Plate. ix.
The first impression of the wind greatly affects the velocity, because the resistance of the water might destroy it, since the velocity being but small at first, the resistance of the water which depends on it will be very feeble: but as the ship increases her motion, the force of the wind on the sails will be diminished; whereas on the contrary the resistance of the water on the bow will accumulate, in proportion to the velocity with which the vessel advances. Thus the repetition of the degrees of force which the action of the sails adds to the motion of the ship, is perpetually decreasing; whilst on the contrary the new degrees added to the effort of resistance on the bow are always augmenting. The velocity is then accelerated in proportion as the quantity added is greater than that which is subtracted: but when the two powers become equal, when the impression of the wind on the sails has lost so much of its force, as only to act in proportion to the opposite impulse of resistance on the bow, the ship will then acquire no additional velocity, but continue to sail with a constant uniform motion. The great weight of the ship may indeed prevent her from acquiring her greatest velocity; but when she has attained it, she will advance by her own intrinsic motion, without gaining any new degree of velocity, or lessening what she has acquired. She moves then by her own proper force in vacuo, without being afterwards subject either to the effort of the wind on the sails, or to the resistance of the water on the bow. If at any time the impulsion of the water on the bow should destroy any part of the velocity, the effort of the wind on the sails will revive it, so that the motion will continue the same. It must however be observed, that this state will only subsist when these two powers act upon each other in direct opposition; otherwise they will mutually destroy one another. The whole theory of working ships depends on this counter-action, and the perfect equality which should subsist between the effort of the wind and the impulsion of the water. Bouguer, Traité du navire.
The effect of sailing is produced by a judicious arrangement of the sails to the direction of the wind. Accordingly the various modes of sailing are derived from the different degrees and situations of the wind with regard to the course of the vessel.
To illustrate this observation by examples, the plan of a number of ships proceeding on various courses are represented by fig. 3. plate [IX]. which exhibits the thirty-two points of the compass, of which C is the center: the direction of the wind, which is northerly, being expressed by the arrow.
It has been observed in the article Close-hauled, that a ship in that situation will sail nearly within six points of the wind. Thus the ships B and y are close-hauled, the former being on the larboard tack, steering E. N. E. and the latter on the starboard tack sailing W. N. W. with their yards a b braced obliquely, as suitable to that manner of sailing. The line of battle on the larboard tack would accordingly be expressed by C B, and on the starboard by C y.
When a ship is neither close-hauled, nor steering afore the wind, she is in general said to be sailing large. The relation of the wind to her course is precisely determined by the number of points between the latter and the course close-hauled. Thus the ships c and x have the wind one point large, the former steering E. b N. and the latter W. b N. The yards remain almost in the same position as in B and y: the bowlines and sheets of the sails being only a little slackened.
The ships d and u have the wind two points large, the one steering east and the other west. In this manner of sailing, however, the wind is more particularly said to be upon the beam, perpendiculaire du vent, as being at right angles with the keel, and coinciding with the position of the ship’s beams. The yards are now more across the ship, the bowlines are cast off, and the sheets more relaxed; so that the effort of the wind being applied nearer to the line of the ship’s course, her velocity is greatly augmented.
In e and t the ships have the wind three points large, or one point abaft the beam, the course of the former being E. b S. and that of the latter
W. b S. The sheets are still more flowing; the angle which the yards make with the keel farther diminished; and the course accelerated in proportion.