YouSail

Loading this page…

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

Clear all filters

89 terms

Page 1 of 3

Electronic Charts & ECDIS
S-100 and S-101
The next generation chart data framework replacing the current standards
S-100 is a flexible geospatial data framework, aligned with mainstream ISO geographic information standards, under which many marine data products are specified. S-101 is the next generation ENC product specification within that framework, while sister specifications cover bathymetric surfaces, water levels, surface currents, marine protected areas and route information. The design allows dynamic data to be overlaid on the chart within one consistent system, with a period of dual operation alongside S-57.
Read definition →
Electronic Charts & ECDIS
S-52 Presentation Library
The rulebook that decides how chart data is drawn on screen
S-52 specifies the colours, symbols, line styles, text placement and display priorities used by every type-approved ECDIS, together with the day, dusk and night colour tables. It is why the same ENC looks essentially identical on different manufacturers' equipment, and why a navigator trained on one system can read another. It also defines the simplified and traditional paper-chart symbol sets that the user may select.
Read definition →

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

Electronic Charts & ECDIS

S-57

The international file standard that ENC chart data is delivered in

S-57 is the IHO transfer standard, in long-standing edition 3.1, that defines the object catalogue, attributes and encoding rules for ENC data. Base cells carry the .000 file extension and sequential updates follow as .001, .002 and so on. Its rigid structure is the reason ENC data from any hydrographic office in the world can be loaded into any type-approved ECDIS without translation by the user.

Read definition →

Electronic Charts & ECDIS

S-63 Data Protection Scheme

The encryption and licensing system that protects official chart data

S-63 encrypts ENC cells so they can only be decrypted by equipment holding a valid permit, and provides digital signatures that authenticate the data source and prove it has not been corrupted in transit. Each installation has a unique user permit derived from its hardware identifier, which the chart supplier uses to issue cell permits with defined expiry dates. An expired permit stops updates and eventually stops the chart displaying.

Read definition →

Radar Navigation

S-band Radar (10 cm)

A longer wavelength radar that sees better through rain and clutter

S-band operates around 3 GHz with a wavelength near ten centimetres. It penetrates precipitation and heavy sea clutter far better and holds longer detection ranges in bad weather, at the cost of a wider beam for a given antenna size and weaker returns from very small targets. Larger ships are required to carry both bands so the navigator can compare pictures; the antenna is too big for most yachts.

Read definition →

Safe Water Mark

Buoy with navigable water all around, often a landfall marker

Painted in red and white vertical stripes, with a single red sphere topmark if the body is a pillar or spar; spherical bodies need no topmark. The light is white and may be isophase, occulting, one long flash every 10 seconds, or Morse A. It marks a fairway, mid-channel or landfall position and may be passed on either side. A landfall buoy showing LFl 10s is a common first waypoint for an offshore arrival.

Read definition →

Electronic Charts & ECDIS

Safety Contour

The depth line separating water you may use from water you must not

The safety contour is the mariner-set depth that divides the display into navigable and non-navigable water, drives the anti-grounding alarm, and is drawn as a bold emphasised line. If the requested value does not exist in the ENC, the system automatically selects the next deeper available contour, so asking for 7 m in a chart holding 5 m and 10 m contours yields 10 m. The default on power-up is typically 30 m.

Read definition →

Electronic Charts & ECDIS

Safety Depth

The setting that makes shallow spot soundings stand out in black

Safety depth governs the presentation of individual spot soundings: those equal to or shallower than the set value are shown emphasised in black, while deeper soundings are drawn in a lighter grey. Unlike the safety contour it generates no alarm and moves no contour line. Setting it to the same value as the intended safety contour keeps the picture consistent; the power-up default is commonly 30 m and must be reviewed for the vessel's draught.

Read definition →

Points of Sail & Sailing Manoeuvres

Sailing by the Lee

Running with the wind slightly on the same side as the boom

This occurs when the boat bears away past a dead run so the wind crosses the stern from the same quarter the mainsail is set on. Airflow over the mainsail reverses and the boom becomes liable to slam across in an unexpected gybe. Warning signs are the leech starting to lift and the burgee or wind indicator crossing the centreline, and the cure is to luff back onto the correct side promptly.

Read definition →

COLREGs - Lights, Shapes & Sound Signals

Sailing Vessel Red over Green Lights

Optional pair of all-round lights at a sailing yacht's masthead

In addition to sidelights and a sternlight, a sailing vessel under way may exhibit two all-round lights in a vertical line at or near the masthead, red above green. They are entirely optional and may not be combined with a tricolour lantern, so a yacht showing them must also carry separate sidelights and a sternlight. Their practical value is limited, since many watchkeepers confuse them with the status lights of working vessels.

