the provision of their structural strength. In doing so, it will be possible to illustrate the distinctive features in the principal classes of ships, to compare the relative efficiencies of various methods of construction, and to contrast the degrees of importance attaching to different parts of the hull of any ship. All that will be assumed is that the reader has a general acquaintance with the names of the different parts; and in most cases even that extent of knowledge will scarcely be requisite in order to follow the discussion.

All ships may be said to consist of two principal parts: (1) the water-tight skin forming the covering of their bottoms, sides, and decks, if they have decks; (2) the framing or stiffening fitted within the skin to enable it to maintain its form. There are many ways of forming the skin in different classes of ships; some of these will be described. Wood, iron, and steel are the three materials at present used for the purpose in sea-going ships; brass skins have been fitted to some small vessels designed for smooth-water services. A skin is an essential part of every ship; and much care and skill are required in its arrangements. Vessels have been built with little or no framing; but these are not ordinary cases, and probably the greatest varieties of practice are to be found in the arrangement of the framing, which constitutes a very important element of the structural strength. In constructing both skin and framing, and considering every detail of the hull, the shipbuilder should seek most fully to combine strength with lightness. To do this, he must possess an intelligent acquaintance with the causes and character of the strains to be resisted, their possible effects upon different parts of the structure, and the principles of structural strength. He is then able to choose from among the materials obtainable those best adapted for his purpose; he can duly proportion the strength of the material to the strains on the various parts, massing it where requisite, or lightly constructing parts subject to little strain; and so far as

the requirements of convenience and accommodation, or of fighting efficiency, permit, he can approximate to an ideally perfect structure, in which each part is equally strong as compared with the strain it has to bear. No structure is stronger than its weakest part; consequently a bad distribution of the materials can only be made at the sacrifice of strength, which might be secured if the material were distributed more in proportion to the straining forces.

Another important practical matter is that of the connections and fastenings of the very numerous pieces making up the hull of a ship. Unless great care is taken, the ultimate strength of these pieces will never be developed, and the structure may fail through lack of rigidity, even when it contains an amount of materials which would be ample if they were properly combined. The character of these connections must bear an intimate relation to the

qualities of the materials. With wood they are necessarily different from what they would be with iron or steel. In fact, the builder has to consider this feature in making the choice of his material; having regard not merely to the ultimate resistance of a single piece to tensile or compressive strains, but also to the possibility of making a combination of two or more pieces efficient against such strains. Having made his choice, he has to effect the best possible connections and combinations, often at no small cost, in order to secure the joint action of the various pieces, and the rigidity of the structure considered as a whole.

In the present chapter it will be convenient to assume that the best possible results have been secured by the builder in each class of ship, and then to investigate their resistances to the principal bending strains, tending to alter the longitudinal and transverse form. Local strains have received in the preceding chapter all the attention that can be given them; and in the succeeding chapter we shall illustrate the capabilities of wood, iron, and steel as materials for shipbuilding.

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The severest strains to which ships are subjected are those tending to produce longitudinal bending; and therefore the greatest strength is requisite to prevent change of form in that direction. If the ship were subjected to excessive bending moments, developing strains greater than her strength could resist, their ultimate effect would be to break her across at the transverse section where the strains reach their maximum; and this section would usually be situated near the middle of the length. Unfortunately, cases are on record where this ultimate effect has been produced, and vessels, when very severely strained, have actually broken across; but ordinarily, instead of actual fracture, we have only to consider a tendency to produce fracture at any cross-section of the ship, the structural strength being ample in proportion to the strains. In either case one thing is clear, viz. that resistance to longitudinal bending or cross breaking at any transverse section of a ship can only be contributed by those pieces in the structure which cross the probable line of fracture, i. e. the particular transverse vertical section of the ship which is being considered. Pieces lying longitudinally or diagonally in the ship may fulfil this condition, and therefore contribute to the longitudinal strength; but pieces lying transversely, such as a transverse rib or frame or beam adjacent to the line of fracture, do not cross it, and therefore do not contribute to the longitudinal strength. By this simple rule it is, therefore, easy to distinguish those parts of the hull which are efficient against the principal bending moments.

One of the most singular cases on record is that of the Chusan iron steamer, which broke in two outside Ardrossan, a year or two ago, one part of the vessel floating into the harbour, while the other sunk outside. It is only proper to add that this ship was not built for sea-going service, being designed for

Her

the shallow waters of China. length was 300 feet, beam 50 feet, and depth in hold only 11 feet. Another case in point is that of the Mary, which broke in two in the Bay of Biscay; she was also a shallow-draught vessel of great length.

Chief among these may be mentioned the skin planking or plating on the outside of the ship; the planking or plating on the decks; and the longitudinal frames, keelsons, shelfpieces under beams, water-ways, side-stringers, and diagonal iron riders. For any transverse section of the ship, the enumeration of all these parts and the estimate of their respective sectional areas are very simple processes, upon which the calculation of the strength of the ship at that section is based.

The greatest bending strains being experienced at or near the midship section, let it be assumed for purposes of illus

tration that the ship is upright, and that it is desired to ascertain the strength of the midship section against crossbreaking strains. In performing this calculation, it is usual to construct an "equivalent girder" section, similar to that shown in Fig. 99. On the left is drawn an outline of the midship section of an iron ship with a double bottom, and with longitudinal frames between the outer and inner skins, these latter being indicated by the strong black lines. On the decks, the planking, plating, and stringers will also be distinguished from the transverse beams upon which they are supported. The effective areas of all these pieces which cross the

midship section, and extend to some distance before and abaft it, are represented in the "equivalent girder" on the right. The deck planking and plating on the upper deck are concentrated in the flange A; those of the middle deck in the flange B, and those of the lower deck in the flange C. The inner and outer bottom plating, longitudinal frames, &c. from the turn of the bilge downwards, are concentrated in the lowest flange or bulb D; the vertical or nearly vertical plating on the sides, together with the longitudinal stiffeners worked upon it, form the vertical web EE, connecting the flanges. It will be observed that the depths of the girder and midship section are identical, and all the corresponding pieces in both are situated at the same heights, the vertical distribution of the pieces on the midship section being maintained in the girder.

There are many important matters connected with the work of constructing equivalent girders; but one or two only of the most important can be mentioned. First, it is necessary to distinguish between the total sectional areas of the longitudinal pieces on the midship section, and their effective areas which are shown on the girder. A very simple illustration will show the character of this distinction. In wood ships it is usual to arrange the "butts" of the outside planking so that at least three planks intervene between consecutive butts lying on the same transverse section. Fig. 100 shows this arrangement; b and I are two butts placed on the same timber; and the probable line of fracture of the planking between these butts is indicated. Against tensile strains tending to pull the butts open on any section such as bl, the butted strakes have little or no strength; therefore, in order to allow for this weakening on the midship section, one-fourth of the total sectional area of the outer planking must be deducted. Further, there must be bolts or wooden treenails driven in the unbutted planks, to secure them to the ribs of the ship; and the holes cut for these fastenings at any cross-section may be taken as equivalent