which operates on the externally threaded shaft is rotatable by a motor 8 the nut 6 is held captive by the action of the weight of the shaft 4 and the load it carries which is reacted by the nut 6 via a thrust bearing 10 which in turn operates against a single or multipart fixed load bearing structure 12 which transmits loads to the wider ship structure motor 8 is also mounted to structure 12 either integrally or as an additional bolt on component in the preferred embodiment the training and raise lower motor 8 is of rim driven construction and comprises a rotor the centre of which forms the nut 6 on the thread of strut 4 the nut 6 is connected to the magnet carrying rim of the rotor 32 by spokes 34 but depending on the torque required the magnets could be mounted directly on the nut 6 the structure 12 has a sleeve portion that extends downwards over the threaded portion of shaft 4 the main radial bearing for the motor is provided by the interaction of shaft 4 and the inner bore of the sleeve accordingly for this embodiment the form of the thread is chosen such that the outer periphery of the thread forms a reasonable radial bearing surface when it operates in conjunction with the sleeve with which it has a close sliding fit for an application where.