Target length is not used to compute speed. It was NEVER used, either by the Americans or the Germans. Quite simply, in order to use it at all you have to accurately identify the target. Less than 40% of targets sunk were identified properly. Not only that but the length of the target is very short compared to the method of timing positions of the entire ship over a 3 minute or longer run.
The longer the distance you time the more accurate your speed measurement is. Timing for a mere ship's length, even if you get lucky and ID it properly, results in significant error.
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