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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.

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

642 terms

Page 17 of 22

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.
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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.
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Celestial Instruments & Sight Reduction

Shades and Filters

Coloured glasses that protect the eye when observing the sun

Swing-in index shades sit between the index mirror and the horizon mirror to attenuate the body, while horizon shades reduce glare from a bright sea surface. Never observe the sun without adequate index shading, since the concentrated reflected image can cause permanent retinal damage. Polarising filters offer variable attenuation. Choose the lightest combination that still gives a crisp limb, because excessive filtering blurs the edge and degrades the sight.

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Depth Sounding & Bathymetry

Shallow and Deep Depth Alarm

An audible warning set on the sounder for chosen depth limits

A settable threshold on the echo sounder that sounds when displayed depth falls below or rises above preset values. The shallow alarm is normally set to the planned minimum safe depth, that is intended under keel clearance plus draught, referenced to whatever datum the display is calibrated to, and should be reset for each leg as the plan's margins change. A deep alarm is useful when following a contour or when anchored, warning that the vessel has dragged into deeper water off the selected bank.

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AIS & Vessel Tracking

Ship Reporting System (SRS)

Scheme requiring ships to report position and details in a sea area

Adopted by the IMO under SOLAS Chapter V Regulation 11 and detailed in the relevant Admiralty Sailing Directions and List of Radio Signals. Reports use standard formats with lettered fields, for example A for ship name and call sign, B for time, C or D for position and E and F for course and speed. Examples include mandatory reporting in the Dover Strait and Torres Strait. Voluntary schemes such as AMVER support search and rescue coordination.

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Nautical Publications

Ship's Log Book

The running written record of the vessel's navigation and events

Entries are made at regular intervals and at every significant event, recording time, position with its source, course steered, log reading, speed, wind, barometric pressure, weather, sea state, engine hours and any change of watch or unusual occurrence. It is the primary evidence for reconstructing a dead reckoning position if electronics fail, and is a legal document in the event of an incident or insurance claim. Entries should be made in ink and never erased.

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Meteorology for Navigators

Shipping Forecast

The regular broadcast forecast for named sea areas around Britain

Issued by the Met Office for 31 sea areas around the British Isles and broadcast on BBC Radio 4 long wave four times daily, with gale warnings, the general synopsis, then area forecasts in a fixed order and format. Each area gives wind direction and force, weather, and visibility, using defined terms: good exceeds 5 nautical miles, moderate is 2 to 5, poor is 1000 metres to 2, and fog is below 1000 metres.

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Depth Sounding & Bathymetry

Shoal Draught Handrail Technique

Following a chosen depth contour to stay clear of danger

Rather than aiming at a mark, the navigator steers to keep the echo sounder reading within a band, for instance holding between 8 and 12 metres to parallel a coast clear of an offshore shoal. Because the sounder gives a continuous independent measurement, it cross-checks GNSS and remains valid if position fixing fails. It works best where contours run roughly parallel to the track and the seabed slopes steadily rather than in steps.

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COLREGs - Lights, Shapes & Sound Signals

Short Blast and Prolonged Blast

The two standard whistle lengths used in every sound signal

A short blast lasts about one second; a prolonged blast lasts from four to six seconds. Every manoeuvring, warning, narrow channel and restricted visibility signal is built from these two units, so timing matters, because a hesitant prolonged blast is easily read as a short one. Vessels of 12 metres or more are required to carry a whistle, and vessels of 20 metres or more must carry a bell in addition.

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Depth Sounding & Bathymetry

Side Scan Sonar

Towed sonar that produces a picture of the seabed's texture

A towfish emits fan-shaped beams to each side and records echo strength against time, building an image where hard, rough or raised features return strongly and shadows reveal height. It maps objects such as wrecks, boulders and cable routes far better than it measures depth, since it gives limited depth information directly. It is commonly run alongside multibeam so that contacts detected in the imagery can be assigned an accurate least depth.

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COLREGs - Lights, Shapes & Sound Signals

Sidelights

Red port and green starboard lights covering the forward sectors

Green to starboard and red to port, each showing an unbroken arc of 112.5 degrees from right ahead to 22.5 degrees abaft the beam. Screened inboard so neither is seen across the bow, they combine with the 135 degree sternlight to cover the full circle. A vessel under 20 metres may combine them in one lantern on the centreline. Ranges are 3 miles at 50 metres or more, 2 miles from 12 metres, and 1 mile below.

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Celestial Navigation Concepts

Sidereal Hour Angle (SHA)

A star's fixed angular position measured westward from the First Point of Aries

Since the stars keep essentially constant positions relative to each other, each navigational star is given an SHA and a declination that change only slowly through the year. The almanac lists both for 57 selected stars. Star GHA equals SHA plus GHA of Aries. Arcturus, for instance, has an SHA near 146 degrees; the exact value drifts by a fraction of a minute of arc over a season, so the current almanac must be used.

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Celestial Instruments & Sight Reduction

Sight Form / Sight Reduction Worksheet

A printed form guiding you through each step of working a sight

A structured layout that sequences the extraction of GHA and declination, the increments, the corrections to altitude, the LHA computation, the table entry and the final intercept and azimuth. Its value is error prevention: each quantity has one box, and blank boxes reveal omitted steps. Different forms exist for sun, star, planet and moon sights, since the moon uses a second altitude correction from the additional table and both v and d corrections.

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Nautical Publications

Sight Reduction Tables (NP 401 / Pub 229, NP 303 / Pub 249)

Precomputed tables turning a sextant sight into a position line

NP 401, the American Pub 229, tabulates calculated altitude and azimuth for whole degrees of latitude, declination and local hour angle, and suits any body to high precision. NP 303, the American Pub 249, is the air series: Volume 1 gives selected stars for a given latitude and hour angle of Aries, while Volumes 2 and 3 handle bodies with declinations up to 29 degrees. Both replace the spherical trigonometry with lookups.

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Page 17 of 22

Sailing reference

The YouSail sailing glossary

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