To illuminate on this further, the methods being used (among others) are AIS transmissions from your own vessel. and dated, stamped time entry and exit, using the D/T=S formula. This institution has advanced mapping, and plotting technology and KNOWS exactly how fast you are traveling, when you enter, exit, and your speed not just averaged, but max and complete transit time. Even if you are slowing down to 9.8 (and we all know how we like to maintain our speed!) as your speed 'bounces' up and down, whether due to pitch, roll or other inaccuracies, ANY data you transmit becomes THEIR data to skewer you.
From my experience I have found the best way to avoid this is to definitely slow down to 9.2 kts (for my vessel) and to judiciously watch for changes due to current or wind which may push the speed up. They don't give a darned about you trying to maintain 10 knots. They are giving heck about exceeding it by even 0.1 Knot though! (ask me how I know!)
A special note to ATB guys: Notice how your speed bounces in a heavy seaway on pitching or rolling. sometimes mine jumps almost a half a knot in a really heavy roll or pitch. An educated mariner would realize (and accept) that this is an inaccurate reading, but the ais transmits everything, in real time. This is THE data they are using to patrol the zones. An afterthought to the speed/data transmission rate. If your AIS speed input can be averaged out to dampen the data that may help, but on my particular vessel the same GPS that gives data to the AIS sends it to the Radar and the plotter. For maneuvering I want instant updates, not dampened data. a conundrum for sure.




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