With many of our correpondent's views of the subject, we entirely coincide, and do not wish to be understood as occupying the ground, either of endorsers or disputants, exclusively, of his positions on Marine science. There is no accounting for all the opinions an architect may chance to hold at any time in regard to the philosophy of ship-building. We do not therefore, hold ourselves in readiness to correct every man's notions of science on every occasion, for it were not only a great but an ungrateful task. For these reasons we welcome the labors of every architect who endeavors to exhibit improvements in designing the bodies of ships of whatever class. In regard to modelling vessels for a particular class or service, we lay down the proposition that it is easier to produce a successful vessel for a special than for general service, we care not what may be the exigencies of that service, from the steamship of war down to the fishingboat. Indeed, the general freighting ship, required to be successful in the hands of owners and masters of every description, is one more difficult than any other for a builder to do himself credit in constructing.
With the accumulation of capital, and the organization of industry, which is quietly and steadily progressing as years lapse away, the general freighting ship is giving place to ships designed for special classes of business. Instead of chartering, parties having uses for vessels, prefer to build or buy, and thus supply their wants upon a legimate basis for economy and profit. Few vessels, compared with times formerly, are now built without a particular trade in view; and to this fact no little merit is due for influencing the recent improvements in ship-building.
By reference to page 258, vol. III, No. 4, it will be seen that the author of Papers of Naval Architecture has defined his ideal for a Cotton ship as follows:—
"We want a wide ship, in proportion to depth, with a very easy bilge; with little or no rise to the floor, with as much length in proportion to the breadth, and strength, and a sufficiency of stability, and ease of working will allow; we would give her a hollow or concave bow, in order to have a good steering ship, with the lines all rounding to the sternpost, both horizontal and diagonal. Our great length would not render it necessary to have our ends too full: still I would not recommend having the lines inside of an angle of 30° from the centre line, for a sailing ship at the load-line, as I do not think anything is to be gained thereby, especially in this class of ships. Suppose we assume the dimensions which we presume to be adapted to the form just described: length, say 210 feet at the water-line; breadth, 44 feet; depth in hold, 24 feet, and we would have a ship constructed to carry, say 118,000 cubic feet. From comparison, and our present experience, we would locate the centre of buoyancy at about 1/30 of the length forward of the centre in sailing ships, at which point, or a little forward, we would fix our widest part."
Upon examination of the lines in the engraving it will appear that the design does not altogether correspond with the dimensions set down in the above extract: the dimensions of the design being, length, on the load-line, 205 feet, moulded breadth, 42 feet, and hold about 26 feet. We prefer the former dimensions to the latter. In another particular does the description differ from the design: the "centre of buoyancy" and the "widest part" was proposed to be nearer midships in the former than we find it in the latter; dead-flat is located 14½ feet forward of mid-length of load-line. The minimum angle of load-line with middle-line was set down at 30°, the angle of the same in the design is about 34°. Whatever excellencies may belong to the design, it is our belief its author had in view an improvement upon it, when he sketched the decription of a Cotton ship in January last. Our impressions would appear correct from the following passage contained in a letter, dated April 28, 1856.
… "I likewise send you a tracing for a Cotton ship answering to my idea, from which two ships have already been built, [they] being 4 feet deeper — which is taken from the top-sides, the bottom remaining the same. — The ships are very successful as they are, but I do not choose to disclose their names."We have, at this time, a single objection to urge against the design which our valued friend and corresponden has favored us with; and in bringing it forward we have no other motine than to communicate to the public, as he has done, a single principle which should govern the modelling of vessels for deep and shallow draught of water; it is this, that while the former class require an easy bilge in order to maintain the most advantageous altitude for the center of buoyancy to insure stability and ease of motion when loaded; the latter description of vessel demands a fair development of bilge, in order to secure capacity, while the diminished draught, by reducing the distance between the centre of buoyancy and the centre of gravity of the vessel, (and of the cargo,) avoids the demand which is otherwise made on narrow ships of a deep draught of water. Not only so, the vessel or proportionate depth and breadth, dispensing with the unprofitable necessity for ballast to aid her in equilibriating, absolutely requires a fair amount of bilge for stability, if she would sail upon her bottom when without cargo, as every ship should be capable of doing.
Did not the good sense of carriage-makers provide a sufficiently of breadth between the wheels, it would become necessary to ballast land vehicles as well as water, especially over rough country roads. Vessels depending on breadth for stability require it with no stinted measure. What nature has furnished in her principles of forces costs us nothing; she has furnished the cheapest highway in the world, and, we believe, the cheapest stability also.
We object to a very easy bilge for a light draught vessel, because we want capacity and stability when carrying sail without cargo; because developing the bilge enlarges the lateral, while it aids to properly distribute the direct resistance, and because, when judiciously formed, it diminishes the rolling of light draught vessels.
A barrel bottom is held in repudiation by good builders of shallow vessels, everywhere. Steamboats are the only shallow vessels benefited by an easy bilge. As a general principle it may be laid down that in proportion as we condemn the deep and adopt the shallow vessel, in commerce, we may lessen deadrise and develop the bilge. The man who would model and proportion vessels of increased beam upon the same rules that he designs those of contracted breadth, must fail of success till his experience dictates an improvement in his models. The bottoms of our dep sea-going ships, if the top-sides were removed, would, in most cases, make inferior coasters, for the reasons above.
We are aware that these remarks apply with but little force to the design of Cotton ship before us, inasmuch as she cannot justly be pronounced as of light draught.
Without meaning to depreciate the labors of others, we may be allowed in this article to name what we would deem proportionate dimensions for a Cotton ship between New-Orleans and Liverpool. We do not expect to be called upon for the model for several years yet.
The ship would draw about 15 feet of water, with 2 feet outstanding keel, and if dimensions were arbitrarily insisted upon without reference to displacement and capacity, would be about as follows:— Length on the loadline, 234 feet; beam moulded not less than 52 feet; hold, 17 feet, with two decks; ship-rooms on the upper deck. The midship section would be full, and be located where its name imports. The breadth of load water-line section would be carried forward of midships; would care less about the sharpness of water lines than the relative ease of vertical section lines; and should overhang the bow and stern the least possible. Very few features of the present ship type model would be found in our Cotton ship; and if old builders should pronounce her an overgrown schooner, it would not in the least disconcert us. We would be obliged to any one who would point out to us the essential differences, if any, required between vessels adapted to freighting lumber, hay, and cotton cargoes, which demand space, rather than displacement, and, hence, call for breadth; as a basis for stowage, which shall not contravene the laws of gravity, and compromise the safety of navigation. Adequate strength would be derived from mechanical sources, not from disproportionate dimensions.
Transcribed by Lars Bruzelius
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