It's a very nice chrono, though you can use any stopwatch in the following manner;
Ship length divided by stopwatch bow-to-stern time, multiplied by 2 == target speed in knots.
Why 2? It's a rounding up of the conversion; 1 meter per second is equal to 1.94384 knots (nautical miles per hour, if you will), given that our target ship lengths are given in meters.
Example;
Target ship length is 120 meters, we time from the bow-to-stern and get 38 seconds, 120/38 = 3.2* multiplied by 2 == 6.3** knots
Figures rounded up/down and yet we still don't really need this level of accuracy, as the TDC only takes integers, so we can round the final figure off and bias our shot fore/aft depending on whether we've gone with a faster/slower target speed.
*Rounded up from 3.15789473684
**Rounded down from 6.13578947368
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