Michael, once again my apologies for a dumb question. On second thought, there are so many variables involved - cross currents and fore and aft currents affecting the ship's speed in addition to the effects of winds, also from all directions. The bicycle speedometers depended on a direct drive off one of the wheels and more or less in relation to RPM's, which leads us back to reckoning speed by way of the engine RPM's. (The famous 78 RPM on Titanic, for example.)It would seem that some kind of computer to take all these effects into account and come up with the true speed would be the only solution to the problem of a "ship's speedometer."