>>would be amazed if somebody didn't think of trying to build such a device. The problem is that the best you could have done at the time would have been something which would have measured the flow of water through something like a tube, or turns of a prop on a logline. It couldn't possibly have accounted for factors such as current, slip, drift, wind, wave and so on. Terra firma doesn't move, but water does even when you're sitting still.<<
So many factors are involved that only some kind of computer taking inputs on all the data...from water, wind, etc....would be able to give a true instantaneous speed indicator. Are there any such devices on modern ships nowadays ?
Bicycle speedometers worked on a gear off one of the wheels and more or less just converted RPM's to MPH's, so we get back to the reckoning of ship's speed by RPM's. Bicycle wheels ran on solid ground...As you said the ground doesn't move but water is constantly in some kind of movement all the time and from all directions.
Incidentally.....In reference to Titanic's speed and the temperature just before the collision, the wind chill factor in the crow's nest would have been something close to 10 Degrees Fahrenheit. It seems as if that would have some effects on the lookouts ability to see things.