If a vessel is going astern, and the rudder is turned off from the line of the keel, the water, striking against the back of the rudder, pushes the stern off in the same direction in which the rudder is turned. For instance, if sternway is on her, and the helm is put to the starboard, the rudder turns to port, the water forces the stern in the same direction, and the ship's head goes off to the starboard. Therefore, when sternway is on a vessel, put the helm in the same direction in which the head is to be turned. A current or tide running astern, that is, when the ship's head is toward it, will have the same effect on the rudder as if the ship were going ahead; and when it runs forward, it will be the same as though the ship were going astern. It will now be well to show how the sails act upon a ship, with reference to her centre of rotation. Suppose a vessel to be rigged with three sails, one in the forward part, one at the centre, and the third at the after part, and her left or larboard side to be presented to the wind, which we will suppose to be abeam, or at right angles with the keel. If the head sail only were set, the effect would be that the wind would send the vessel a little ahead and off to the starboard on her centre of rotation, so as to bring her stern slowly round to the wind. If the after sail only were set, the vessel would shoot ahead a little, her stern would go off to the starboard and her head come up into the wind. If only the centre sail were set, the effect would be the same as if all three of the sails were set, and she would go ahead in a straight line. So far, we have supposed the sails to be set _full_; that is, with their tacks forward and their sheets aft. If they were all set _aback_, the vessel would go astern nearly, if the rudder were kept steady, in a straight line. If the head sail only is set and aback, she will go astern and round upon her axis, with her head from the wind, much quicker than if full. So, if the after sail alone were set and aback, she would go astern, and her head would come suddenly into the wind.