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Reporting wind speeds
Reporting Wind Speeds
"The forecast said we'd get a near gale with 30 knot winds - we got a full storm with 50".
Well ... maybe, but there are some major difficulties in measuring wind speeds aboard small boats. Firstly, forecast wind speeds are for a height of 10 metres above sea level. Fortunately, this happens to be about the average height of a sailing boat mast but an anemometer here will be influenced by the boat rolling. At lower levels, these effects are reduced though speeds will be more influenced by flows around sails and superstructures.
A further difficulty is that while the boat is moving, the speed and direction of the wind felt aboard is affected by the boat's own course and speed. As an example, a boat anchored in a wind of 20 knots would be experiencing a fresh breeze, but if motoring at 9 knots into the wind would feel an on-deck wind speed of 29 knots or near gale. On the opposite course at the same speed, the apparent wind would be reduced to 11 knots, giving only a moderate breeze.
There can be vast differences between apparent wind and the actual or true wind. Meteorological agencies are concerned only with true wind though the mast head instruments fitted to many cruising boats measure the wind as it appears to the vessel moving throught the water. Correcting this for the boat's own speed is an easy job if you have a computer linked to your wind instruments, log and GPS. It is not a trivial process but without this it can be carried out by plotting and adding the vectors on graph paper or with a scientific calculator, or perhaps with a simple program. Here is a selection of Java scripted code and applets that carry out convertion.
Code for the apparent wind calculator is written in Java Script. How it appears is influenced by the script interpreter on your browser. It has not been tested on all browser types so performance may vary.
Wind Angles Applet
For a given boat speed, true wind strength and angle of approach, this applet plots the resultant apparent strength and direction. As an example, type in a boat speed of 6 knots, apparent speed of 10 and direction of 045 degrees. Click the 'Compute' button and the true and apparent winds are calculated along with a pair of red and green arrows against the boat outline. Tail lengths are proportional to the wind speed. Notice how, for any true wind speed and direction, the apparent wind is always closer to the bow than the true wind; thus confirming what every sailor already knows - that the wind is always on the nose.
Apparent Wind Diagrams
Here's another wind applet, but instead of plotting a single wind strength and direction, this one computes them at 5 degree intervals for port or starboard sides. The diagram may take a while to appreciated but as an example begin by entering a boat speed of 10 knots, true wind speed of 20 knots, then click 'Compute'. The horizontal line at 20 knots indicates the true wind speed which remains constant no matter how the boat is turned. At 5 degree intervals, lines from ths axis are joined to their respective apparent wind point which is marked with a red circle. The angle of the line (always sloping towards the bow) indicates the wind shift. If it is above the true wind line, it strength is increased and if below, it's strength is reduced.
First introduced by Admiral Beaufort in 1808, the Beaufort scale is still in wide and regular use and forms a reliable basis for estimating wind and sea conditions without instruments. The underlying principle is that when the wind has been steadily blowing from the same direction for an hour or more, the open sea conditions assume a recognisable state that can be directly related to the truewind speed. To use the system, simply note the sea state from the following table and estimate the wind speed from the range given.
A Beaufort report consists of two parts ie.the direction and force. So for example, a simple report of NW 4 conveys in compact form that the wind is from the north west blowing at between 11 and 16 knots, with 1 metre waves from the same direction.