CHAPTER XII.

PROPULSION BY SAILS.

THE efficient management of a ship under sail furnishes one of the most notable instances of skilful seamanship. In different hands the same ship may perform very differently. Changes in stowage and trim also affect the performance; but such changes as an officer in command can make are necessarily limited in their scope and character; and some ships can never be made to sail well, having some radical fault in their designs. Without intruding upon the domain of seamanship, the naval architect requires, therefore, to study very carefully the conditions of sail-power, and the distribution of sails in a new design, if the completed ship is to be fairly successful. His success or failure greatly depends upon the possession of information respecting the performances and sail spread of ships of similar type and rig; having such information, the process by which the total sail spread and the distribution of the sail are determined in the new ship is by no means difficult or complex. Taking, the exemplar ships, and the reports on their sailing qualities, an analysis is made of the sail areas, the distribution of the sail longitudinally and vertically, the transverse stability, and some other particulars. Furnished with these data, and having regard to the known qualities of the completed ships, it is possible to secure similar, or perhaps improved, performance in the new design. Apart from such experience, however, the naval architect would be unable to be equally certain of obtaining good results; and in cases where great

strides are taken in a new design, away from the sizes and proportions or sail plans of existing ships, the arrangement of the sail-power cannot but be, to a large extent, experimental. Illustrations of this are to be found in the earlier ironclads of the Royal Navy, such as the Achilles and Minotaur classes, in which the sizes, lengths, and proportions of length to breadth were all much greater than in preceding ships. When first fitted with four masts, the Achilles did not perform well under sail; but as now arranged with three masts, she stands high among the ironclads. The Warrior, on the other hand, a ship of the same class as the Achilles, proved successful under sail from the first; having only three masts. In fact, although the general principles of propulsion by sails were long ago formulated, and although many eminent mathematicians and naval officers have endeavoured to assist the naval architect by constructing general rules for guidance, there is even now no accepted theory fully representing the conditions of practice. In this chapter attention will be confined to a few of the fundamental principles of propulsion by sails, and to the simple rules which are commonly observed by naval architects in arranging the sails of a ship.

Suppose a fixed plane surface to be exposed to the action of a wind of known velocity, blowing steadily at right angles to the plane; the pressure upon it will then be represented by the expression

Pressure on plane (in lbs.) = C × (velocity)2 × area of plane, where C is a constant quantity determined by experiment. This law is not exact, but is sufficiently so for practical purposes. Some doubts exist as to the proper value to be assigned to C; if the velocity is expressed in feet per second and the area in square feet, then, according to the best data hitherto available,*

See a table of wind speeds and pressures at page 90 of Shipbuilding, Theoretical and Practical; also the

"wind scales" at page 41 of the Barometer Manual of the late Admiral FitzRoy.

= 0·00234 lb. × (velocity)2.

Pressure on one square foot

}

of area on plane . Lieutenant Paris, of the French navy, conducted an extensive series of experiments on the pressures of winds of various speeds, in connection with his observations on waves, and the mean coefficient deduced from his published results very nearly agrees with that stated above, being 0.00239.* These observations on ship-board, however carefully conducted, must be subjected to many disturbing causes; and, moreover, there must be great difficulty in determining accurately the velocity of the wind past the ship and past the anemometer. For these reasons it appears preferable to substitute for the motion of the wind past a plane the motion of a plane through still air, in order to obtain accurate unit pressures. This method has been employed by Mr. Froude, with the aid of his delicate automatic apparatus for measuring resistance, and we have been favoured with some of the results. The coefficient thus obtained is about 28 per cent. less than that obtained by previous experimenters; and it gives

= 0.0017 lb. x (velocity).

Pressure on one square foot of area on plane According to the earlier experiments, a pressure of one pound per square foot of surface corresponds to a speed of wind of about 12 knots per hour; according to Mr. Froude, that speed would be about 14 knots per hour. The latter result appears more trustworthy; and it indicates that in a "fresh breeze" or "top-gallant sail wind" (Force 5 to 6) the normal wind pressure per square foot is about 1 lb.

