Any object floating on the water will have a tendency to drift before the wind; but a boat, with its scientifically constructed hull, sails, and rudder, can be so guided as to sail with the wind on her quarter or abeam, or even close-hauled, as it is called, that is, with the wind meeting her at an angle of about forty-five degrees.
Fig. 20 represents the deck plan of a boat sailing close-hauled under two sails. The sails A and B are drawn aft with the sheets till they form an acute angle with the line of the keel. The wind, whose direction is indicated by the arrow W, strikes the sails at a very acute angle, so that they do not shake, but are just full.
The result of this pressure on the sails is that the boat is propelled forward and also sideways away from the wind, making leeway, as it is called.
Fig. 20.
If a boat has a deep keel, her lateral resistance to the water will cause the leeway to be insignificant. If the boat is of very shallow draught and so offers little lateral resistance to the water, she will not go ahead at all, and the entire force of the wind will be expended in driving her bodily to leeward. Lee-boards and centre-boards are fitted to shallow boats in order to obviate this.
The pressure of the wind on the sails, in addition to producing the above effects, heels a boat over. A sailing-boat is so constructed as to resist this tendency to capsize. Either she is made narrow and deep and is weighted with ballast as far as possible below the water-line, or she is shallow but of considerable beam. The deep and weighted boat will heel over more readily than the beamy shallow boat, but the further she heels the greater pressure of wind is necessary to make her heel still more, for the leverage of her ballast increases as she heels, and many boats with lead upon their keels are practically uncapsizable. On the other hand, the beamy shallow boat does not heel so readily, but after she has heeled to a certain angle she will capsize.
The pressure of the wind on the sails not only propels, drives to leeward, and heels over a boat, but, unless the sails are absolutely balanced, it tends to turn her in one direction or the other.