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 effect 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 perpendicular 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.
Rolling-tackle, a pulley or purchase fastened to that part of a sail-yard which is to the windward of the mast, in order to confine the yard close down to the leeward, when the sail is furled.
It is used to prevent the yard from having a great friction against the mast in a high sea, which would be equally pernicious to both.
ROPES, cordes, (rap, Sax. reep, Dutch) a general name given to all sorts of cordage, above one inch in circumference, used in the rigging a ship. See Cable, Hauser, Towline, and Warp.
Ropes are either cable-laid or hauser-laid: the former are composed of nine strands, viz. three great strands, each of which is composed of three smaller strands; and the latter is made with three strands, each of which contains a certain number of rope-yarns, in proportion to the size of the rope required.
Rope-bands, rabans, pronounced roebins, certain pieces of small rope, or braided cordage, used to tie the upper edges of the great sails to their respective yards. They are inserted through the eyelet-holes in the head of the sail, being generally of a sufficient length to pass two or three times about the said yard.
Rope-yarn, fil de caret, the smallest and simplest part of any rope, being one of the threads of which a strand is composed; so that the size of the latter, and of the rope into which it is twisted, are determined by the number of rope-yarns.