Read definition →

AIS & Vessel Tracking

Satellite AIS (S-AIS)

Receiving ship broadcasts from orbit to track vessels far offshore

Low earth orbit satellites carry AIS receivers whose footprint spans hundreds of nautical miles, extending coverage well beyond terrestrial range. The wide footprint causes message collisions in busy sea areas because SOTDMA only deconflicts locally, so detection is probabilistic and updates may be hours apart. Two channels, 156.775 and 156.825 MHz, are allocated for long range AIS to help. Commercial tracking websites blend satellite and shore-station feeds.

Read definition →

GNSS & Satellite Navigation

Satellite Compass

A heading sensor using two or more aerials instead of magnetism

By measuring the carrier phase difference between antennas on a fixed baseline, a satellite compass derives true heading directly, with no deviation, no variation correction and no settling error on turns. Accuracy of the order of one degree or better is typical, improving with a longer baseline. It also supplies rate of turn, pitch and roll for radar and AIS stabilisation, but it fails whenever satellite reception fails, so a magnetic standby remains essential.

Read definition →

Compass & Magnetism

Satellite Compass (GNSS Compass)

A heading sensor using two or more GNSS antennas

By measuring the carrier phase difference of the same satellite signals arriving at two or three antennas on a fixed baseline, the unit computes the orientation of that baseline in space and hence true heading, typically to a fraction of a degree with a baseline of a metre or more. It is immune to magnetic disturbance and needs no swinging, but depends on satellite reception, so signal blockage, jamming or spoofing removes the heading output entirely.

Read definition →

GNSS & Satellite Navigation

Satellite-Based Augmentation System (SBAS)

Extra satellites that broadcast correction data over a whole region

SBAS uses a network of monitoring stations to generate wide-area corrections and integrity data, uplinked to geostationary satellites that rebroadcast it on the GPS L1 frequency so an ordinary antenna can receive it. Regional systems include EGNOS in Europe, WAAS in North America, MSAS in Japan and GAGAN in India. Typical horizontal accuracy is one to two metres, and the receiver usually indicates a differential fix quality when it is in use.

Read definition →

Anchoring & Mooring

Scope

Ratio of cable let out to the water depth below the bow

Calculated as veered cable divided by depth measured from the bow roller, so charted depth plus height of tide at high water plus freeboard. Common working guidance is a minimum of 4:1 for all chain and 6:1 to 8:1 for rope with a chain leader, increased in heavy weather. Example: 6 m charted, 3 m rise, 1.5 m freeboard gives 10.5 m, so veer about 42 m of chain.

Read definition →

Wind, Waves & Sea State

Sea Breeze and Land Breeze

Daily coastal winds caused by land heating and cooling

Differential heating draws air onshore during the day, typically setting in mid to late morning and reaching 10 to 15 knots, sometimes more, and extending perhaps 10 to 20 nautical miles offshore before dying in the evening. Coriolis effect causes it to veer through the afternoon in the Northern Hemisphere. The reverse land breeze at night is much weaker, usually under 10 knots, and confined close inshore.

Read definition →

Emergency Navigation & Distress

Search and Rescue Transponder (SART)

A device that paints a bright marker line on rescuers' radar

A radar SART lies dormant until triggered by an X-band radar in the 9 GHz band, then sweeps a response that appears on the interrogating display as a line of twelve blips extending outward from its position, spaced 0.64 nautical miles apart. As the searching unit closes, the blips widen into arcs and then circles. Detection range from a ship is about five miles and considerably more from a search aircraft.

Read definition →

Great Circle & Ocean Routing

Seasonal Passage Window

The safe time of year to make a particular ocean crossing

Ocean planning starts with timing rather than track, because tropical revolving storm seasons, monsoon reversals and high-latitude ice all confine safe passages to defined months. North Atlantic trade wind crossings are conventionally made from late November onwards, after the hurricane season, while insurers commonly impose their own latitude and date limits. Routeing charts and sailing directions provide the climatological basis for fixing the window.

Read definition →

Depth Sounding & Bathymetry

Second Trace and False Echo

A misleading sounder reading caused by stray or repeated returns

Erroneous depth indications arising when a return from one transmission arrives after the next pulse has been sent, when the transducer detects fish shoals, aeration, thermoclines, a soft mud layer or the vessel's own wake, or when the display locks onto a strong side lobe return. In deep water a second trace can show a fraction of the true depth. Cross checking against the charted depth, the trend of successive soundings and a graphical echogram display, rather than a bare digital figure, helps identify them.

Read definition →

Tidal Calculations

Secondary Port

A smaller harbour whose tides are worked out from a nearby big port

Published as a set of differences applied to the standard port: time differences for high and low water and height differences at MHWS, MHWN, MLWN and MLWS. Differences are tabulated for two reference times and two reference heights, and intermediate values require linear interpolation. Errors compound quickly, so the result should always be sanity-checked against the range and against local knowledge or a chart plotter prediction.

