The water speed indicator is simply a water pressure gauge calibrated in miles per hour. The water pressure pick up tube fastens to the transom so it picks up pressure 2 or 3 inches below the bottom of the boat. One of our members brought part of a case of them to our January swap meet. I was given one he had left at the end of the day. Accuracy varies. The ones we used testing props were calibrated over a measured mile. . . 😕