The classification of winds is a subject lying outside our present field, but it may be stated that authorities agree in assigning a speed of from 60 to 100 knots per hour to a hurricane" (Force 12), the corresponding pressures being

* See Chapter V. page 177, and the full description of the observation in vol. xxxi. of the Revue maritime.

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from 18 to 50 lbs. per square foot. The "storm-wind" (Force 11) would have a speed of 45 to 50 knots, and a pressure of from 11 to 13 lbs.; the "heavy gale" (Force 10) would have a speed of about 40 knots, and a pressure of 8 to 9 lbs. ; the strong gale" (Force 9), a speed of about 34 knots, and a pressure of about 6 lbs. ; the "fresh gale" (Force 8), a speed of about 28 knots, and pressure of about 4 lbs.; the "moderate gale" (Force 7), a speed of about 23 knots, and a pressure of about 23 lbs. ; the "strong breeze" (Force 6), a speed from 15 to 20 knots, with a pressure from 1 lb. to 2 lbs.; and the "fresh breeze" (Force 5), the upper limit of 1 lb. pressure, corresponding to a speed of 14 knots as above. All these pressures are supposed to act on a plane area of one square foot placed at right angles to the direction of the wind.

If this plane area were placed obliquely to the direction of the wind, the pressure upon it would diminish; but the law of the decrease does not appear to have been ascertained. Formerly it was supposed that for any angle of inclination a of the plane to the direction of the wind, or "angle of incidence,"

Wind pressure=normal pressure (due to velocity) x sin2 a.

This law, however, is known to be incorrect, making the wind pressure less than the true pressure, especially for small angles of incidence. Mr. Fincham (in his treatise on Masting Ships) gave the results of experiments made by the French Academy of Sciences, on the comparative resistances of water to the motion of wedge-shaped bodies, the plane surfaces of the wedges having been set at various angles of obliquity; and he proposed to use these results in estimating the effective pressure of the wind acting obliquely on the sails. According to these experiments, the law of the "square of the sine of the angle of incidence" above stated held very closely up to 70 degrees, but for smaller angles of incidence the actual pressure exceeded the pressure given by this law, and gradually approached the product of the normal pressure by the sine of the angle. When the angle of

incidence was about 25 degrees, the plane being inclined 65 degrees from the normal position, the law of the sine held. exactly. The results for greater inclinations need not be given; they appear inexplicable. In fact, these experiments do not, and could hardly be expected to, furnish any trustworthy information as to the effective pressure of wind on the sails.*

Apart from experiment, it seems reasonable to suppose that the wind pressure on a plane placed obliquely should vary nearly as the sine of the angle of incidence, within the limits which are important in practice; and although this law may not be exact, it will be sufficiently so for practical purposes.

A sail acted upon by the wind either normally or obliquely would not remain plane, but would become concave on the windward side, or would "belly out." According to the experiments of M. Thibault (quoted by Professor Rankine), "the impulse of the wind upon a sail of the usual concave "figure is very nearly equal to its impulse on an equal area "of plane surface." But experience appears to show that the more nearly plane the surface of a sail can be kept, the greater will be the propelling force derived from the wind pressure upon it. "All slack canvas," says Mr. Fincham, "whether sailing by the wind or large, lessens the effect of "the sail; and even before the wind, when the slack reef is "out, the power which acts on the sail will be reduced very "considerably on the curved surface; less even than the "base of the same curve, or than if the sail were set taut-up, "but reduced to the same hoist or distance between the yards as when slack."

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Sails attached to ships are not fixed in position like the plane and sail just considered, but necessarily move with the ship. Hence, in dealing with the propulsive effect of a

* See the details given by M. Bourgois, in his Mémoire sur la Résistance de l'Eau. The experi

ments were conducted by Bossut, Cetondorct, and D'Alember.