watch the target through the periscope while your speed is as close as possible to zero.
Id the target
point the periscope at a point slightly ahead of the target ship.
when the front of the target ship touches the vertical cross hair in the periscope start your clock and do not move the periscope.
when the back of the target ship crosses the vertical cross hair in the periscope stop your clock.
find the target ship's lenght in the recognition manual..
length of ship divided by time in seconds multiplied times 1.94 = speed in knots
round up to the nearest knot. for example if your calculation revealed a speed of 4.12 knots... call it 5 knots.
thats a quick "on the fly" method of calculating speed. and its pretty darned accurate.
for convoys, just "clock" the nearest ship, and its a safe bet that every ship in the convoy is moving at the same speed
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