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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 19 of 22

Tidal Calculations
Standard Port
A major port with full daily tide predictions published for it
Standard ports in Admiralty Tide Tables and nautical almanacs have tabulated times and heights of high and low water for every day of the year, plus a mean spring and neap tidal curve for interpolation between them. Predictions are given in the port's time zone with a stated zone correction, so daylight saving must be applied by the user. All secondary port calculations are referred to a nominated standard port.
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Anchoring & Mooring
Standing Moor
Laying two anchors while drifting back on the tide
The reverse sequence to a running moor. The vessel stops over the downstream position, lets go the first anchor and drops astern with the stream while veering about double scope, then lets go the second anchor upstream of her. Cable is then heaved on the first and veered on the second until the vessel rides midway. It suits situations where making sternway under control is easier than manoeuvring ahead.
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Celestial Instruments & Sight Reduction

Star Finder 2102-D

A plastic disc for identifying and pre-selecting stars for a sight

A base plate with the 57 navigational stars plotted by SHA and declination, used with transparent latitude templates and a blank template for planets, which must be plotted from current almanac data. Set for the LHA of Aries at the intended observation time, it predicts the approximate altitude and azimuth of each visible star. This allows the sextant to be preset, making it far easier to find a star in the brief twilight window.

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Celestial Navigation Concepts

Star Fix (Multi-Body Fix)

A position from several star sights taken close together at twilight

Three to five stars observed within a few minutes give position lines that cross at a point. Selecting bodies spread evenly in azimuth, ideally about 120 degrees apart for three, minimises the effect of systematic errors and produces a well-conditioned fix. Sights taken more than a few minutes apart must be advanced for the run. A consistent offset in one direction in all lines typically indicates index error or an incorrect height of eye.

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Starboard-hand Mark

Lateral buoy marking the right edge of a channel inbound

In Region A a starboard-hand mark is green with a conical buoy body, or a green cone topmark point up on pillar and spar bodies, and shows a green light of any rhythm apart from composite group flashing (2+1). In Region B the equivalent mark is red with a red light and red cone topmark. Numbering commonly runs upward from seaward, with even numbers on one side and odd on the other according to national practice.

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AIS & Vessel Tracking

Static and Voyage Related Data

The slowly changing details a ship broadcasts about itself and its trip

Message 5 for Class A carries IMO number, call sign, ship name, ship and cargo type, overall dimensions expressed as distances from the GNSS antenna to bow, stern, port and starboard, static draught, destination and estimated time of arrival. It is transmitted every 6 minutes and on request. Class B uses messages 24A and 24B for name, type and dimensions. Destination and ETA are entered manually and are frequently stale or wrong.

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Speed, Log & Distance

Statute Mile (sm)

A land mile of 5280 feet, used on some inland and lake charts

The statute mile is 1609.344 m, or 5280 feet, against the international nautical mile of exactly 1852 m; one nautical mile therefore equals about 1.151 statute miles, so 10 kn is roughly 11.5 mph. Statute miles and miles per hour appear on United States Great Lakes and inland waterway charts, in some canal guides, and in shore-based weather bulletins, so units must be checked before any distance or speed is plotted.

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Emergency Navigation & Distress

Steering by Polaris

Holding a rough northerly course by the pole star at night

In the northern hemisphere Polaris sits within about one degree of the celestial pole, so its bearing gives true north closely enough to steer by when compass and electronics have failed, and its altitude in degrees approximates the observer's latitude. It is located by extending the pointers of the Plough about five times their separation. In the southern hemisphere the Southern Cross axis extended is used instead to indicate the south point.

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Compass & Magnetism

Steering Compass

The main compass the helmsman uses to hold a course

Normally a liquid-filled card compass mounted in a binnacle or bulkhead housing on or near the centreline, with a lubber line aligned exactly fore and aft, gimbals or a card floating in damping fluid, and internal lighting. On yachts it is typically read to the nearest degree or two. Siting matters more than quality: keeping it at least a metre from engines, loudspeakers, chart plotters, handheld VHF radios and mobile telephones limits deviation far more effectively than later correction.

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Tidal Streams & Currents

Stemming the Tide

Motoring or sailing against the stream just fast enough to hold position

Steering into the stream at a speed through the water equal to its rate, so that speed over the ground is nil and the vessel holds station relative to the land. Against a 3 kn ebb, 3 kn through the water gives zero SOG. It is used while waiting for a bridge, lock or berth to clear, and in a narrow channel it preserves steerage and rudder authority far better than lying stopped.

