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WAR-SHIPS

CHAPTER I.

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.

1. 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

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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

I TON

6 TONS

36 TONS

10 FEET

FIG. 1.

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

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