With the general reference already made to Mr Froude’s invaluable labours in connection with the resistance of vessels the brief statement of the agencies through which progress has been made during the present century may be considered as brought down to the period coming within the scope of the term “Modern,” as used in this work. The more difficult task of chronicling the progress made during the period in question, both in the science of naval architecture purely, and in the application of science to practice, must now be attempted. The plan upon which it is proposed to accomplish this is to show wherein and to what extent scientific methods in designing and observing the behaviour of ships have been regarded, and indicating generally where still further improvement may be looked for. To accomplish this in such a way as to take appreciative account of the most salient features, and yet to avoid difficult technical terms and unnecessary elaboration, may involve some omissions and slight inaccuracies, important enough from a strictly scientific point of view, yet which do not materially affect the faithfulness of the record.[5]
As preparing the way for references to those more special points in connection with which scientific progress has taken place during recent years, the following general and elementary outlines of the principal scientific problems in ship design and construction may be helpful to many readers:—
DISPLACEMENT AND CARRYING CAPABILITY.
A vessel floating at rest displaces a volume of water whose weight equals her own total weight.
For vessels floating in sea water the number of cubic feet of water displaced per ton of weight is, as nearly as possible, thirty-five. For vessels in fresh water—i.e., lakes or rivers—the cubic feet per ton of weight is thirty-six.
By calculating the volume of under-water portion of the vessel’s hull, the number of cubic feet displaced by the vessel when floating at any given draught is obtained. This result, divided by 35 or 36, according as the water is salt or fresh, gives the number of tons weight displaced, and consequently the total weight of the vessel.
Calculations being made of the volume of the vessel’s hull to intermediate distances between the keel and the maximum load line, it is thus possible to construct a “curve of displacement” from which the actual amount of displacement at any intermediate draught can be obtained.
From this curve a set of scales—usually set up alongside a vertical scale of feet and inches, representing the vessel’s draught-marks—are constructed, showing—1st, the tons “displacement” at any draught; 2nd, the tons of “dead-weight” capability—i.e., the tons displacement due to the weight of cargo, coal, ballast, stores, fresh water, spare gear, &c.—at any draught above the vessel’s light-draught: “light-draught” being that at which the vessel floats with holds clean-swept, bilges dry, water in boilers, and with such spare gear on board as is required by Board of Trade; and 3rd, the amount of “freeboard”—i.e., the distance in feet and inches from any particular draught line to the top of the deck amidships.