MSD50 Mechanical Stern Drive. On stern drive boats (inboard/outboard) with power assisted steering, the Simrad MSD50 stern drive unit will operate the servo valve to automatically steer the boat. The MSD50 is attached to the servo unit and operates the servo valve in the same way as the push-pull (“Teleflex”) cable from the helm. The MSD50 generates a feedback signal to the autopilot which is relative to the motion of the actuator rod and can only be operated from autopilots that are using the AC10, AC20, AC40 autopilot computers or J300X, J300X-40 and J3000X junction units.