If you happen to come across the contact while surfaced, take a visual range and bearing while he is still on the horizon. Plot the position on the map. Then wait three minutes and take another range and bearing Submerge right away. Plot the second position on the map. If you timed 3 minutes precisely between fixes then simply measure the distance in yards between the two fixes and divide by 100 to get the target's speed. Course is found simply by connecting the fixes with a line extending in the direction of travel.
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