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about in the distribution of weight and buoyancy. At the ends. of ships there still remains an excess of weight, exaggerated in many cases by the adoption of very fine under-water lines in combination with heavy bows and sterns above water; but the distribution of weights in the fuller parts of the ship becomes much changed. How great the change has been we may infer from the fact that at present merchant steamships are in actual employment of which the length is 400 feet, and the proportion of length to breadth exceeds 10 to 1, both length and proportion having been more than doubled since the introduction of iron into ship construction and steam into ship propulsion.
We usually find the weights of engines, boilers and coals concentrated at some part of a ship. In a paddle-steamer they are found near the middle of the length, in full-powered screwsteamers rather abaft the middle, and in auxiliary screw-steamers very far aft. Wherever they come, their weight obviously increases the downward pressure at that part very considerably; in some cases they cause, while in others they exaggerate, an excess of weight over buoyancy, and in others they bring up the weight very nearly to an equality with the buoyancy. No general law can now be laid down for the strains of all ships, and no general statement can be made to include all the conditions in which any particular ship may be placed by means of variations in her stowage or in the weights she has on board. Having given the details of the weights and buoyancies of various parts, however, the calculation of the resulting still-water strains is practicable, but involves considerable labor. We have taken the cases of one or two typical ships, and have had the distribution of the weight and buoyancy very carefully calculated and graphically recorded. Each example is a ship of modern type, and the results are wholly unlike any which have before been published. In fact, owing to the great labor involved, or to some other cause, only the most meagre and unsatisfactory attempts to discover and exhibit the actual strains of ships have previously been made and recorded.
The first case represents the conditions of long, fine paddlesteamers, of high speed, employed as yachts, or blockade-runners, or on other services where great cargo-carrying power is of comparatively minor moment. The case we have selected is that of the royal yacht Victoria and Albert, and Fig. 30 has been prepared in order to indicate the distribution of weight and buoyancy. In making the calculations required for this purpose, the
total length (300 feet) has been divided into 20-feet spaces, and transverse planes of division have been supposed to be drawn, in order to form the foremost and aftermost boundaries of the spaces. For each division of the ship, the buoyancy, the weight of the hull, and the weight of the equipment have been determined; and the sum of the two latter qualities, of course, gives the total weight of ship and lading for any particular 20-feet space. A base-line, AB (Fig. 30), has been taken to represent the ship's length, and a series, of equidistant ordinates has been erected, each ordinate representing, in position, the centre plane of a 20feet space.
The positions of the imaginary planes of division in the ship are indicated in the figure at the middle points of the parts of AB, lying between the feet of the ordinates; and the distance between consecutive ordinates is, we need hardly say, 20 feet on the scale by which AB is set off. Upon these ordinates, there have been set off, on a certain scale of tons per inch*~(1), a length representing the buoyancy of the division of the ship, with which the ordinate corresponds, divided by the length of the division; the ordinate will therefore represent the average buoyancy of the division per unit of length; (2), a length representing in a similar way, and on a similar scale, the average weight of hull per unit of length for that division: (3), a length similarly representing the weight of hull and equipment for that division. Through the three sets thus obtained, three curves have been drawn. The curve DD represents the displacement or buoyancy, the curve HH represents the weight of hull, and the curve WW represents the total weight of hull and equipment. From this explanation it will be obvious that, by choosing a proper scale, the areas lying above the line AB, and inclosed by the various curves as well as by any two ordinates, may be taken as representatives of the buoyancy, total weight, and weight of hull, respectively, for the corresponding part of the ship. Hereafter it will appear preferable to adopt the latter mode of representation, and in the various diagrams of a character similar to Fig. 30, this plan is followed.
These curves are not minutely accurate representations of the distribution of weight and buoyancy; but for our present pur
8,000 tons. For lengths along line
* For areas of curves, 3 square inches = 8,000 tons. AB, 3 inches = 200 feet.
