ROLLING

ROLLING, therefore, is a sort of revolution about an imaginary axis, passing through the center of gravity of a ship: so that the nearer the center of gravity is to the keel, the more violent will be the rolling-motion; because the center about which the vibrations are made, is placed so low in the bottom, that the resistance made by the keel to the volume of water which it displaces in rolling, bears very little proportion to the force of the vibration above the center of gravity, the radius of which extends as high as the mast-heads.

But if the center of gravity is placed higher above the keel, the radius of vibration will not only be diminished, but an additional force to oppose the motion of rolling will be communicated to that part of the ship's bottom, which is below the center of gravity.

So far as relates to the effects of rolling, when produced by the quality or stowage of the ballast, and to the manner by which it may be prevented, viz. a change of the quantity or disposition of the ballast, we shall endeavour to explain under the article TRIM. It may, however, be necessary to remark, that the construction of the ship's bottom may also contribute to diminish this movement considerably.

To illustrate this by an example, let us suppose the section of a ship perperpendicular to the keel to be exactly circular, plate VIII. fig. 8. it is evident, that if this be agitated in the water, it will have nothing to sustain it, because the rolling or rotation about its center displaces no more water than when it remains upright: consequently the rolling motion must be very great in a high sea. But if a plank is fixed below it edgeways, or perpendicular to the surface, as low as e, throughout the whole length of the ship, it is plain that the plank e will displace a volume of water to the right, when the ship is inclined to the left, which will retard her motion; and this obstruction will always act contrary to her heeling or inclination to one side, and greatly diminish the vibration or rolling; although it will add very little to her stiffness: For, admitting the ship to incline to one side, as in fig. 8. the plank d e would produce a very weak effort to bring her upright. But the depth of the keel, the rising of the floors, and the dead wood fore and aft, as in fig. 9. plate VIII. will answer the same purpose as the plank d e. Many fatal disasters have happened to ships, arising from a violent rolling; as the loss of the masts, loosening of the cannon, and straining violently on the decks and sides, so as to weaken the ship to a great degree. See BALLAST, LABOURING, and PITCHING

Plate VIII

Reference Works