An electromagnetic speedometer suitable for use in a boat with a horizontal boat hull having a lower exterior surface exposed to the water, and an upper interior surface, and at least one through opening; the electromagnetic speedometer comprising: - a tubular penetrator member (302, 402, 1002, 1102, 1202, 1302), suitable to be mounted concentrically within said hull open ing; - a large diameter electromagnetic coil, suitable for being mounted on the hull for establishing an electromagnetic field in the water adjacent the hull lower surface and extending beyond a boundary layer within the water resulting from movement of the boat hull, the electromagnetic coil comprising a coil (304, 404 1010, 1110, 1210, 1310) mounted concentrically about said tubular penetrator member (302,402, 1002, 1102, 1202, 1302); - a plurality of electrodes (314, 414, 1011, 1111, 1211, 1311) having first ends provided with tips which engage with the water, said electrodes having second ends; - electrode support means containing guide passages (307, 407, 1007, 1107, 1207, 1307) removably supporting said electrodes within said tubular penetrator member (302, 402, 1002, 1102, 1202, 1302) to effect engagement of the electrode tips with the water adjacent said electromagnetic field and to permit removal of said electrodes and cleaning of said electrode tips when the boat is in the water, said electrode support means further including closure means comprising valves (312, 412, 1012, 1112, 1212, 1312) connected with said guide passages, respectively, and operable between a closed and an open position, said valves in said closed position preventing water from entering the boat hull when said electrodes are removed; and - velocity measuring circuit means arranged above said hull upper interior surface, said velocity measuring circuit means including a plurality of conductors respectively connected with said electrode second ends. - a tubular penetrator member (302, 402, 1002, 1102, 1202, 1302), suitable to be mounted concentrically within said hull open ing; - a large diameter electromagnetic coil, suitable for being mounted on the hull for establishing an electromagnetic field in the water adjacent the hull lower surface and extending beyond a boundary layer within the water resulting from movement of the boat hull, the electromagnetic coil comprising a coil (304, 404 1010, 1110, 1210, 1310) mounted concentrically about said tubular penetrator member (302,402, 1002, 1102, 1202, 1302); - a plurality of electrodes (314, 414, 1011, 1111, 1211, 1311) having first ends provided with tips which engage with the water, said electrodes having second ends; - electrode support means containing guide passages (307, 407, 1007, 1107, 1207, 1307) removably supporting said electrodes within said tubular penetrator member (302, 402, 1002, 1102, 1202, 1302) to effect engagement of the electrode tips with the water adjacent said electromagnetic field and to permit removal of said electrodes and cleaning of said electrode tips when the boat is in the water, said electrode support means further including closure means comprising valves (312, 412, 1012, 1112, 1212, 1312) connected with said guide passages, respectively, and operable between a closed and an open position, said valves in said closed position preventing water from entering the boat hull when said electrodes are removed; and - velocity measuring circuit means arranged above said hull upper interior surface, said velocity measuring circuit means including a plurality of conductors respectively connected with said electrode second ends.