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circumstances which have already been mentioned as producing strains upon a ship considered as a whole may and do produce severe local strains. For example, a heavy load concentrated in a short length, not merely contributes to the longitudinal bending moment previously described, but also tends to push outwards that part of the bottom upon which it rests. Similarly, the thrust of a screw propeller not only tends to rack the ship as a whole, but produces considerable local strain on that part of the ship to which the "thrust-bearer" is attached. Again, the downward thrust of a mast, besides tending to alter the transverse form of the ship as a whole, produces a considerable local strain on the step, and on the frame of the ship which carries the step. And these are only a few illustrations of a general principle. When the ship is treated as a whole, it is virtually assumed that these local strains have been provided against; so that the various parts of the structure can act together and lend mutual assistance. As a matter of fact, however, it is not at all uncommon to find local failure supervening long before the limit of the strength of a ship considered as a whole has been realised. The case of the Independenzia, previously quoted, well illustrates this; when she stopped in launching, her general structural strength was ample even against the severe bending moments experienced; but while her longitudinal form remained almost unchanged, the very exceptional local strains on a small portion of the bottom forced it inwards, disturbing the decks, &c. above it. Many similar examples might be added, but enough has been said to show how important it is to provide carefully against local strains in arranging the structure of a ship.

One of the chief causes of local straining has already been mentioned; viz. a great concentration of loads at certain parts of a ship; and the converse case is also important— that where there is a great excess of buoyancy on a short length. Examples have been given of such concentration of loads; one of the most notable is that for the Devastation,

in wake of the turrets (see Fig. 85), where there is an excess of weight over buoyancy of 550 tons on a length of about 30 feet. Still more concentrated is the load of armour on a battery bulkhead, weighing perhaps 60 or 80 tons, and lying athwartships. Immediately in wake of such concentrated loads the bottom tends to move outwards from its true shape; the local strain which is developed tending to produce simultaneously both longitudinal and transverse change of form. Many similar causes of straining will occur to the reader; it is only necessary to mention the cases of a vessel with a heavy cargo, like railway iron, stowed compactly, or of a vessel with heavy machinery carried on a short length of the ship, or of the parts adjacent to the mast step of a sailing ship.

Surplus buoyancy on a ship afloat is not usually found so much concentrated as surplus weight; but in some instances the excess of buoyancy produces a considerable local strain tending to force the bottom upwards. When a ship grounds, her supports are often so few and small that the bottom cannot withstand the concentrated upward pressure, which then produces considerable local damage. This is, however, an uncommon case, and one which the shipbuilder can scarcely hope to provide against satisfactorily.

To prevent local deformations of the bottom in wake of excesses either of weight or buoyancy, the shipbuilder employs a very simple and well-known device. The concentrated load or support is virtually distributed over a considerable length by means of strong longitudinal keelsons, bearers, &c. In not a few cases these longitudinal pieces are additions to the main framing or structure of the ship; in other cases they form part of the main structure, being effective against the principal strains as well as against local strains. The latter plan is preferable, where it can be adopted, favouring, as it does, lightness and simplicity of construction. These longitudinal bearers and strengthenings can only distribute loads or upward pressures when they are individually possessed of considerable strength; and

this is easily secured. Generally the longitudinals must be continued through a length sufficient to connect and secure the mutual action of parts where there is an excess of weight with others where there is an excess of buoyancy. But in very many ships, and especially in iron ships, there are cross-sections, like those at bulkheads, where alteration of the form is scarcely possible. In such cases the bearers distributing a concentrated load or pressure frequently extend from one of the strong cross-sections to the next: just as the girders of a bridge extend from pier to pier, and, if they are made sufficiently strong, can transmit a concentrated load placed midway between the piers to those supports without any sensible change of form.

The Great Eastern furnishes a good example of the lastmentioned arrangement. In the lower half of her structure there is very little transverse framing. Numerous and strong transverse bulkheads supply the strength requisite to maintain the transverse form unchanged. Strong girders, or frames, extend longitudinally from bulkhead to bulkhead, and transmit the strength of the bulkheads to the parts lying between them. Arrangements of a similar, but not identical, character are also made in the ironclad ships of the Royal Navy, and will be illustrated in the following chapter. The engine and boiler bearers in many iron steamers are also arranged on this principle.