pose they are sufficiently close approximations to such representations. Our chief interest centres in the comparison of the curve of buoyancy with the curve of total weight of hull and equipment; but the curve HH of weight of hull has an interest attaching to it also, as it enables us to determine the strainingeffect of the equipment, and to illustrate the importance of careful stowage of the weights carried. For the present we shall only make an examination of the distribution of the weight and buoyancy, and for this purpose shall compare the curves WW and DD. These curves, it will be noticed, cross each other at four points marked R', R2, R3, R', in Fig. 30; at these stations the weight equals the buoyancy, and the ship is there "waterborne." Before the foremost water-borne section R'R', which is 50 feet from the bow, the weight exceeds the buoyancy by 85 tons; between this section and the water-borne section R2R next abaft it, a length of about 68 feet, the buoyancy exceeds the weight by 225 tons; between the two water-borne sections, R'R' and RR, a length of 82 feet of the midship length (in which come the engines, boilers, and coals), the weight exceeds the buoyancy by 210 tons; and from R3R3 to R1R1, a length of 70 feet, the buoyancy exceeds the weight by 130 tons; while abaft R*R*, which is 30 feet from the stern, the weight exceeds the buoyancy by 60 tons. These excesses and defects of buoyancy are graphically represented by the areas of the spaces inclosed by the two curves DDD and WWW between their various points of intersection. The hydrostatical conditions of equilibrium are, of course, satisfied by the distribution of the weight and buoyancy.
These figures will show the vastly different condition of many modern steamships as compared with the older types of sailingships, which had an excess of weight only at the extremities.
Some modern ships, however, have a distribution of weight and buoyancy similar in kind, although extremely different in degree, to that of their predecessors; and, as an example of these, we have taken the iron-clad frigate Minotaur. This ship is armored throughout the length; or, to use a more common phrase, is completely protected," and may be considered a fair representative of extremely long, fine ships so protected, with V-shaped vertical transverse sections at the bow. Her length is 400 feet; the heavy weights of engines, boilers, water, powder, and provisions are distributed over a considerable portion of the length; the guns are
also distributed along the broadside; and the weight of hull is nearly uniform, except at the extremities. We should naturally expect, therefore, that the weight would considerably exceed the buoyancy at the bow and stern, and that the buoyancy would exceed the weight throughout the amidship section. The curves in Fig. 31 show that this is actually the case. They are constructed and marked similarly to those of the Victoria and Albert. In this case there are only two water-borne sections, R' R', R' R2. The first is about 80 feet from the stem, and before it the weight exceeds the buoyancy by about 420 tons; the second is 70 feet from the stern, and on this length there is an excess of weight of about 450 tons; between R' R' and R2 R3, a length of 250 feet, the buoyancy exceeds the weight by the sum of these excesses870 tons. It will be observed that at the stern the curve of buoyancy DD is ended at some distance before the curve of total weight WW, in Fig. 30. The overhang of the stern above water is the cause of this method of ending the curves; and in the Minotaur the distance between the points where they terminate is greater than in the Victoria and Albert, because she is a larger ship, and has a screw propeller.
MEASURE OF FIGHTING EFFICIENCY OF SEA-GOING IRON-CLADS.
It is very difficult to express by a coefficient the several elements which constitute efficiency in a ship of war; but the principal elements of fighting efficiency in a sea-going iron-clad may, we think, be said to be
1. The weight of armor per ton of ships' measurement. 2. The weight of the protected guns and ammunition carried. 3. The height of the battery port-sills above the load-line.
4. The speed in knots at the measured mile.
5. Handiness and quickness in manœuvring.
MEASURE OF FIGHTING EFFICIENCY OF SEA-GOING IRON-CLADS. 97
A coefficient of fighting efficiency may be constructed from these elements if the following assumptions are made:
First. That the efficiency will vary directly as the first three of these elements. If an objection is made against the third on the ground that in a turret-ship the guns have the advantage of being withdrawn further from the water in rolling by being placed centrally, and that this advantage is not shown, it may be answered that neither is the advantage possessed by the broadside ship of having a greater number of guns, and that these advantages may be set off one against the other.
Secondly. That the efficiency will vary as the cube of the speed. This power of the speed is taken because the difference in speed among the ships compared are very small; but even small differences may have great results in an engagement.
Thirdly. That, other things being the same, handiness and quickness in manoeuvring will vary inversely as the length of the ship.
On these assumptions we get the following measures of fighting efficiency :
Proposed approximate measure of fighting efficiency in fullyrigged iron-clads, as given by the expression
A × G × H × S3
Where A is the weight of armor per ton of ship's measurement.
NOTE.-Extract from report of the English Naval Construction Bureau, as given before the Committee appointed to examine designs upon which ships of war have recently been constructed.