi from May to September and describes it on the Meltemi page.

Mistral

A cold, dry north to north-westerly wind that pours down the Rhône valley into the Gulf of Lion and onto the coast of Provence and can reach Corsica, Sardinia and the Balearics. It often blows for several days under clear skies, can reach gale force and builds up a short, steep sea. Sailseer recognises Mistral situations in the consensus and describes them on the Mistral page.

Nautical mile (nm)

The unit of length at sea, called Seemeile (sm) in German: one international nautical mile is exactly 1,852 metres, roughly one minute of arc along a meridian. That is why distances on a chart are measured on the latitude scale at the side of the chart. Sailseer gives distances in nautical miles, for example the range from a harbour on the map.

Neap tides

The days around half moon (first and last quarter), on many coasts with a delay of one to two days, when the moon and sun stand at right angles to the earth and their tidal forces partly cancel out. The tidal range is then at its smallest and the tidal streams are at their weakest. Sailseer shows the moon phase with springs and neaps next to the tide curve (Tides and tidal streams).

Polar diagram

A diagram showing how fast a boat sails at a given wind strength and angle to the true wind. From it you can read passage times, the best angle when beating and the VMG. Sailseer calculates boat speed from a polar that scales with the boat profile and can be adjusted from your own GPX track or the boat instruments (Beating to windward).

Reef

Reducing sail area as the wind increases: on the mainsail in stages (first, second, third reef), on the headsail usually by furling. You reef for the gusts, and better too early than too late. Sailseer recommends the sail plan for each leg, down to the third reef, based on the expected gust, the course to the wind and the boat size (Gusts and thunderstorms).

Restricted area

A sea area in which navigation is permanently or temporarily prohibited or restricted, for example because of military firing exercises, nature conservation or safety zones; the times are published in Notices to Mariners and radio broadcasts. Sailseer routes no passage through restricted, crisis or protected areas with a navigation ban, factors in firing times for Germany and Denmark, and avoids firing ranges without known times wherever there is another way (Map).

Sailing trip

A trip on a sailing or motor boat, from a day sail to a cruise of several weeks with crew. Sailseer plans multi-day trips by daily budget with reserve days, ports of refuge and the tidal window at the day's destination on the map, and with the float plan your shore contacts know when the boat is due to arrive.

Schengen Area

The group of European states without passport checks at their shared borders. Anyone arriving by boat from a state outside it, such as the United Kingdom, Turkey or Montenegro, has to clear in at a harbour with border control; Ireland and Cyprus belong to the EU but not to the Schengen Area. Sailseer shows at every border crossing on the route whether it is a Schengen border (Clearing in).

Sea fog

Fog over the water that forms when moist, warm air flows over colder water and cools to its dew point; it can persist for days and also occurs with wind. It is typical in early summer on the North Sea and the Baltic, when the water is still cold. Sailseer estimates the fog risk for each hour from the dew point spread, wind and water temperature and shows visibility as a layer on the map.

Speed and course over ground (SOG, COG)

The speed and direction of the boat relative to the seabed, as measured by GPS (Speed and Course over Ground). They differ from speed and course through the water as soon as a current sets or the boat makes leeway. In navigation mode Sailseer shows both with a COG line on the map, and in planning the tidal stream goes into the speed over ground (Tides and tidal streams).

Spring tides

The days around new and full moon, on many coasts with a delay of one to two days, when the moon, sun and earth are in line and the tidal forces add up. The tidal range is then at its greatest, high water rises higher, low water falls lower, and the tidal stream runs at its strongest. Sailseer marks springs and neaps with the moon phase on the tide curve (Tides and tidal streams).

Swell

Waves that have left the wind field in which they formed and travel on over long distances; they are long, regular and have a long period. Swell can be high even in a flat calm and often comes from a different direction than the wind sea. Sailseer shows swell and wind sea separately, with height, period and direction of travel, from the wave models of the DWD, Germany's national weather service (Weather models).

Tack

The manoeuvre in which a sailing boat turns its bow through the wind and so changes from one tack to the other; when beating, one tack follows another. The tacking angle of cruising yachts is usually around 90 degrees, more with leeway and current, and every tack costs speed. Sailseer factors in a tacking loss by boat size and shows where the tacks turn on the map (Beating to windward).

Tidal inlet

A channel between two islands through which the Wadden Sea exchanges water with the open North Sea on every tide. Strong streams run in tidal inlets, and over the bars off them dangerous breakers can build with wind against tide or a strong onshore wind. Sailseer knows the approach buoys of the tidal inlets on the North Sea coast and calculates when the inlet and the channels behind it have enough water (Tides and tidal streams).

