The 3min15sec rule stems from the yard/knot relationship used mostly by the US navy, or whoever also uses yards as a measurement unit. Well, you needed to in SH1 anyway. And thesedays in SC and DW.
1 nautical mile is roughly 2000 yards. (2025 to be more precise) In 3 minutes (1/20th of an hour) an object going 1 knot would have moved about 100 yards. So it is easy to calculate the speed if you can measure it's distance travelled in 3 minutes, just measure in units of hundred yards.
Since SH3 uses the metric system the 3 minute didn't hold anymore. Meters are about 10% longer that a yard. So the time period had to be enlarged a bit. The correct time would be 194.4 seconds as allready shown above. But rounding it up to 15 seconds is just so much easier in practice. The seconds handle moves a quarter of a circle forward on the dial after every interval. 3m15s, 6m30s, 9m45s, 13m. Easy to remember. Luckily the minute handle of the SH3 stopwatch goes to 12 minutes. Just another minute waiting on a full lap by the seconds handle and you've got 4 time intervals. Good enough for 0.25 knots accuracy. And you can't enter much more precise in the TDC anyway.
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