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COLREGs - Lights, Shapes & Sound Signals

Sternlight

White light showing astern over an arc of 135 degrees

A white light placed as near the stern as practicable, showing an unbroken arc of 135 degrees, that is 67.5 degrees either side of right aft, so that it exactly fills the gap left by the two sidelights. Minimum range is 3 miles for vessels of 50 metres or more and 2 miles for smaller vessels. A sailing vessel under 20 metres may combine it with her sidelights in a single tricolour lantern.

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Wind, Waves & Sea State

Swell

Long smooth waves arriving from distant weather

Waves that have travelled out of their generating area and become regular, long-crested and long-period, typically 8 to 20 seconds or more. Longer waves travel faster in deep water, so swell sorts itself by period and the longest arrives first, which is why a rising long swell can precede a distant storm by a day or more. Swell direction may differ greatly from the local wind, producing an uncomfortable cross sea.

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Depth Sounding & Bathymetry

Swept Area and Wire Drag

Survey proving no obstruction shallower than a stated depth exists

Historically achieved by towing a horizontal wire between two vessels at a set depth to snag any pinnacle, and now more usually by full coverage multibeam. Charts annotate such areas with a depth figure and a distinctive symbol indicating the area has been swept to that level. A swept depth is a positive statement of clearance rather than merely the shallowest sounding recorded, so it carries greater confidence for deep draught passage.

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Anchoring & Mooring

Swinging Circle

The area a boat can sweep while lying to her anchor

A circle centred on the anchor with radius equal to the veered cable plus the vessel's length overall, plus any offset between the bow roller and the GPS antenna. Neighbouring boats on different scope, or on rope rather than chain, will swing to different radii and at different rates, so anchor with regard to what others have laid rather than only to their present position.

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Compass & Magnetism

Swinging the Compass

Turning the boat through all headings to measure compass error

The vessel is steadied on a series of headings, usually in calm water with no tidal stream, while the bearing of a distant transit, a shore object of known bearing, or the Sun's calculated azimuth is compared with the compass reading. The difference on each heading, less the known variation, is the deviation. A professional adjuster will then fit corrector magnets and Flinders bar to reduce it before recording the residual values on a new card.

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Meteorology for Navigators

Synoptic Chart

A weather map showing pressure, fronts and systems at one time

A plot of surface observations reduced to a common time, showing isobars, fronts, highs and lows, and often plotted station circles giving wind, cloud, pressure and tendency. Analysis charts depict the observed situation; prognosis charts, labelled T plus 24, T plus 48 and so on, give forecast positions. Navigators use a sequence of charts to judge how systems are moving and deepening rather than reading any single chart in isolation.

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Electronic Charts & ECDIS

System Electronic Navigational Chart (SENC)

The internal working database the ECDIS builds from chart cells

The SENC is the ECDIS's own optimised database, created by loading base cells, applying every official update in sequence, adding any mariner's manual updates, and converting the result into a form the display engine can draw and query quickly. Alarms, route checks and cursor pick reports all interrogate the SENC rather than the delivered files. Some suppliers deliver data already converted, which shortens loading time on large folios.

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Points of Sail & Sailing Manoeuvres

Tack (Coming About)

Turning the bow through the wind to change sides

The helm calls ready about, then lee-oh or hard-a-lee, and the boat is luffed from close hauled through the eye of the wind onto the opposite close hauled course, a heading change of roughly 90 degrees for a cruising yacht. The headsail sheet is released as the sail backs and taken up smartly on the new side. Adequate speed must be carried into the turn to avoid ending in irons.

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Points of Sail & Sailing Manoeuvres

Tacking Angle

The total heading change from one upwind board to the other

The tacking angle is the sum of the close hauled angles on each side of the true wind, typically 80 to 90 degrees for a cruising yacht and as little as 70 degrees for a racing keelboat. It is measured over the ground rather than through the water when tidal set is present, so a cross tide can make one board far more productive than the other and must be allowed for when planning boards.

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Position Fixing

Three-Bearing Fix

A fix taken by compass bearings of three separate charted landmarks

The navigator takes three compass bearings in quick succession, converting each from compass to true by applying deviation and variation, then plots the reciprocal from each object. Shoot the object changing bearing most slowly first, usually the one nearest the bow or stern, and the fastest-changing beam object last, so the elapsed time causes least error. Objects should be spread across roughly 120 degrees of horizon to give strong cuts. Label the fix with the time of the last bearing.

