An Universal Dictionary of the Marine / Or, a Copious Explanation of the Technical Terms and Phrases Employed in the Construction, Equipment, Furniture, Machinery, Movements, and Military Operations of a Ship. Illustrated With Variety of Original Designs of Shipping, in Different Situations; Together With Separate Views of Their Masts, Sails, Yards, and Rigging. to Which Is Annexed, a Translation of the French Sea-terms and Phrases, Collected from the Works of Mess. Du Hamel, Aubin, Saverien, &c.

by William Falconer

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 height of these filaments, viz. That the weight of a column of any fluid will be in proportion to the specific gravity of the fluid and the height 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 sufficient 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.

Several circumstances are also to be particularly considered with regard to the quality, weight, and stowage of the ballast. The center of gravity being placed too high, will render the ship incapable of carrying a sufficient quantity of sail; and by having it too low, she will be in danger of rolling away her masts. When it is placed too far forward, the ship will pitch, and labour heavily; and when too far aft, she will occasionally be exposed to the dangerous circumstance of a pooping sea. These extremes being carefully avoided, it remains to proportion the contents of every part of the hold to its capacity, and to place the lightest materials uppermost. See Stowage.

Trim, when applied to the sails, denotes the general arrangement which is best calculated to accelerate the ship’s course, according to the direction of the wind. See the article Sailing.

If the ship were always to sail before the wind, it would be a very simple operation to trim the sails; because nothing else could be required than to dispose them so as to receive the greatest possible effort of the wind, which is evidently performed by arranging them at right angles with its direction. But, when the current of wind acts more directly upon the ship’s side, it necessarily falls more obliquely on the surface of the sails, so as to diminish their effort to push the ship forward; and to augment their tendency to make her incline to one side. Hence we may conclude, that an increase of the wind, when accompanied with a variation unfavourable to the ship’s course, will by no means augment her velocity; because the force previously employed to push her forward, will afterwards operate to overturn her; and because this impression renders it necessary to reduce the quantity of sail; the effort of which is farther diminished by the obliquity of the action of the wind upon its surface.

By this theory it appears, that the effect of the wind to advance the ship decreases in proportion to its obliquity with any sail upon which it operates.

The mechanical disposition of the sails, according to every direction of the wind upon their surfaces, is copiously described in the articles Close-hauled, Large, Sailing, and Tacking.

Trim, when expressed of the masts, denotes their position with regard to the ship and to each other. Thus, in the latter sense, they should neither be too near nor too far apart; and, in the former, they should not be too far forward or aft; and, according to the situation or quality which communicates a greater velocity to the vessel, they should either be upright, or inclining aft, or forward.