When a ship floats upon the stream, it is evident that her upper-works will be separated from the bottom by the surface of the water, which will accordingly describe an imaginary horizontal line upon the bottom from the stem to the stern-post.
The most elevated of those lines is termed the load-water-line, which is supposed to be drawn by the surface of the water on the upper part of the bottom, when she is sufficiently laden for a sea-voyage. For if we suppose this surface a rule, and thereby describe a corresponding black line along the vessel’s bottom, that line will be distinguished upon the bottom, which is white, and represent what is called the load-water-line.
If the ship is lightened of any part of her lading, and preserves the same difference in her draught of water at the two ends, or, what is the same thing, if she is lightened so as to preserve the same equilibrium of the keel with regard to the surface of the water, it is evident that she will rise higher out of the water, so that the black line already described will be elevated above it, and another black line may be delineated upon the bottom, close to the surface of the water, which will exhibit a second water-line parallel to the first, but nearer the keel in proportion to the number of feet which the ship has risen.
Thus by lightening a ship gradually, and at the same time preserving the direction of her keel, or the angle which the keel makes with the surface of the water, a variety of water-lines may be drawn parallel to each other, and to the load-water-line. See a farther illustration of these lines in the article Water-Line. See also their figure on a ship’s bottom, plate [I]. fig. 5.
The ribbands are likewise of great utility in ship-building; they are narrow and flexible planks placed on the bottom at different heights, so as to form a sort of mould for stationing the inferior timbers between the principal ones. They differ from the water-lines, inasmuch as the latter have only one curve, which is horizontal, whereas the ribbands, besides their horizontal one, have a vertical curve. To convey a just idea of these curves, which cannot be represented on one draught at their full length, without an oblique section of the ship’s length, it will be necessary to have recourse to two planes; that of the elevation, which exhibits their vertical curve; and to the floor-plane, upon which the horizontal curve is expressed. See Ribbands.
These different lines are extremely useful in exhibiting the various curves of a ship’s bottom, that as they are gradually diminished, their uniformity or irregularity may be discovered by the skilful artist.
We have already observed, that the qualities required in a ship ought to determine the figure of the bottom: a ship of war therefore should be able to sail swiftly, and carry her lower tier of guns sufficiently out of the water. A merchant-ship ought to contain a large cargo of merchant-goods, and be navigated with few hands; and both should be able to carry sail firmly; steer well; drive little to leeward; and sustain the shocks of the sea without being violently strained.
The first thing to be established in the draught of a ship is her length; and as a ship of war, according to her rate, is furnished with a certain number of cannon, which are placed in battery on her decks, it is necessary that a sufficient distance should be left between their ports to work the guns with facility, and particularly to leave space enough between the foremost gun and the stem, and between the aftmost gun and the stern-post on each side, on account of the arching, or inward curve of the ship towards her extremities.
When the length of a ship is determined, it is usual to fix her breadth by the dimensions of the midship-beam. On this occasion the shipwrights, for the most part, are conducted by rules founded on their own observation; for having remarked, that some vessels which by repeated experience have been found to answer all the purposes of navigation, have a certain breadth in proportion to their length, they have inferred that it would be improper to depart from this proportion: but as other ships have been constructed with different breadths, which were equally perfect, a variety of different general rules have been adopted by these artists, who are accordingly divided in their opinions about the breadth which ought to be assigned to a ship relatively with her length, whilst each one produces reasons and experience in support of his own standard. Those who would diminish the breadth alledge, that a narrow vessel meets with less resistance in passing through the water; 2dly. That by increasing the length she will drive less to leeward; 3dly. That according to this principle, the water-lines will be more conveniently formed to divide the fluid; 4thly. That a long and narrow ship will require less sail to advance swiftly; that her masts will be lower, and her rigging lighter; and, by consequence, the seamen less fatigued with managing the sails, &c.
Those, on the contrary, who would enlarge the breadth, pretend, 1st. That this form is better fitted to preserve a good battery of guns. 2dly. That there will be more room to work the guns conveniently. 3dly. That by carrying more sail the ship will be enabled to run faster; or, that this quality will at least overbalance the advantage which the others have of more easily dividing the fluid. 4thly. That, being broader at the load-water-line, or place where the surface of the water describes a line round the bottom, they will admit of being very narrow on the floor, particularly towards the extremities: and 5thly. That a broad vessel will more readily rise upon the waves than a narrow one.