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THE STRENGTH OF SHIPS.
Before considering in some detail the structural arrangements of various classes of ships in the Royal Navy, it will be of use to consider briefly the general nature of the strains to which a ship's structure is likely to be subject.
These strains may broadly be divided into two classes, viz.—
1. Structural strains, coming on a ship regarded as a complete structure, and
2. Local strains, which only affect a particular portion of a ship.
A ship may have great structural strength with small local strength; for example, a small torpedo-boat is strong enough to stand being lifted bodily out of the water on to a ship's deck, while the plating is very thin and easily damaged. A full-sized ship could not be thus lifted (supposing it to be feasible) without the probability of serious damage to the structure.
I. Structural Strains.—The most important of the structural strains to which ships are subject are those strains tending to bend them in a fore-and-aft direction. The ship may be regarded as a huge beam or girder. In order to bring out the various points in connection with this fore-and-aft bending and the proper arrangement of the material to make the ship strong enough, we shall first consider some properties of beams.
Beams.—If we take a plank, say 12 in. wide and 2 in. thick, and place it on supports 10 ft. apart, a ton weight placed at the middle would be as much as the plank could bear without
breaking. If, now, we placed six such planks as in Fig. 1, the planks could stand 6 tons. If instead of simply resting on one another the planks are tightly clamped or bolted together, we should find that the beam thus made would stand far more than the six unconnected planks. If we carried the process a step further, we should have a log 12 in. wide and 12 in. deep of homogeneous material, and we should find that such a beam would stand 36 tons before it would be on the point of breaking, or six times the weight that could be carried by the six unconnected planks.
We notice that the upper layers of such a beam are being
compressed and the lower layers are being stretched, and it is the resistance offered by the fibres of the beam, to this compression and this stretching, which enables the beam to withstand the bending. The middle layer of the beam is unaltered in length, and none of the upper layers are compressed to the extent that the top layer is, and none of the lower layers are stretched to the extent that the bottom layer is. So that if the timber of the beam is just strong enough to stand the tension at the bottom and the compression at the top without rupture, none of the layers between the top and the bottom are being used to their full strength. Thus the portions of the beam furthest away from the centre of the section contribute most to the strength of the beam.
The following will illustrate the same principle. Suppose it is desired to make an iron beam 12 ft. long, having a sectional area of 16 square inches. How should this area be disposed to admit of the beam standing the greatest weight at the centre, without exceeding a stress of 10 tons per square inch on the material? If the beam is, as (a) Fig. 2, 8 in. wide 2 in. deep, it will stand a weight of about 1'5 tons; with a section, as (b), 4 in. wide 4 in. deep, it will stand about 3 tons; as (c), 2 in. wide 8 in. deep, about 6 tons, and as (d), 1 in. thick with flanges at top and bottom 5 in. wide, 10 tons. In the last case the bulk of the material composing the section is disposed most effectively in being away from the centre of the section, and although it has the same sectional area as (a), the beam can stand between six and seven times the weight.1 A familiar instance of this principle is seen in the construction of many bridges. The upper and lower flanges are made exceedingly strong, and the web is often formed
of lattice-work. (Charing Cross Railway Bridge in an instance of this construction.)
Longitudinal Strains.—In arranging the structure of a ship so that it shall efficiently resist the bending in a fore-and-aft direction, we arrange the material on the above principles. Special attention is paid to the sufficiency of the strength of the upper and lower parts of the ship, as the upper deck and the side plating adjacent, and the bottom plating, keel, longitudinals, etc., at the lower part.
The longer the ship is in proportion to her depth, the more necessary does it become to pay attention to these portions of the structure. Thus the upper-deck plating and side plating adjacent, and the structure at the keel, are made much stronger in a cruiser of 14,000 tons than in a battle-ship of 15,000 tons, because the proportion length -f- depth is so much greater in the former case than in the latter. Fig. 3 shows in comparison the keel structure in these two cases.
1 The thinning down of the web cannot, of course, be carried on indefinitely.
In attempting to make these strains the subject of calculation we are confronted with the difficulty that it is not possible to
Keel Of Battleship —— Keel Of Cruiser
accurately determine the maximum strains the ship may reasonably be expected to withstand. An extreme case would be to assume that the ship is caught amidships on a rock, with the ends unsupported, or, again, if the ends only were supported. It would, however, be impossible to so construct a ship that the structure could stand anything so extreme as either of these conditions. The weight involved for the hull structure would be prohibitive.
For purposes of calculation, the following assumptions are made—(a) the ship is supposed to be momentarily poised on the crest of a wave of the same length as the ship, Fig. 4, and (6) astride
the trough of a wave of the same length, Fig. 4. Under the first assumption, the ends of the ship would tend to drop relative to the middle, and we should have the upper works tending to tear apart, and the lower works to buckle up. Under the second, the reverse would be the case, viz. the keel and lower parts of the structure would tend to tear apart, and the deck and upper parts to buckle. The first is termed hogging, and the second sagging.
In a well-constructed ship these are only tendencies, and the material is able to withstand the strains thus brought to bear upon it. If, however, the ship is not strong enough, these tendencies will show themselves by the giving way of the structure at some point. In some destroyers, for instance, the compressive strain on the upper deck has caused the plating to buckle between the beams, and in these vessels it is most important to provide fore-andaft stiffening to the deck, so that the plating will be able to stand up to the strain without deformation.
Even in still water there are strains on a ship, because the weight and the support of the buoyancy vary along the length. At the ends, for instance, there would be an excess of weight over buoyancy, because of the fineness of the ends, and at other portions of the length the reverse may be the case. These strains, however, are quite small in amount compared with those which might come on a ship in a seaway.
If the results of calculation on a certain ship on the above assumptions give a certain stress on the material, and the ship is found to show no signs of straining on service, it is safe to proceed with another ship, which by a similar process of calculation is found to be equally strong. The stresses on the material thus found are not regarded as absolute values. There are many conditions in the problem which cannot be taken into account, but the stresses thus calculated form a valuable means of comparison from one ship to another.
One feature in the construction of recent cruisers has been the adoption of special steel of high tensile strength. Instead of the tests specified for ordinary mild steel, viz. 26 to 30 tons per square inch, this special steel has to stand a tensile test of between 34 and 38 tons per square inch. This steel naturally is somewhat expensive, and is only used in special places, as e.g. for portions of the upper deck, and the upper and lower portions of the outer bottom plating. These are portions which, as seen above, are the most severely strained when the ship is hogging or sagging. Torpedo-boat destroyers of recent construction are built with decks and outer bottom plating of a steel of still higher tensile strength, viz. between 37 and 43 tons per square inch.