Abstract: An apparatus, according to the invention, detects the relative speed of the apparatus with respect to the fluid into which it extends. The apparatus includes a streamlined body portion having an upper end, a lower end, a front edge, a back edge, and a pair of sides. A conduit extends through the body portion and is exposed to the fluid at an orifice located beneath the fluid line. The other end of the conduit is coupled to a pressure transducer which produces an output proportional to the pressure acting on the orifice and thus proportional to the speed of the apparatus relative to the fluid. This apparatus can be connected to an appropriate speedometer and is particularly useful for measuring the speed of a watercraft to which the apparatus is mounted.

Abstract: A marine speedometer system is disclosed which utilizes a short pitot tube or air column. The speedometer system includes an inlet having a short air column, a transducer, a control circuit, and a display. The electric signal from the transducer is representative of the pressure in the short air column. The control circuit preferably utilizes a microprocessor and a look-up table to generate a display signal in response to an electric signal from the transducer. The look-up table includes T-Buoy values and count values. The count values are used to drive a frequency signal for an air core meter. The display provides a needle and a metered background which is easy to read. The microprocessor is coupled to receive calibrate up and calibrate down signals. The microprocessor causes the needle to jump when it receives a signal to provide feedback to the driver. The microprocessor includes an auto zero subroutine which automatically calibrates the speedometer at start up.