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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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A portable compass for taking bearings of shore objects
A handheld unit with a prism or sighting notch that lets the user read the bearing of a landmark while sighting it, usually to within about one to two degrees when used from a stable position. It should be used well clear of steel rigging, winches and electronics, and different positions aboard should be checked for consistency against a known transit. Because it is not fixed to the vessel, no deviation table applies, though local ferrous influence remains a real risk.
Read definition →Following a natural or charted line such as a contour or shoreline
A linear feature, typically a depth contour, a coastline, a river bank or a line of buoys, is followed as a guide. Contour handrailing is practical with an echo sounder: hold the vessel on, say, the 20 metre line by steering to keep the sounding constant. The technique demands care where the contour is steep-to or where the same depth occurs on both sides of a channel, which can produce a plausible but wrong track.
Read definition →Following a depth line or shore as a guide
Using a linear feature, most often a charted depth contour such as the 10 metre line, to bound or guide the track. The echo sounder then gives continuous confirmation: while the sounding stays deeper than the chosen contour, corrected for height of tide, the vessel remains in safe water and any shoaling is an immediate warning. The same principle covers keeping a shoreline, a wall or a line of buoys at a fixed distance to one side.
Read definition →Shallow bank across a harbour entrance that breaks in swell
A ridge of sand or shingle deposited across an estuary or river mouth where the outgoing stream meets the sea and loses velocity. Depth over it is the least on the approach and it shifts, so charted soundings may be out of date. With swell running against an ebb stream a bar can break heavily and become impassable even when depth is adequate; entry is normally planned for the last of the flood, with the stream fair.
Read definition →The individual cosine waves that are added to predict a tide
Each constituent has a fixed astronomical period and a port-specific amplitude and phase lag derived from long records of observation, ideally a year or more. Principal ones are M2 (principal lunar semi-diurnal, 12.42 h), S2 (principal solar semi-diurnal, 12.00 h), N2 (larger lunar elliptic), K1 and O1 (diurnal). Modern predictions sum tens to hundreds of constituents; shallow-water overtides such as M4 and M6 model distorted estuarine curves.
Read definition →The older logarithmic method of solving the navigational triangle
The haversine of an angle equals half of one minus its cosine, a function chosen because it is always positive, which removes sign ambiguity, and because it behaves well for small angles. Traditional reduction used log haversines with Norie's or Burton's tables, adding logarithms rather than multiplying. The method remains a valid backup and appears in some examination syllabuses, but it demands careful bookkeeping and is slower than either modern tables or a calculator.
Read definition →A distance formula that stays accurate for very short legs
The haversine of an angle is (1 minus cosine) divided by two. The formula states hav(d) equals hav(lat2 minus lat1) plus cos(lat1) cos(lat2) hav(difference of longitude). Because it works with half-angle sines rather than cosines near unity, it avoids the rounding loss that spoils the cosine rule over a few miles. Most navigation software and calculator routines use it, or the equivalent Vincenty method on the ellipsoid for higher precision.
Read definition →Distant object steered towards to hold a straight track
A single conspicuous feature, such as a chimney, tower, church or notch in the skyline, lying on the extension of the intended track and steered directly at during an approach. It is less accurate than a two-object transit because a cross-set produces no visible change until the bearing has shifted appreciably, so a head mark should always be backed up by a depth check or a clearing bearing on a second object.
Read definition →Navigating visually by what you see ahead rather than the chart
The navigator prepares a simple ordered list of expected features, headings and depths before entering, then conducts the approach by eye, using the chart only for confirmation. It suits fast craft and busy harbour approaches where plotting is impractical. Each leg carries an escape plan and an abort point. The technique relies on positive identification of marks, since a misidentified buoy in a head-up sequence propagates the error through the rest of the approach.
Read definition →Parking the boat almost stationary using backed sail and opposed helm
From close hauled the boat is tacked without releasing the headsail sheet, so the jib ends up aback, then the helm is lashed to leeward and the mainsail eased. The backed headsail tries to push the bow away while the rudder and main try to bring it up, so the boat settles at roughly 50 to 60 degrees off the wind, forereaching slowly and drifting to leeward and leaving a slick to windward.
