Abstract: Means are provided for coupling at will a driven shaft (2), intended more particularly for adjusting the orientation of the blades of a boat propellor, either to the output shaft (1) of a motor or to a hand operable member (3). To this end there are provided three coaxial inwardly splined ring-wheels juxtaposed axially and integral respectively, the first two (13,14) with the two shafts and the third one (16) with said member, an outwardly splined hub (15), a tangent wheel (22) integral with said member and coacting with an endless screw (23) operable by means of a crank and screw means actuated by the rotation of a crank mounted on a head (28) and arranged so as to control the axial sliding of the hub by means of thrust bearings (32) and springs (33).

Abstract: A tensioning device for a run of a chain comprises a toothed roller, for engagement with the chain, rotatably mounted on a crank-pin which is rotated about an axis by resilient means to bring the roller into tensioning contact with the chain. Angular movement of the crank-pin is limited by means carried by the output member of a freewheel. This output member is urged by further resilient means which are weaker than the first mentioned resilient means in the direction to tension the chain.