When a ship has passed a wave, the afterpart falls into the hollow of the w ayes, by the mean vertical di rection of the water acting on the foreside of the cen tre of gravity. This action, which is called scending is affected by the disposition of the weights similarly to the pitching.
The form of the fore and after parts of a ship de termines, in a great degree, these actions of pitching and scending; but as other circumstances frequently require a form not the best calculated to regulate them, it becomes the more necessary that the best disposi tion of the moveable weights should be made for this purpose. It is therefore necessary to bring as many of the moveable weights as possible near the middle of the ship, to reduce the depth of the pitching and scending.
4. Holding a steady a body moves through any fluid, it is necessary that the lateral re sistance abaft the centre of gravity should be greater than before it, to prevent the body having a continual tendency to turn round. This disposition in a ship to turn from the direct course, is technically called yaw it increases the difficulty of steering, and retards the sailing. To prevent this bad quality in a ship, the weights should be so placed that the centre of gravity may be before the middle of the ship's length, by which the moment of the lateral resistance abaft the centre of gravity will be increased, and the moment forward diminished.
5. ardency of a ship, or its tenden cy to fly up into the wind, depends on the mean di rection of the water, the ship sailing by a wind, and the position of the centre of effort of the sails. When a ship is fully stored and properly trimmed, the mean direction of the water passes a little before the cen tre of gravity. By the loss of the consumable stores the trim may, by improper stowage, be so much al tered, that a ship, which at first possessed a weather ly quality in a proper degree, may either lose it alto gether, or have it altered so much as to destroy the excellency of this important quality. The stowage should, therefore, be so disposed, that the consuma ble stores should be taken in such proportions from the fore and after parts of a ship, that the good quali ties at first possessed may be retained when lightened.
This requires great acquaintance with the qualities of the ship to be stowed, as well as great judgment in the disposition of the ballast and stores.
6. resistance a ship experiences, in coming about, depends on the lateral resistance of the parts before and abaft the centre of gravity. This re sistance Avill be proportional to the squares of the lengths of the parts before and abaft the centre of gra vity, which will be a minimum when the centre of gra vity is in the middle of the length.
7. -action of the the rotation of a ship must always be referred to the axis that passes through the centre of gravity, the momentum of the power of the rudder to turn a ship is proportional to the dis tance of the centre of the mean resistance of the rud der from the centre of gravity. This consideration would lead to the moveable weights being placed so that the centre of gravity of the ship should be before the middle of the length.
8. Strain of the inequality between the weights in different parts of a ship, and the ver tical pressure of the water at the corresponding parts, causes a continual strain on the ship longitudinally, which produces an arching, sometimes technically called hogging. To equalise these two actions, is the mode immediately suggested by the consideration of the cause of arching, as the best method of preventing it. Circumstances, however, prevent the establish ment of the equilibrium; great weights will always necessarily be at the extremities of the ship, and the buoyancy of the corresponding parts of the body must always be very inadequate to their support, from the leanness of the fore and after parts of the body. As far, however, as circumstances will admit, the prin ciple should be attended to, of placing the weights where the buoyancy of the body is best able to sus tain them. This requires the ballast and heaviest stores to be placed in the full parts of the body, to wards the midship section; reserving, however, the immediate vicinity of the mainmast free from the heaviest weights.