One of the many instances of the want of fixed principles in naval architecture, is that of determining the positions of masts. The practice and experience of a long series of years have established rules, according to which the masts of ships are generally placed; yet the frequent recurrence of ships requiring the position of their masts to be altered, shows that these rules are not of general and certain application. The different forms of ships, and particularly the relative degrees of fineness of the fore and after bodies, are necessary elements to be included in all rules for determining the position of the masts. To obtain such rules as will always determine their position in relation to the properties of ships, will require a combination of experimental and theoretical knowledge, greater, perhaps, than the present state of the knowledge of naval architecture admits.
Among other difficulties of this subject, the rake given to masts of ships is frequently a question concerning which there is considerable difference of opinion, and the propriety of which is often doubted.
In some instances the sailing of ships whose masts are too far forward is improved by the rake of the masts being increased, which carries the surface of sail further aft; but the desired effect would be better gained, as far as regards this consideration, by carrying the masts wholly further aft. The advantage of raking masts must be considered on other principles.
The French, and the Americans especially, have carried the practice of raking masts to a greater extent than the English; and as some of their vessels, in which the masts have been much raked, have been remarkable for their superior sailing, it naturally becomes a question, whether or not the peculiar manner of placing the masts has, in any degree, contributed to produce this excellency?
In examining this question, it will be necessary to consider in what way the surface of the sails is subjected to the action of the wind, both when the masts are upright and when they are inclined.
Suppose the masts of a ship to be placed perpendicularly to the surface of the water; then, if the yards were braced up, the tacks and sheets close-hauled, and the ship in an upright position, the planes of the sails would be vertical; and if the ship, when sailing by the wind, could be prevented from heeling, the planes of the sails would receive the impulse of the wind in the most advantageous manner possible. But as it is impossible to prevent the heeling, the planes of the sails cannot remain vertical when they receive the action of the wind.
If the masts, instead of being upright, were to be inclined towards the stern, the planes of the sails, when they are close-hauled, and the ship is upright, would be inclined to the horizon; and as the ship is heeled by the force of the wind, this inclination of the planes of the sails is gradually diminished, and it may be reduced to nothing, if the inclination of the masts was to be in a given proportion to the angle of heeling. Thus, if the angle to which the ship may generally be permitted to heel is 10°, if the rake of the masts was such that the inclination of the common section of the sails, when close-hauled, with an athwartship vertical plane, was also 10° when the ship became inclined, the sails would become vertical.
Again, when the masts are placed perpendicularly, the area of sail presented to the wind, when the ship is upright, is greater than when the ship is heeled, in the proportion of radius to the cosine of the angle of heeling; but when the masts are raked, the area increases as the ship is heeled, in the proportion of the cosine of the inclination of the ship to the radius.
It appears, therefore, in order to make a ship sail by the wind with the greatest degree of velocity, the masts should be placed at a certain angle of inclination towards the stern, and not in a vertical position.
In the above reasoning, the sails of a ship have been considered as plane surfaces; it has also been supposed that the wind acts uniformly all over their surfaces. Both of these hypotheses, however, are inaccurate. It must be considered in what manner this circumstance affects the above conclusion.
When a ship is sailing by the wind, the curvature of the sails is very small on the weather side, although it is very considerable near the leech on the lee-side; also, as the particles of air impinge very obliquely on the surface of the sails, and as each particle, in gliding off after impact, takes off a part of the action of some of the more leewardly particles, the effective action of the wind on the sails must be gradually diminished from the weather-side to the lee-side. The rush of air across the ship may also probably produce a diminution of pressure on the fore-side of the sail near the weather-leech, which is not the case in the vicinity of the lee-leech.
From these considerations, it appears that the pressure of the wind, on a sail that is trimmed sharp, is greater on the weather-side, where the surface is nearly coincident with a plane, than it is on the lee-side, where the surface is much curved, and that the diminution of pressure is gradual, from the one side to the other. This is confirmed by the well-known fact, that when a ship is on a wind, the tension on the weather braces is always greater than that on the lee ones; and also, by the general proposition, that a plane surface, which is acted on by a fluid in an oblique direction, always endeavours to assume a position perpendicular to the line of action.
It appears, then, from the above observations, that the inaccuracy of the hypotheses admitted respecting the surface of the sails, and the manner in which they are acted on, cannot materially affect the justness of the former conclusion; for the part of the sail which receives the greatest part of the force of the wind does not differ greatly from the circumstance supposed in the reasoning, and the part least agreeable to the supposition does not require any reasoning which opposes the principle of the argument.
The writer is aware that there are other considerations relating to the raking of ships' masts, and that objections are sometimes made to the practice, particularly by the increased strain brought on the materials by it; he is, however, the more desirous to offer arguments in its favour, as it has received the sanction of experience, and considers that the above reasoning, as far as it goes, is correct in its conclusion; that masts, to produce the best effect on ships' sailing, should be placed at a certain inclination towards the stern.
Papers on Naval Architecture and other Subjects Connected with Naval Science. Volume I.
Whittaker, Treacher, and Arnot, London, 1826. pp 45-47.
Transcribed by Lars Bruzelius.
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