YouSail

Loading this page…

Australian sailing — learn, improve, connectManage a listing Join YouSail

Marine Services Retailers Marine Hardware Sailing Clubs

Map Directory Marinas Charters Passages Guides

FAQs Glossary Sailing Schools Courses Videos Boats Boats for sale Academy

Brokers Boat Insurance Online Retailers Brands

Brands Meet the Sailors

Search YouSail

Search YouSail

Explore YouSailClose menu⌄

Search YouSail

Directory⌄

All Directory Marine Services Retailers Marine Hardware Sailing Clubs

Destinations⌄

All Destinations Map Directory Marinas Charters Passages Guides

Learn⌄

All Learn FAQs Glossary Sailing Schools Courses Videos Boats Boats for sale Academy

Marketplace⌄

All Marketplace Brokers Boat Insurance Online Retailers Brands

Explore⌄

All Explore Brands Meet the Sailors

Australian sailing intelligence for learning, planning and finding the local people who can help.

Explore

Directory Charters Sailing clubs Marinas Marine Services

Guides & tips FAQs Glossary Videos

Marine Insurance

Sailing YouTubers

Meet the Sailors

More from YouSail

Sailing Passages

A safer starting point

YouSail supports decisions; it does not replace current charts, official warnings, local notices or the skipper’s judgement.

Join YouSail Manage a listing

Australian marine directory

Browse every marine service

Search the directory →

Supplies & chandlery

Chandlery
Marine hardware
Safety equipment
Ropes, canvas & upholstery
Online sailing store
Boat trailers

Marinas & boatyards

Marina service
Berthing & moorings

© 2026 YouSail · Learn, improve, connect · About · Sitemap · Contact

Haul-out / slipway

Boat cleaning & detailing

Mooring service

Sails & rigging

Sailmaker
Sail repair
Rigger
Rig inspection

Mechanical & electrical

Engine service
Marine diesel service
Outboard service
Marine electrical
Marine electronics
Navigation electronics
Marine communications
Marine refrigeration
Watermakers
Marine batteries

Hull & deck

Boatbuilding & repairs
Fibreglass & composites
Antifouling & painting
Boat builder

Professional services

Yacht brokerage
Marine surveyor
Marine insurance
Boat transport
Sailing charter company
Boat dealer / brand sales

Training

Sailing academy / training
Skipper & safety training
Sailing club

views

Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.

All A B C D E F G H I J K L

642 terms

Page 20 of 22

Tidal Streams & Currents
Tidal Stream
The horizontal flow of water caused by the rise and fall of tide
Expressed as a set, the true direction toward which the water flows, and a drift or rate in knots. It is periodic and predictable, reversing or rotating over the tidal cycle, and is distinguished from ocean currents, which are largely non-tidal. Rates are strongest in constrictions and over headlands, and off north-west Europe can exceed 8 knots at springs in places such as the Pentland Firth.
Read definition →
Tidal Streams & Currents
Tidal Stream Atlas
A booklet of hourly arrow charts showing streams across an area
Admiralty tidal stream atlases present thirteen chartlets, one for each hour relative to high water at a reference port such as Dover. Arrows show direction, and paired figures give neap and spring rates in tenths of a knot, so 19,34 means 1.9 knots at neaps and 3.4 at springs. Weight of arrow indicates relative strength. They give a far better regional picture than isolated diamonds.
Read definition →
Dead Reckoning & Estimated Position

Tidal Stream Vector for an Hour

The set and rate drawn on the chart as a single arrow for one hour

For each hour of a passage leg, the tidal stream is represented by an arrow drawn in the direction of set with length equal to the rate in nautical miles, carrying three arrowheads by convention. Where a leg spans several hours the vectors are laid end to end from the starting position and the sum used as one resultant. Hours are counted relative to high water at the reference port, not clock hours from departure.

Read definition →

Tidal Calculations

Tidal Window

The span of time when there is enough water to pass safely

Found by entering the tidal curve with the required height of tide and reading off the times at which the rising and falling curve crosses it, giving the earliest and latest safe transit times. The window must be reduced by the passage time across the shoal and by a safety margin. For a bar entry needing 2.0 m on a tide peaking at 3.4 m, the window may span several hours around high water.

