A sail support and control system designed for use as sail assist propulsion on a cargo vessel. The system provides complete powered control of the amount of exposed sail, the angle of the sail in relation to the vessel, and clew outhaul and downhaul tension so that no manual handling of rigging is required. An unstayed mast is rotatably mounted on the ship to enable furling and unfurling of the sail by mast rotation. A cantilever boom is mounted on the mast support so as to be horizontally rotatable thereon. Boom positioning winch means is mounted on the boom to pay out and take in sheet lines, which are dead ended on the deck, for swinging the boom to a desired angular position in relation to the ship and maintaining it in said position. Means is provided for rotating the mast in relation to the boom so that the sail may be reefed without changing the position of the boom, and so that swinging of the boom also causes a corresponding rotation of the mast. A change in position of the boom does not, therefore, change the amount of exposed sail. Tension on the clew of the sail is provided by an outhaul and a downhaul which operate independently of each other. An outhaul tension line is payed out and taken in under continuous tension in conjunction with the rotation of the mast to take in or let out sail. The downhaul mechanism is mounted on the boom and connected to the clew, and travels in and out along the boom with movement of the clew, maintaining a continuous downhaul tension during such movement. All motions of components and tensions on lines are preferably provided by hydraulic motors or cylinders controllable from the bridge, so that the manual handling of the rigging is required.