I like the 3 minute rule, or actually the 3:15 rule (more accurate).
Make a mark on your target ships position and immediately start the chronometer (if you're zoomed in close and want precision, make the mark a little ahead of the ships stern or bow and start the chrono when the ship passes the mark). Wait 3 minutes and 15 seconds, and make a new mark (but don't stop the chrono). Measure the distance (i prefer the compass rather than the ruler). Now, the speed in knots is the distance in km multiplied by 10. I.e, the ship moved 1450 meter, then the speed is 14.5 knots.
Since you didn't stop the chrono at 3:15, make another mark at 6:30 and multiply the distance by 5 to get an even more accurate reading.
If you're using imperial instead of metric units, just multiply by ten and add a tenth, since a yard is roughly 0.9 meters.
|