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See also Course to Steer and Estimated Position
There are several terms relating to the boat's course:
Water Track is the path of the boat through the water derived from its Heading. If this is a Compass Heading, it needs compensation for Deviation, Variation and Leeway before plotting on a chart. (All chart information and plotting uses TRUE bearings). When plotting on a chart the line has one arrow, which you could consider as the bows of the boat.
Course Over Ground - COG is the path of the boat over the ground. On a plot it has 2 arrows - 2 feet on the ground.
Course to Steer - is the required heading of the boat to achieve a given COG. When found by plotting, the true bearing of the CTS must be corrected for Leeway, Variation and Deviation to find the compass course you give to the helm.
Leeway is the sideways effect of the wind on the boat's course. It varies with the boat's angle to the wind, and is at a maximum when a sailing yacht is beating to windward when most of the wind force on the sail is to the side. It may be a 5 degrees for a very efficient yacht, maybe up to 10-15 degrees. On a reach it decreases, and on a run it is zero. Some inexperienced helms will almost compensate for leeway by continually luffing up to windward. Wind and rough water will increase leeway further.
On a motor boat leeway is highest when the wind is on the beam.
Leeway will increase with angle of heel, and low boat speed will increase the leeway angle. Heavy weather will increase leeway to as much as 30 degrees or more, depending on the boat. A yacht's keel is designed to reduce leeway.
Leeway can be difficult to determine, but can be estimated by comparing the centre line of the boat with the wake:
With experience it gets easier, particularly if you check positions and consider if leeway plotted should have been different.
When finding a Course to Steer, apply Variation, then apply leeway BY STEERING INTO THE WIND - this counteracts the leeway. Then apply Deviation last, as the other errors may change the compass heading enough to alter the deviation.
When plotting an Estimated Position, find the true heading of the boat via deviation and variation, then apply Leeway AWAY FROM THE WIND, to find the true track to plot on the chart.
sailing yacht leeway course to steer estimated position
21st May 2011