angles. In some cases where special strength is desired, this duplicate arrangement of the riders is carried right fore and aft, as in her Majesty's ship Caledonia; but the more common plan is to have one system only. It will be observed that, as usually arranged, these iron riders are very efficient aids against hogging strains, which are those most injurious to wood ships. When hogging takes place, the ends must drop relatively to the middle, a change of form which would bring the iron riders under tensile strains, the kind of strains which they are best fitted to resist. Against compressive strains these thin narrow bands of iron cannot be nearly so efficient as against tensile strains, so that, as commonly fitted, riders are not of much service against sagging strains, except amidships, where the two systems overlap one another. Of course it is amidships that the severest strains are experienced, so that the crossing of the riders there is a great advantage; and it has been suggested that, if the duplication of the systems were carried through, say, one-third or one-half of the length amidships, there would be a further gain in strength, owing to the circumstance that the riders would then assist against sagging as well as hogging.

Composite ships of the mercantile marine are usually built with a single thickness of planking, and consequently need diagonal strengtheners. One common plan of fitting these is to have rider plates riveted outside the iron frames, and inclined 45 degrees to the vertical. The upper ends of those riders are attached to the sheer strake, and the lower to another detached longitudinal tie, formed by a strake of plating worked at the bilge.

The composite ships of the Royal Navy are built with their outside planking in two thicknesses. The edge-seams of the planks in the inner thickness are each covered by a plank of the outer thickness; the seams of the outer thickness being similarly covered by the planks of the inner thickness. A strong edgewise connection is thus made in the double skin, and consequently diagonal rider

plates are dispensed with. It should he added that this plan of working the planking in two layers is principally adopted because these vessels have their bottoms covered with copper sheathing, and any injurious galvanic action of the copper on the iron hull can thus be avoided.

Other composite ships have been constructed with the outside planking in two thicknesses, one or both of which had the planks worked diagonally; it was then unnecessary to fit diagonal rider plates to assist the skin against racking strains.

This diagonal system of planking has also been adopted in some special classes of wood ships with great success. The royal yachts are examples of this system of construction, and Mr. White, of Cowes, has applied it in many vessels built at his yard. Three thicknesses of planking are employed, the two inside being worked diagonally, and the outer one longitudinally. The two diagonal layers are inclined in opposite directions, and the skin thus formed possesses such superior strength to the skin of an ordinary wood ship that there need be comparatively little transverse framing above the bilges. Direct experiments with models, and the experience gained with ships built on this plan, have demonstrated its great superiority in the combination of strength with lightness. The royal yacht Victoria and Albert, built on this plan, with her unusually powerful engines and high speed, is subjected to excessively great sagging moments, but has continued on service for twenty years with complete exemption from signs of weakness. Like many other improved systems of construction, this is found rather more expensive than the common plan; but if wood had not been largely superseded by iron, probably much more extensive use would have been made of the diagonal system.

*

It may be mentioned that the large steam and sailing launches employed in the Royal Navy are built on a some

See the remarks at page 278.

what similar plan; the skin planking is in two thicknesses, worked diagonally, with the two layers inclined in opposite directions. These boats answer admirably, and have only frames on the flat of the floor, where the wear and tear of grounding have to be borne.

Iron ships have outer skins formed by numerous plates, each of which is strongly fastened at the edges, as well as the butts, to the plates adjacent thereto. Such a combination is very strong against longitudinal racking strains, and needs no supplementary strengthening such as the diagonal riders of wood or composite ships. Many proposals have been made, and several plans have been patented for using diagonal strengthenings in iron ships, the superiority of an iron skin, and its capability of resisting and transmitting strains in all directions, not having been apprehended. From the bilges upwards, the outside plating forms the principal part of the web of the equivalent girder section in ordinary iron ships like that in Fig. 103; and when properly stiffened, it acts this part most efficiently when the ship is upright. When she is considerably inclined, some parts of the same plating contribute strength to the flanges of the girder-section for that position, as already explained. Vessels with double bottoms extending far up the side, or with wing-passage bulkheads like that in Fig. 104, gain much on vessels with single bottoms, since the additional skin contributes to the strength of the web of the girder for the upright position, and to the strength of the flanges of the girders for inclined positions. Any other longitudinal bulkheads which extend over a considerable length in the ship may also be regarded as contributing to the longitudinal strength, and one of the most valuable additions of this kind that can be made to a ship is a middleline bulkhead like that shown in Figs. 18-25 (page 30) for an ironclad of recent type. The longitudinal bulkheads fitted in the Great Eastern add greatly to her longitudinal strength. It need hardly be said, however, that such bulkheads are fitted primarily with a view to increase in safety

or accommodation; the increase in structural strength being a secondary consideration.

Mention may also be made, in passing, of a plan upon which a few iron ships have been built, intermediate in character between ships with transverse frames and others with longitudinal frames. The main frames in these special vessels lie diagonally, somewhat after the fashion of riders, and therefore cross the probable line of fracture of the plating in ordinary iron ships, which line, it has been said, would lie in a transverse plane. It is hoped, therefore, either to divert the line of fracture from this transverse plane to some longer and stronger diagonal line or else to make the diagonal frames add to the strength of the transverse section which gives the smallest effective sectional area to the bottom plating. The plan has not found favour with shipbuilders, nor does it seem comparable to the longitudinal system, either in cheapness and simplicity of construction or the combination of lightness with strength.

Vessels designed for service in shallow waters often have their hulls strengthened longitudinally by girders. It has been shown that the depth of any cross-section of a vessel has a great influence upon the amount of its resistance to bending strains; and in these special vessels the depths of the hulls are so small as to render supplementary strengthenings essential. The American river steamers before mentioned furnish good examples. Their hulls are extremely shallow, and have to carry an enormous superstructure of saloons, &c., although they have in themselves little longitudinal strength. To supply this, what is termed a "hog frame" is constructed. It consists of a strong side keelson fitted along the flat floor of the vessel, at some distance out from the keel. Upon this keelson are erected a series of timber pillars, and along over the heads of the pillars a strong continuous timber beam or tie is carried, diagonal struts being fitted between it and the keelson. A light but strong timber girder of considerable depth is thus

firmly combined with the shallow hull, and made to help it efficiently against hogging. In other light-draught vessels built for river or coast service, with iron or steel hulls, arrangements have been adopted similar in principle to the foregoing, iron or steel lattice girders having been substituted for the more cumbrous and less efficient hog frame. These vessels, being designed for smooth-water service, are not subjected to longitudinal strains of so severe a character as those experienced by ships at sea, and, what is still more important, their strains remain nearly constant in character as well as intensity. Hence their case is much more easily dealt with in the manner described, than is that of a sea-going ship which has to bear rapid and extreme variations of longitudinal bending strains while she rolls from side to side in a seaway. At the same time, there is considerable range for the exercise of ingenuity in securing the lightness of construction demanded by the shallow draught. The conditions of the problem resemble more closely those of bridge construction than those connected with the construction of sea-going ships, with which we are more especially concerned.

Figs. 105 and 106 furnish illustrations of this class; being respectively a side view and cross-section of a tug-boat built

* See a very interesting paper on the subject by Mr. Norman Russell, in vol. ii. of the Transactions of the Institution of Naval Architects.