Read definition →Extra compass error that appears when the boat leans over
When a vessel heels, the geometry of the vertical component of the Earth's field relative to the ship's own magnetic structure changes, altering the deviation. On a sailing yacht heeled 20 to 30 degrees the effect can amount to several degrees and reverses sign on the opposite tack, which is one reason a course held on one tack may not repeat on the other. It is largest at high latitudes where the vertical field component dominates.
Read definition →The level from which charted land and light heights are measured
Vertical clearances and the elevations of lights, hilltops and other conspicuous objects on Admiralty charts are referenced to Mean High Water Springs, whereas soundings and drying heights use chart datum. US charts commonly use mean high water instead. Because MHWS is well above chart datum, actual clearance under a bridge at a given moment equals the charted clearance plus MHWS minus the present height of tide, and only equals the charted figure at spring high water.
Read definition →How high your eye is above the water, which shifts the horizon
The visible horizon lies below the true horizontal by the dip angle, approximately 1.76 times the square root of the height of eye in metres, expressed in minutes of arc. At 2.5 metres in a small yacht, dip is about 2.8 minutes; at 9 metres on a ship's bridge wing, about 5.3 minutes. Dip is always subtracted from the sextant altitude. An error of one metre in the assumed height costs a few tenths of a mile.
Read definition →How far the water surface stands above chart datum right now
Always a vertical height above chart datum, never a depth. Add it to the charted sounding to get actual depth, and subtract it from a charted drying height to test whether a bank is covered. For example, a charted 2.4 m sounding with a 3.1 m height of tide gives 5.5 m actual depth; a drying height of 1.8 m with the same tide is covered by 1.3 m.
Read definition →The highest level tides reach astronomically, used for bridge clearances
The counterpart of LAT, derived over the same 19-year span. Vertical clearances under bridges and overhead power cables on Admiralty charts are measured from HAT, giving the worst-case least clearance; US charts generally use Mean High Water. Elevations of lights are measured from MHWS on Admiralty charts. Using the wrong reference when calculating air draught clearance produces an error of the order of the spring range.
Read definition →How well the seabed material grips an anchor
Charts and pilots record the bottom quality using abbreviations such as M for mud, S for sand, Cy for clay, R for rock and Wd for weed. Firm sand and stiff mud give good holding for most modern anchors; soft ooze, shingle, kelp and smooth rock give poor holding. Where holding is doubtful, use more scope, a heavier anchor, or an anchor type suited to the specific bottom.
Read definition →How wide the radar beam spreads sideways as it sweeps round
Horizontal beamwidth is the angular width of the transmitted beam and depends on antenna length and wavelength: a longer open array gives a narrower beam. A large open array may achieve around one degree, while a small yacht radome is more typically four to six degrees. The beamwidth is added to the true angular width of every target, which is why land and buoys always paint wider on screen than they really are.
Read definition →A precise fix from measuring the angles between three charted objects
Two horizontal angles between three objects define two position circles whose second intersection is the vessel. Plotted with a station pointer or by constructing the circles geometrically, this technique is largely independent of compass error and was the standard for hydrographic survey work. The fix fails when the vessel lies on or near the circle passing through all three objects, the so-called revolver or swinger, so select objects with the middle one nearer the vessel than the outer two.
Read definition →The rough speed limit of a displacement hull set by its length
Approximated by 1.34 multiplied by the square root of the waterline length in feet, or 2.43 times the square root of the length in metres, giving speed in knots. A 30 foot waterline yields about 7.3 knots. It marks where the wave the hull generates has a length equal to the hull and drag rises steeply. Light displacement and planing hulls exceed it readily, so treat it as a guide rather than a barrier.
Read definition →Water rushing through a channel because sea level differs at each end
In a strait or canal joining two bodies of water whose tides differ in time or range, the difference in head drives a flow largely independent of the local vertical tide. Slack water occurs when the two levels equalise, not at local high or low water, so timings must be taken from a stream table rather than inferred from a tide table. Hell Gate, the Cape Cod Canal and parts of the Pentland Firth behave this way.
Read definition →Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.