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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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.
Read definition →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →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 →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.