Abstract: A through-hull speed sensor of the magnetized paddlewheel sensor type wherein a paddlewheel is rotatably supported in a cavity adjacent a magnetic sensing device which generates electrical pulses corresponding to paddlewheel speed as the wheel rotates when the vessel moves through water. The sensor has improved linearity provided by a flow channel recessed in a downstream surface of a sensor mounting member. The recessed flow channel raises the trailing edge above the leading edge, thereby reducing pressure on a back wall of the cavity where the pressure is due to fluid flow through the paddlewheel.