Tidal range

The difference in height between high and low water of a tide. It varies with springs and neaps and greatly from place to place: barely measurable in the Baltic, around 2.5 to 4 metres on the German North Sea coast. Sailseer shows the tidal range for each harbour from the tide curve, far from the North Sea too, for example up to 4 m spring range at Nosy Be in Madagascar (Tides and tidal streams).

Tidal stream

The horizontal current produced by the tide: the flood stream landwards or upriver, the ebb stream seawards, with the turn of the tide and slack water in between. In tidal inlets, estuaries and straits it can reach several knots, and when it runs against the wind the sea becomes short and steep. Sailseer factors the tidal stream into the speed over ground, waits for the turn of the tide when too much stream runs against the boat, and flags wind against tide (Tides and tidal streams).

Tides

The regular rise and fall of the water level caused by the attraction of the moon and sun; in the North Sea, high and low water alternate roughly every six hours. The Baltic and the Mediterranean, by contrast, have very little tide. Sailseer calculates tide and tidal stream from the Copernicus Marine ocean models and shows the gauges of the BSH (Germany's Federal Maritime and Hydrographic Agency); in the Mediterranean it shows tide in the northern Adriatic and in the Gulf of Gabès (Tides and tidal streams).

Traffic separation scheme (TSS)

A sea area with separate one-way lanes for shipping, similar to a motorway with a central reservation. Under Rule 10 of the International Regulations for Preventing Collisions at Sea (COLREGs), you use a traffic lane only in its direction of traffic and cross on a heading as nearly as practicable at right angles to the direction of traffic, and vessels under 20 m as well as sailing vessels must not impede power-driven vessels following a traffic lane. Sailseer checks the route's crossing angle at traffic separation schemes and places no tacks inside them (Beating to windward).

Turn of the tide

The change of the tidal stream from flood to ebb or the other way round; around this time the stream is at its weakest (slack water). German uses the same word, kentern, for a boat capsizing; the two are not to be confused. If the boat cannot make headway against the stream, Sailseer's planning waits for the turn of the tide (Tides and tidal streams).

Vertical clearance

The clear height under a bridge or overhead power line above the water; the boat fits if the mast height above the waterline plus a safety margin stays below it. In tidal waters it refers to a defined high water level, and the chart states which one. Sailseer avoids fixed bridges that the mast cannot pass under according to the boat profile (Bridges and locks).

VHF

Marine radio on very high frequency (UKW in German) between about 156 and 162 MHz, with channel 16 for distress, urgency and safety traffic and for calling, and channel 70 for digital selective calling (DSC). In Germany you need a radio certificate to operate it, on recreational boats usually the SRC. Sailseer does not replace a radio in an emergency: use VHF channel 16 or DSC; the float plan is only a safety net ashore.

VMG

Velocity Made Good, the speed at which a boat actually closes on a destination or makes ground to windward. When beating, the highest course is not the fastest; the angle with the best VMG is. Sailseer uses the VMG from the polar when beating, for example when no tacks can be laid in open water (Beating to windward).

Water depth

The actual depth of the water at a given place and time: the charted depth relative to chart datum plus the current water level. It changes with the tide, wind setup and air pressure, and in creeks and on bars also through silting. Sailseer shows depths from 0 to 20 m from EMODnet as a map layer and checks creek depths against the water level, but it does not replace a chart (Tides and tidal streams).

Weather window

A period in which wind, sea state and visibility suit a particular route and a particular boat, for example for a crossing or the return to your home port. If you are waiting for a weather window, you only cast off once the forecast is right for the whole route. Sailseer rates every full hour of the next 120 hours as a departure and sends a window alert when a window of 55, 70 or 80 points or more opens (Weather window).

Wind sea

The waves the local wind is producing right now; they are shorter and steeper than swell and travel with the wind. Their height depends on wind strength, duration and fetch, the open stretch of water over which the wind blows. Sailseer shows wind sea and swell separately as map layers and works out from wavelength and angle to the course how rough the passage will be (Weather window).

Wind setup

The rise in water level when an onshore wind pushes water against the coast; an offshore wind lowers it accordingly. On the North Sea and the Baltic, wind setup can shift the water level by a metre or more from the prediction, and in a storm surge by several metres. Sailseer shows wind setup at the BSH gauges and in the tide curve (Tides and tidal streams).

Windward and leeward

Windward is the side facing the wind, leeward the side away from it, for a boat as for an island or coast. In the lee of a coast the sea is calmer, whereas a lee shore, onto which the wind pushes the boat, is dangerous. Sailseer rates the shelter of anchorages for each wind direction, that is, whether they lie in the lee (Map).