Read definition →

Lights & Light Characteristics

Sector Light

Light showing different colours over different arcs of bearing

A sector light uses coloured screens or filters so that a mariner sees white in the safe approach sector and red or green over hazards, red conventionally on the port hand seen from seaward. Sector limits are given on the chart as bearings from seaward toward the light, in degrees true. Sectors are quoted clockwise, and the boundary is not razor sharp: an arc of a degree or two of colour mixing is normal, so do not navigate on the cusp.

Read definition →

Emergency Navigation & Distress

Securite

The safety call announcing an important navigational or weather message

Securite, pronounced say-cure-i-tay and spoken three times, introduces a safety message about navigation or meteorological warnings, for example an unlit buoy off station, a container adrift, gale warnings or a large vessel manoeuvring in a narrow channel. The announcement is made on channel 16 and the message itself is then passed on a working channel to keep the distress frequency clear. It has the lowest of the three priorities.

Read definition →

GNSS & Satellite Navigation

Selective Availability (SA)

The old deliberate degradation of civilian GPS accuracy, now switched off

For many years the United States deliberately dithered the civil GPS signal so that standalone civilian accuracy was only about one hundred metres, ninety-five per cent of the time. This was the original reason differential systems became widespread at sea. Selective Availability was set to zero in May 2000 and later generations of satellites were built without the capability, which is why unaided receivers now routinely deliver accuracy of a few metres.

Read definition →

Semi-Diurnal Tide

Two roughly equal high waters and two low waters each day

The dominant regime around north-west Europe, driven principally by the M2 constituent with a period of 12 hours 25.2 minutes, so successive high waters occur about 12 h 25 m apart and the pattern advances roughly 50 minutes per day. Successive highs are of similar height because the diurnal constituents are weak relative to the semi-diurnal ones. Most UK and Atlantic European ports are classified as semi-diurnal.

Read definition →

Dead Reckoning & Estimated Position

Set

The compass direction towards which the tidal stream flows

Set is expressed in degrees true and always names the direction the water is going to, in direct contrast to wind, which is named for the direction it comes from. A stream setting 270 true carries the vessel westward. Set is read from tidal stream atlases, from tidal diamonds on the chart with their hourly tables referred to a standard port high water, or from electronic tidal prediction, and it is interpolated between spring and neap columns.

Read definition →

Tidal Streams & Currents

Set and Drift

The direction the water flows and how fast it moves

Set is quoted as the true direction toward which the water moves, the opposite convention to wind, which is named from where it blows. Drift, or rate, is in knots, or occasionally as a distance in nautical miles over a stated interval. In the estimated position triangle, the water track and boat speed are laid off from the dead reckoning start and the tidal vector applied to give the ground track.

Read definition →

Anchoring & Mooring

Setting the Anchor

Making sure the anchor has dug properly into the seabed

After lowering the anchor to the bottom and dropping back under wind, tide or gentle astern power while veering cable, the engine is used astern at moderate revolutions once the full scope is out. Transits or a bearing are watched to confirm the vessel is stationary and the cable comes taut and steady rather than juddering. Never let the anchor pile on top of itself, as it will not dig in.

Read definition →

Celestial Instruments & Sight Reduction

Sextant Adjustable Errors (Perpendicularity, Side Error, Collimation)

The three misalignments a user can correct with the mirror screws

Perpendicularity error occurs when the index mirror is not square to the frame, detected by viewing the arc's reflection for a step. Side error arises when the horizon mirror is not perpendicular to the frame, shown by a horizontal displacement of a star image. Collimation error is a telescope axis not parallel to the frame and is rare in modern instruments. These are removed in order, since adjusting one can reintroduce another; whatever remains is index error.

Read definition →

Celestial Navigation Concepts

Sextant Altitude, Apparent Altitude and True Altitude (Hs, Ha, Ho)

The three stages of a measured height as corrections are applied

Hs is the raw reading off the instrument. Applying index error and dip gives Ha, the apparent altitude. Applying the main correction for refraction, semi-diameter and parallax gives Ho, the true or observed altitude used in sight reduction. The Nautical Almanac's inside front cover combines refraction, semi-diameter and parallax into a single altitude correction table for the sun, stars and planets, entered with Ha.

Read definition →

Anchoring & Mooring

Shackle of Cable

A standard length of anchor chain used on ships

In current British and NATO practice a shackle is 15 fathoms, that is 90 feet or 27.4 metres, and the same length is called a shot in United States usage. Chain lengths are joined by detachable links and marked with turns of wire or paint so the officer on the forecastle can report cable veered, for example three shackles in the water. Older Royal Navy practice used a shackle of 12.5 fathoms.

Read definition →

Page 1 of 3

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.