Vessels with few transverse bulkheads, or with none, have strong keelsons, binding strakes, stringers, and other longitudinal strengthenings on the flat of the bottom below the bilges, these pieces distributing loads and adding to the structural strength. This is the common arrangement in wooden ships of all classes, as well as in iron sailing ships. Recently, however, in the wood-built ships of the Royal Navy and the French navy iron bulkheads have been constructed, and, in some cases, iron bearers and keelsons have been fitted. The wood-built American river steamers

See Fig. 104, page 331.

furnish curious illustrations of the connection of parts of a ship having surplus buoyancy with others having surplus weight. Besides strong longitudinal keelsons, the builders have recourse to the "mast-and-guy" system. Poles or masts are erected at parts of the structure having surplus buoyancy; these masts are stepped upon strong timber keelsons. Chain or rod-iron guys are then secured to the heads of the masts and connected at their lower ends to parts of the vessel where considerable weights are concentrated, thus hanging these parts on, as it were, to the buoyant parts. In this fashion, the long fine bows and sterns are prevented from dropping; and, in wake of the machinery, tendencies to alter transverse form are similarly resisted. Such arrangements are, of course, only applicable to vessels employed in smooth water, not subjected to the changes of strain to which sea-going ships are liable. The guy-rods can transmit tension, but not thrust; and the plan is said to have answered admirably in these long fine vessels, having great engine-power and high speed.

Grounding is another cause of more or less severe local strains, the intensity depending upon the amount and distribution of the supports. Very concentrated supports, as has already been shown, may crush up the bottom; distributed support such as a ship obtains when docked or fairly beached produces strains which can be easily met. Every provision described above for giving stiffness to the bottom of a ship is also efficient in helping her when aground. In fact, to these provisions shipbuilders mainly trust, making few special arrangements against local strains due to grounding, and these almost wholly at the extremities. Nor is this surprising, for it is impossible to foresee all the conditions of strain, or to provide against them. Such accidents to any individual ship are comparatively rare, and in iron ships the damage which results, even when it is very serious, can be repaired much more easily than is possible in wood ships. Examples will be given in Chapter X. illustrating this difference.

Penetration of the skin of a ship ashore often takes place without any serious crushing up of the bottom; and this danger is of peculiar importance to iron ships, having skin plating never exceeding an inch in thickness, and in the great majority of cases less than half that thickness. Sharp hard substances, such as rocks, will penetrate the plating more readily than they will penetrate the much thicker bottom of a wood ship. This superiority of wood ships in sustaining rough usage ashore without penetration of the bottom is well known; and some persons have attached such importance thereto as to advocate the construction of ships with wooden floors and bottom planking, but otherwise of iron. The plan has, however, obvious disadvantages, and has not found much favour with shipbuilders, who prefer to accept this occasional disadvantage of iron, rather than to sacrifice its superiority in other respects to wood.

*

It is sometimes assumed that iron bottoms are more inferior to wood in their resistance to penetration than is really the case. To the experiments of the late Sir W. Fairbairn, more exact knowledge on the subject than was previously accessible; in these experiments, a few comparative tests were made of the resistances of wood planks and iron plates to the punching action of a very concentrated support. Under the experimental conditions an oak plank 3 inches thick was found equal in resistance to an iron plate inch thick; and a 6-inch plank to a plate 1 inch thick. Planking appeared to offer a resistance proportional to the square of the thickness; whereas iron plating offered a resistance proportional to the thickness only. The largest iron ships have, therefore, bottom plating about equivalent to a 5-inch or 6-inch oak plank. This would be quite as thick as, or thicker than, the average bottom planking of large wood ships; but within this planking the wood ship

* See the account of the experiments given in Sir W. Fairbairn's work on Iron Shipbuilding.

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