I am trying to learn a more realistic way to obtain speed, never learned trig, but if all else fails, try using your map.
What I do is when I have the target in sight, go to the map and with your marker mark the position of the target and your position. Zoom in pretty tight when you do this. After a period of time remark both and measure distance covered by both.
From there it is easy, just compare a known speed (yours) to obtain the targets speed.
Let's say over a set time period you have covered 200 yards at 4 knots and your target has covered 600 yards.
4/200=X/600
200X=2400
X= 12Knots, your target's speed.
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