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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 15 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. 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

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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 depth is so much greater in the former case

1 The thinning down of the web cannot, of course, be carried on indefinitely.

than in the latter. Fig. 3 shows in comparison the keel structure in these two cases.

In attempting to make these strains the subject of calculation we are confronted with the difficulty that it is not possible to

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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 (b) astride

HOGGING

ON WAVE CREST

SAGGING

ACROSS WAVE TROUGH

FIG. 4.

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.

In deck-protected cruisers, like Diadem and previous ships, and present second and third class cruisers (see Figs. 21, 22, 24, 27), the heavy protective deck fitted in the region of the waterline, although valuable as a means of stiffening the ship, is not in a position to contribute anything like its fair share to the structural strength, because of its position near the neutral axis of the section. In large cruisers since the Diadem, armour protection has been adopted at the side, and the main deck is made a protective deck as well as the middle deck (see Fig. 23). This is a much better distribution of the material as regards its usefulness in the structure of the ship, as well as for purposes of protection. The latest development in the design of large cruisers has been the adoption of a battery on the main deck instead of casemates, and the upper deck over this battery is made a thick deck. This deck is still better adapted to assist in the structural strength.

In large ships in the Royal Navy it is the established practice to build the structure mainly on the longitudinal system. That is, the longitudinal portions of the structure are made continuous, and the transverse portions are made in short pieces between, or intercostal. This system is carried out from the keel to the lower edge of armour or the protective deck, over the length of the double bottom (about two-thirds the length in a battle-ship). This system is admirably adapted to the formation of a double bottom. At the ends of the ship the vertical keel is still continuous, but the transverse framing is made the continuous part either side of the keel. The longitudinal strength is not so important at the ends, and is obtained by intercostal girders, the various platforms and bulkheads as well as the outer bottom plating. For smaller ships the transverse framing is made continuous and more closely spaced, and the fore-and-aft framing is mostly intercostal. The close spacing of the framing is necessary in order that the outside plating shall be well stiffened to hold it up to its work, as it is necessarily thin in a small ship. In these vessels a double bottom is not fitted. These features of construction will be more fully dealt with when we consider in detail the structural arrangements of various classes of ships.

If we compared two ships of the same size, one a war-ship and the other a merchant ship built to the rules of a registration society, we should find that the scantlings (or sizes of the steel used) of the former would be considerably less than those adopted The neutral axis is a horizontal line through the C.G. of the section.

in the latter case. There are several reasons which contribute to this. Merchant ships carry heavy weights of cargo in large holds, and the transverse framing especially has to be very massive to take the weight of the cargo. Again, merchant ships in some trades frequently ground when loading or discharging their cargo, and this again necessitates a good margin of strength as compared with war-ships, which are more carefully handled in this respect. Also, in war-ships a very extensive system of watertight subdivision is adopted; the large number of bulkheads and flats thus obtained assist very materially in the structural strength. Most merchant ships are built with the transverse framing for the most part continuous, with fore-and-aft girders intercostal. This system, although a very convenient method as regards economy in construction, is not so efficient from a structural point of view for large ships as the method adopted for war-ships. An important point in connection with the scantlings adopted in a ship is the subsequent maintenance. Inspection and care of the structure of war-ships is carried out in a most careful and thorough manner, and in this way the margin for deterioration may be made considerably less than in ships not so carefully looked after. The regulations for inspection of the structure of H.M. ships will be referred to later.

A very important principle in ship construction to be borne in mind is the necessity for avoiding any discontinuity in strength. Any part of the fore-and-aft structure which has to be ended, must be tapered down over several frame spaces, and a deep girder which has to be continued to the end of a vessel in a smaller form must be tapered down gradually to avoid any discontinuity of strength.

A superstructure like a boat deck is very high up, and if made a continuous portion of the structure is likely to be severely

-PLATING OF BOAT DECK

FIG. 5.

strained, and it is not sufficiently strong to stand any severe strains. On this account the plating of this deck is deliberately cut through, and a sliding joint is made as Fig. 5 to save the

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