Read definition →

Tidal Streams & Currents

Tide Rip and Tide Line

A visible line on the sea where two bodies of water meet

A convergence zone showing as a band of ripples, smooth water, foam or accumulated weed and flotsam, often marking a boundary between water masses of differing density, temperature or direction of flow. Useful visually as evidence of stream direction and of eddy boundaries, and often a productive fishing mark. Crossing one can bring an abrupt change in sea state, apparent wind and the vessel's steering behaviour.

Read definition →

Tide-Generating Force

The pull that raises tides, from gravity and orbital motion combined

The residual force at any point on Earth after subtracting the centripetal acceleration of the Earth-Moon (or Earth-Sun) barycentric rotation from the local gravitational attraction of that body. It varies with the inverse cube of distance, not the inverse square, which is why the Moon dominates: the Sun's mass is vastly greater but its distance makes its tidal effect roughly 46 per cent of the Moon's. The residual is directed outward on both the sub-body and antipodal sides, giving two bulges.

Read definition →

GNSS & Satellite Navigation

Time To First Fix (TTFF)

How long a receiver takes to produce a position after switching on

A hot start, where the receiver retains valid ephemeris and a good time and position estimate, can fix within a few seconds. A warm start needs fresh ephemeris and typically takes around thirty seconds or so. A cold start after a long lay-up, a flat backup battery or a large geographical move requires almanac collection and may take several minutes. Poor sky view or a shaded antenna lengthens all three.

Read definition →

Topmark

Small shape fixed above a buoy to confirm its meaning

Topmarks resolve ambiguity when buoy shape is unclear or colour is degraded by daylight, glare or fading. Standard forms are the can and cone for lateral marks, two cones for cardinals, two spheres for isolated danger, a single sphere for safe water, an X for special marks and an upright cross for emergency wreck marks. On pillar and spar buoys, where the body gives no shape information, the topmark is the primary daytime identification and should always be sought before committing.

Read definition →

COLREGs - Lights, Shapes & Sound Signals

Towing Light

Yellow light above the sternlight shown by a towing vessel

A yellow light with the same 135 degree characteristics as a sternlight, mounted in a vertical line directly above it on a vessel towing astern. That vessel also carries two masthead lights in a vertical line, or three when the length of the tow from her stern to the after end of the tow exceeds 200 metres. In United States inland waters a vessel pushing ahead or towing alongside shows two towing lights instead of a sternlight.

Read definition →

Great Circle & Ocean Routing

Trade Wind Route

The classic downwind crossing using reliable belts of steady wind

Sailing routes have long exploited the trade winds, which blow from the north-east in the northern hemisphere and the south-east in the southern, roughly between five and thirty degrees of latitude. The traditional Atlantic passage runs from the Canaries or Cape Verdes south-westwards until the trades are firmly established, typically near twenty degrees North, before turning west for the Caribbean. The route is far longer than the great circle but faster under sail.

Read definition →

Chart Symbols & Abbreviations

Traffic Separation Scheme Symbology

Purple chart markings showing one-way shipping lanes and their boundaries

Lanes are shown by broad purple arrows indicating the established direction of traffic flow, with the separation zone tinted or hatched between them and the outer limits drawn as dashed purple lines. Inshore traffic zones lie between the scheme and the coast. Schemes marked as IMO adopted carry the full force of COLREGs Rule 10; those not adopted by IMO are advisory but still charted. Precautionary areas are ringed and often carry a roundabout symbol.

Read definition →

Speed, Log & Distance

Trailing or Walker Log

A spinner towed astern on a line to record distance run

A rotator streamed on a braided line of perhaps 30 metres turns a mechanical counter mounted on the taffrail, reading distance in nautical miles. It requires no hull penetration and no electrical power, remains accurate over long ocean passages and is unaffected by fouling of the hull, which is why it survives as an offshore backup. Drawbacks include vulnerability to fish strikes and weed, and the need to recover it before manoeuvring astern.

Read definition →

Depth Sounding & Bathymetry

Transducer Offset

The correction applied so displayed depth means what you intend

The instrument measures from the transducer face. Setting a positive offset equal to the transducer's depth below the waterline gives depth below the surface, which is what compares directly with charted depth plus height of tide. Setting a negative offset equal to the distance from transducer to the lowest point of the keel gives clearance under the keel. Whichever is chosen must be known by everyone aboard, since confusion of the two has grounded many yachts.

