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One: Look aft if your helmsman/woman are steering a straight course and see how much your wake is carried to one side. Note; it should be straight and not serpentine.
Two: For a sailing vessel you could use an average of five degrees to leeward, Pending on the type of vessel you have.
Three: It also depends on the amount of wind you have also.
The best way is to look aft and observe your wake.
 

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Multiply the sine of the angle between your heading and your COG by your SOG. Remember that the sine of small angles (in radians) is about equal to the angle. E.g. if the yacht's heading is 255, the COG is 250 and the yacht's speed is 6 knts, the effective "leeway" is about .52 knots. The foregoing does not indicate, however, the source of the leeway, whether set and drift or side-slip. Once can use a vector diagram to calculate effective set and drift if one has an accurate knotmeter. An interesting experiment to give you an idea of your leeway is to tie a string to a cork and trail that behind the yacht 100' or so tied to the back-stay. The angle between the centerline of the yacht and the axis of the string will give you a pretty good sense of you leeway due to "side-slip" (the string averges out the minor course changes one makes when sailing a "straight" line). With a large plastic compass laid against the transom beneath the string, one can actually measure the angle and proceed as above.

FWIW...
 
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