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Tidal Bore

A wave of water that travels up a river against the current

Forms in a funnel-shaped, shoaling estuary with a large spring range, typically exceeding about 6 m, where the flood tide front steepens until it breaks as a travelling hydraulic jump. The Severn, the Qiantang and the Amazon Pororoca are well-known examples. Bores occur only on the larger springs, and small craft should be secured or clear of the channel with allowance made for the abrupt rise and violent turbulence behind the front.

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Tidal Calculations

Tidal Coefficient

A French number from about 20 to 120 showing how big today's tide is

Used in French tide tables and widely quoted on Channel and Biscay coasts, the coefficient expresses the day's range at Brest as a proportion of the mean spring range. Roughly 120 is the maximum equinoctial spring, 95 mean springs, 70 mean tide, 45 mean neaps and 20 the smallest neaps. It gives a quick feel for range and likely stream strength, but it is not a height, so predictions must still be worked properly.

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Tidal Calculations

Tidal Curve

A graph of tide height against time used to work out heights

Each standard port page carries mean spring and mean neap curves, a height scale linking low water to high water, and a time scale in hours before and after high water. To find height at a given time, enter the time scale, run up to the curve interpolated for the day's range, and read across the height scale. Ports with distorted tides, such as Southampton, use curves referenced to low water instead.

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Chart Symbols & Abbreviations

Tidal Diamond

A lettered purple diamond keyed to a tidal stream table

Each diamond marks a position where tidal streams were observed, labelled with a capital letter in a magenta diamond outline. The corresponding table on the chart lists, for each hour from six before to six after high water at a stated standard port, the set in degrees true and the rate in knots at springs and at neaps. Interpolate between the spring and neap columns using the range on the day. The data applies only near the diamond itself.

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Passage Planning

Tidal Gate

Place that can only be passed within certain tidal hours

A headland, sound, bar, bridge or lock that can only be passed safely during a limited part of the tidal cycle, because the stream would otherwise be foul or dangerously strong or the depth insufficient. Planning works backwards from the gate. If a tidal race runs fair from two hours before to three hours after local high water, and the gate lies 18 miles off at a planned 5 knots, departure falls nearly four hours earlier.

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Tidal Calculations

Tidal Height by Harmonic Method

Calculating tide height by adding up the constituent waves directly

Sums terms of the form f times H times cosine of the argument, where H is the constituent amplitude, f the nodal factor, and the argument combines the astronomical phase with the port's phase lag g. Admiralty Tide Tables Part III publishes harmonic constants for many ports lacking daily predictions, worked through the Simplified Harmonic Method form. Electronic almanacs and plotters perform the same summation, which is why they handle distorted curves properly.

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Tidal Prism

The total volume of water flowing into an estuary on one flood

Equal to the surface area of the basin multiplied by the tidal range, adjusted for the sloping intertidal profile. It governs flushing time, sediment transport and the strength of streams in the entrance channel: a large prism forced through a narrow mouth produces fast rates. Harbour engineers use it to assess whether dredged channels will scour naturally or silt, and it explains why narrow-mouthed lagoons run strongly.

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Tidal Streams & Currents

Tidal Race

An area of fast, confused water where streams pile up

Occurs where a strong stream is forced over an abrupt shoal, around a headland, or where two streams converge, generating steep standing waves, overfalls and violent turbulence. Conditions worsen dramatically when wind opposes stream. Charted with the symbol for overfalls and races and usually described in the relevant Sailing Directions, with a recommended inshore passage or a stated offing, commonly several miles, to clear the worst water.

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Tidal Calculations

Tidal Range

The height difference between one high water and the next low water

Calculated for each individual tide as high water height minus low water height, both above chart datum. It determines which tidal curve to interpolate and whether conditions are nearer springs or neaps for the day. Range is not the same as the height of tide, and comparing the day's range with the port's published mean spring and neap ranges is the standard way to judge the state of the cycle.

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Tidal Resonance

Amplification when a basin's natural sloshing period matches the tide

A gulf or shelf sea behaves as an oscillating body of water with a natural period approximated by Merian's formula, T equals 4L divided by the square root of gd for a basin open at one end, where L is length and d depth. Where this approaches 12.42 hours the M2 tide is strongly amplified; the Bay of Fundy reaches ranges above 15 m, and the Bristol Channel exceeds 12 m at springs.

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Page 19 of 22

Sailing reference

The YouSail sailing glossary

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