Read definition →

Position Fixing

Transferred Position Line

An older position line moved forward to match the distance you have sailed

A position line remains valid only at the instant of observation. To combine it with a later observation, every point on it is advanced parallel to itself through the course and distance made good, including tidal set and leeway, over the intervening time. Plotted with a double arrowhead and labelled with the original and transferred times, it underlies the running fix and the celestial sun-run-sun. The transfer inherits all the uncertainty of the estimated run.

Read definition →

Pilotage & Harbour Approach

Transit

Two fixed objects seen in line giving a precise bearing

When two charted objects appear one directly behind the other, the observer lies on the line joining them, extended seaward. A transit is the most accurate line of position available and needs no instrument, so it is used to check compass error, to hold a track, to mark the edge of a danger and to detect a dragging anchor. Sensitivity improves as the separation between the two marks grows relative to the observer's distance off.

Read definition →

Coastal Piloting Techniques

Transit (Range in US usage)

Two fixed objects seen in line, giving a very precise bearing

When two charted objects appear one directly behind the other, the observer lies on the straight line joining them. That line is drawn on the chart and is far more accurate than a compass bearing because no instrument error is involved. Transits are used to check compass deviation, to mark the edge of a hazard, and as leading lines into harbour. Accuracy improves as the ratio of the distance between the two marks to the observer's distance from the nearer mark increases.

Read definition →

Compass & Magnetism

Transit (Range) Compass Check

Using two charted objects in line to test the compass

When two charted marks come into line, the vessel lies on the transit, whose true bearing is measured from the chart. Reading that transit over the compass at the moment of alignment and comparing the two values gives compass error on the heading being steered. Many harbours have published leading lines with their bearing printed on the chart. The method is quick, needs no calculation, and works equally well for checking a steering or hand bearing compass.

Read definition →

Chart Projections & Datums

Transverse Mercator Projection

A Mercator turned on its side, good for narrow north-south areas

The cylinder is rotated so it touches a chosen central meridian rather than the equator, giving minimal distortion in a north-south strip a few degrees wide. It underpins the Universal Transverse Mercator grid and national systems such as the Ordnance Survey National Grid, and it is used for large scale harbour and coastal charts in higher latitudes where standard Mercator scale distortion would be objectionable. Rhumb lines are no longer straight, but over a single sheet the difference is negligible.

Read definition →

Dead Reckoning & Estimated Position

Traverse Sailing

Adding up several short legs mathematically to find one net movement

When a vessel sails a series of short courses, as when tacking, each leg is resolved into northerly and easterly components using departure equals distance times sine of course and difference of latitude equals distance times cosine of course. Summing the signed components gives the net difference of latitude and total departure, from which a single equivalent course and distance follow. Departure is converted to difference of longitude by dividing by the cosine of the mean latitude.

Read definition →

COLREGs - Lights, Shapes & Sound Signals

Tricolour Lantern

Single masthead lantern combining sidelights and sternlight on small yachts

A sailing vessel of less than 20 metres may combine her red, green and white lights in one lantern carried at or near the top of the mast, keeping the standard arcs of 112.5, 112.5 and 135 degrees. Height gives excellent range offshore for very little current draw. It must never be shown together with deck-level sidelights, nor when the engine is driving the boat, and it is less useful inshore against shore lighting.

Read definition →

Meteorology for Navigators

Tropical Revolving Storm (TRS)

An intense tropical low with violent winds circling a calm eye

A warm-core system forming over sea temperatures of about 26 degrees Celsius or more, usually between 5 and 20 degrees latitude. It is classed as a tropical storm at sustained winds of 34 to 63 knots and as a hurricane, typhoon or cyclone at 64 knots and above. Warning signs at sea include a long low swell from an unusual direction, a pressure fall exceeding about 3 hPa below the diurnal normal, and increasing cirrus.

