An assembly for determining speed of a supercavitating underwater vehicle during underwater travel includes a fin mounted on the vehicle aft of a cavitator portion of the vehicle and adapted to be extended outwardly beyond a hull of the vehicle and through a boundary of a gas-filled cavity around the vehicle to form a disturbance in the cavity boundary, which disturbance propagates along the boundary. An acoustic transmitter is mounted on the vehicle and directs acoustic energy toward the boundary and the disturbance. An acoustic receiver is mounted on the vehicle and receives acoustic energy reflected off the disturbance. An autopilot is mounted on the vehicle and clocks times of projection of the fin and acoustic transmission receptions of reflected acoustic energy to determine the speed of the vehicle through the water.