TRIM

Thus the trim of the hold denotes the most convenient and proper arrangement of the various materials contained therein, relatively to the ship's motion or stability at sea. The trim of the masts and sails is also their most apposite situation, with regard to the construction of the ship, and the effort of the wind upon her sails.

As the stowage of the hold, or the disposition of the several articles of the cargo, considerably affects the ship's motion and stability, it will be necessary to give a general idea of the action of a heavy body upon the fluid that supports it, and the re-action of the fluid on the floating body.

The whole weight of any body, then, may be considered as united in its center of gravity; so that, if it were suspended by a line fastened to this center, the line would hang in a perpendicular position, as directed through the center of gravity to the center of the earth. A body which floats in a fluid is not, however, supported by its center of gravity, but by the compression of the surrounding filaments of water: and each of these, being considered as infinitely small, will act upon a very minute portion of the surface of the floating body, with regard to the specific gravity, and conform to a principle applicable to all fluids, in proportion to the heighth of these filaments, viz. That the weight of a column of any fluid will be in proportion to a specific gravity of the fluid and the heighth of the column multiplied by its base.

But as heavy bodies endeavour, by their gravity, to approach the center of the earth, in a vertical line passing through their centers; so the pressure of fluids endeavours to carry bodies in a vertical, tending from the center of the earth towards their surface, and passing through the center of gravity of the submerged part, which forces them towards the surface. So, in any submerged body at rest, these two opposite forces coincide in the same vertical, acting in a direction quite contrary to each other. Bouguer's Traité du navire.

From this theory it results, that the stability or trim of a ship chiefly depends upon her construction, as considering the bottom to be homogeneous. This, however, can only happen when her cargo consists of the same materials throughout, as with corn, salt, or any species stowed in bulk, and when her hold is entirely filled. For if the ship has not sufficient breadth to resist the effort of the wind upon her sails; or if she is built too high, or too sharp in the floor, her center of gravity will be too high, and she will be very crank, i.e. apt to overturn.

But as the stiffness of a ship, or quality to carry sail without danger of overturning, depends very much on the stowage of the hold, the center of gravity may thereby be considerably lowered, by which her stability will be increased in proportion. It is a general maxim amongst mariners, that a ship will not carry sufficiently sail till she is laden so deep that the surface of the water may glance on her extreme breadth amidships. She must therefore have a great deal of weight, as ballast, &c. to bring her to this situation, which is called a good sailing trim.

Reference Works