Read definition →

Wind, Waves & Sea State

True Wind

The actual wind over the water, unaffected by the boat's motion

Wind speed and direction relative to the surface of the sea, which is what forecasts, the Beaufort scale and synoptic charts describe. It is what an anchored or stationary vessel feels. Modern instruments compute it from apparent wind combined with boat speed and heading, so its accuracy depends on correct calibration of the log, the masthead unit alignment and, for full accuracy over the ground, the tidal stream.

Read definition →

Wind, Waves & Sea State

True Wind Angle (TWA)

The angle between the true wind and the boat's heading

Measured in degrees from 0 dead ahead to 180 dead astern, port or starboard. It is the independent variable in a polar diagram, which tabulates predicted boat speed against true wind angle and true wind speed, and is therefore the basis of performance sailing and routeing software. Unlike apparent wind angle, true wind angle does not change as the boat accelerates, which makes it the meaningful measure for tactical decisions.

Read definition →

Pilotage & Harbour Approach

Tug Assistance

Using tugs to push, pull or steer a ship alongside

Tugs are engaged for berthing, unberthing and turning where wind, stream or confined water exceed the vessel's own control, particularly on high-sided ships at low speed when the rudder has little effect. They work made fast forward and aft, or pushing on the shoulder and quarter, and their capability is quoted as bollard pull in tonnes. Escort tugs work astern to steer or retard a laden tanker. Speed must be reduced before a tug makes fast.

Read definition →

Pilotage & Harbour Approach

Turning Basin

Widened harbour area where large ships can swing round

An area at the head of a channel or off a berth, dredged and widened so that a vessel can be turned, usually with tug assistance and often against a stemmed tidal stream. Its diameter is designed around the longest vessel expected, commonly around twice her length, and it is charted with the maintained depth. Small craft must keep well clear while a large vessel swings, since she may occupy the whole basin and cannot give way.

Read definition →

Position Fixing

Two-Range Fix (Range-Range Fix)

Two measured distances from two landmarks cross to give position

A fix obtained by measuring the distance to two charted objects and plotting each as an arc centred on the object, the vessel lying at the intersection. Ranges usually come from radar, and radar ranges are more trustworthy than radar bearings because horizontal beam width smears bearings by roughly half the beam width while range accuracy is limited mainly by pulse length. Choose objects roughly 60 to 120 degrees apart in relative direction for a good cut, and pick steep, isolated, positively identified targets rather than low sloping shores.

Read definition →

Depth Sounding & Bathymetry

Under Keel Clearance (UKC)

The water remaining between the bottom of the keel and the seabed

Calculated as charted depth plus height of tide minus static draught, then reduced by squat, heel, wave-induced heave and allowances for datum and survey uncertainty. Company and port policies commonly require a minimum of ten percent of draught, or a fixed figure such as 0.5 to 1.0 metres for small craft in known ground. For example, charted 3.2 metres with 1.8 metres of tide and 1.9 metres draught leaves 3.1 metres before dynamic allowances.

Read definition →

Celestial Navigation Concepts

Universal Time (UT/GMT) in Celestial Work

The single worldwide time standard all almanac entries are based on

All almanac tabulations are argued on UT1, referred to in practice as GMT and closely tracked by UTC, which is kept within 0.9 seconds of it. Since the sun's geographical position sweeps west at about 0.25 minutes of arc per second of time, a timing error of four seconds displaces a longitude line by roughly one nautical mile at the equator. Watch error and watch rate must therefore be logged and applied.

Read definition →

Radar Navigation

Variable Range Marker (VRM)

An adjustable ring you expand to measure the distance to a target

The VRM is an electronically generated circle centred on own ship whose radius is displayed numerically, and it is the most accurate measurement a radar offers. The correct technique is to touch the ring against the nearest edge of the target echo, not its centre. Most sets provide two independent markers, allowing a radar fix from two or three ranges to well-identified charted features, which is generally superior to a fix by bearings.

Read definition →

Compass & Magnetism

Variation (Var)

The angle between true north and magnetic north at your position

Also called magnetic declination ashore, variation is the local difference between the direction of the Earth's magnetic field and the geographic meridian. It is named East when magnetic north lies clockwise of true north and West when anticlockwise. Values range from zero near an agonic line to more than 30 degrees in some regions and towards very large values near the poles. Variation changes slowly with time, typically a few minutes of arc per year, so charted values need updating.

Read definition →

Page 20 of 22

Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.