To illustrate the principles involved, it will be assumed as a fact that ice boats in America have been timed to sail at the rate of one mile in a minute by the aid of a force due to a velocity of thirty sea miles an hour of the wind. Such a wind force is termed a " fresh gale" by sailors, and a ship would be ender reefed topsails, reefed courses, or possibly fore and mizen topsails furled. In Fig. 118 we will suppose A B to be a boat fixed so that it cannot move. The line w d represents on a scale the direction and force or speed of the wind equal to 30 miles an hour. The line s s represents the balance lug sail of an ice boat. It is obvious that the wind, blowing on the sail from the direction shown, would tend to drive the boat in the direction of its own motion. But the force w d is made up of two components—one, c d, is acting at right angles to the sail, and the other, d k, acting along the plane of the sail without any potent effect, The component c d can be further regarded as a force made up of three other components—one, c x, acting at right angles to the keel of the vessel, and tending to drive her to leeward ; another, x d, acts in a line with the vessel's keel, and tending to drive her ahead. The third force is resolved vertically in a downward direction, and need not be considered.
Hitherto we have only dealt with the "real wind," and we have now to consider what takes place when a boat, under the influence of x d, gathers way, or commences to move in a direction to meet the wind at a more or less acute angle. In Fig. 119 A B is the boat set in motion* by the component of the wind x d, and is proceeding in the direction of the arrow m m ; the difference between the arrow and the keel line represents the angle of leeway, or amount of leeway made. The dotted line to d represents the direction and velocity of the real wind, as in Fig. 118 ; but the wind apparently will now be blowing more ahead, and it is the apparent wind with which we have to deal. The apparent wind is thus determined : On a line parallel to m m set off a distance (see d n, Fig. 119) by scale (same as the wind scale) to represent an opposing motion equal to the speed of the boat (assumed in this case to be sixty miles an hour, or double the velocity of the wind). Join n w, and the dotted line n w will represent the force and direction of the apparent wind.
This apparent wind must now be regarded as the propelling force, and not the real wind as shown in Fig. 118. In Fig. 120 let the dotted line to n represent the direction and force of the apparent wind ; by a parallelogram of forces the line of force to n has two components, one acting in the direction to c, or k n, and the other in the direction c n.
The component c n is farther resolved into three components, as before shown by Fig. 118, represented by c D and D n in Fig. 120, and another acting vertically, not shown. It is the component D n which impels the boat forward. It will be seen that the force D n is very small, or only about one-seventh of the force c D, which is striving to drive the boat to leeward ; but the resistance to leeway is very great, whilst the resistance to headway is very small. Consequently the boat moves a scarcely perceptible distance in a broadside direction, but gathers speed in the direction of her keel, or rather in the direction of the line m m, which includes the broadside motion or leeway. The boat continues to gather way or increase in speed from the rest position shown in Fig. 118 until the resistance she meets with, from friction of the skates on the ice and resistance to the rigging, &c., equals the force shown by D n. The speed of the boat then remains uniform so long as the wind is constant.
• If the speed, with the same wind force, could be increased by diminish ing the resistance, beyond sixty miles an hour, the apparent wind would draw more ahead, and then obviously the effective impulse, D n, would be further diminished, until the apparent wind were brought right ahead, when D n would disappear altogether; the sails would " lift," and a fresh start would have to be made.
From what has been said it will be concluded that on any point of sailing between a quarterly wind and a wind on the bow, if any great speed is realised, the boom must be hauled close aboard, the exact angle with the keel being determined by the speed of the boat or by the direction of the apparent wind, and not by the direction of the real wind. When a point dead to leeward has to be made, it is thought that the point is reached more quickly by hauling up to 45° from the course, and then, when half-way, gybing or tacking, and making for the point to be reached, thus traversing a right angle.
Sailing for a point dead to windward is not such rapid work, and we believe ice boats do not make a course nearer than 34 points of the real wind, and their speed over the ice is not one-half of what it is with a beam wind, but it is believed to just about equal the speed of the wind.
The usual mode of coming to rest from a high speed is by running off to dead before the wind, and then luffing to sharply until head to wind. The boat is " anchored " by turning the "rudder runner" right across the keel.