Abstract: The invention provides a propulsive-force jack installation for producing the continuous movement of an oblong object (1) in the direction of its axis and it is characterized in that it comprises two jack assemblies whose pistons (6, 6') work in seesaw fashion, a piston (6) of one assembly taking in charge said object (1) as soon as the piston (6') of the other assembly ceases this action, offload movement of the piston (6 or 6') and its locking on the object (1) being controlled automatically just before the arrival at the end of travel under load of the other piston (6' or 6) and the offload descent of one piston (6 or 6') being controlled automatically at a time which follows the beginning of the taking in charge of the object by the other piston (6' or 6). The invention may be applied to hoisting, self-raising, prospection, or working platforms at sea.