We dismounted our paddle wheel speed meter because our handheld GPS (and backup) are so much more accurate. I should add that currents in the Great Lakes where we sail are less than 1 knot. (the absolute accuracy of a paddle wheel is seldom better that +- 1 knot.)
I do remember a review of a fish finder and depth gauge that also senses the flow of water from sound reflections off the water. This can be an accurate measurement with no moving parts. It may be worth looking into.
We don't use our paddle wheel knotmeter much, since we aren't racing, just pleasure sailing and I pay more attention to my internal funmeter anyway. The paddlewheel is in the bilge next to the capped thruhull, if I want it I put it in place.
But our autopilot does like to have the speed input from the knotmeter- if it isn't there the autopilot will go off course after maybe 10 minutes. I think it's programmed to vary response with speed, so lacking that input confuses it.
GPS is really not all that precise for instantaneous readings at low speeds, but it's great for average speed or distance covered, or SOG as mentioned. A GPS displaying speed that has not been artificially smoothed can display a speed while sitting on a stationary table. If the location is displayed as accurate to within lets say 10 ft, fairly typical for a WAAS corrected position, that means that your actual location is within a 10 ft radius circle. Let's say for example it updates every second. One update is at one edge of that circle, then a second later it sees itself at the other side. That is 20 ft in one second, roughly 13.5 mph, a significant error, and the unit hasn't actually moved.
That was just to illustrate a point, most of the time the error is much less. I use my GPS most of the time, but I understand it's limitations.
Once a friend and I were bringing a Catalina 27 from San Francisco Bay down to Monterey. We had some big wind and swell behind us, our average speed was over 7.5 knots, pretty good for that boat. But the GPS said that the max speed we reached was 13.5 knots! I don't claim that as real, I find it hard to believe that a Cat 27, even in a strong broad reach and surfing down swells, hit almost double hull speed.
spaceplace.nasa.gov
