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Affordability, redundancy, as well as space, weight, and energy savings are main issues on board ship. Classical oil-powered onboard actuators used for steering gears (rudders, roll-stabilizing fins) are usually cumbersome, weighty, low-efficiency and costly plants, also requiring an heavy maintenance burden. The Italian Navy recently committed a research project to the University of Rome-Sapienza, aimed to define innovative inverter-fed full-electric actuation systems in alternative to the hydraulics. The new actuators had to guarantee the required dynamic performances, i.e. very high torques/forces at very low speeds, besides high affordability, redundancy, and, mostly, high efficiency. This paper proposes a new high-thrust permanent-magnet linear actuator for direct-drive of steering gears, through an appositely designed rotary-prismatic coupling. Mechanical and electrical drive sizing are shown, and force, speed, and efficiency performances are analyzed and compared with those of the original hydraulic plant. The proposed actuator will be verified by tests on a reduced-scale laboratory prototype in the final steps of the research program.