I'm SURE some SHIII naval historical hotshot type can tell me how I'm wrong, but I'm pretty sure that convoys crossing the Atlantic during WWII did so generally at full steam for the slowest boats, which would be 9-12kts, perhaps a bit more, and try to make a zig zag pattern.
The goal was to confuse visual range and solution finding, not sonar and TMA ranging.
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LW

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