We have made little mention, in the foregoing pages, of the actual tonnage or dimensions of ships, for the reason that the figures would be for the most part unreliable or misleading in import. The basis on which tonnage was measured was constantly changing. It was difficult to obtain accurate measurements of the principal dimensions; length, especially, was an indeterminate dimension, and, in the days when a large fore and aft rake was given, the length of keel gave no indication of the over-all length. Even if the over-all dimensions could be accurately measured, they gave small information as to the form of the hull: the fullness or fineness of the lines, the form of the bow-curves and tuck, the position of the section of maximum breadth, both longitudinally and relatively to the water-line—proportions on which the sailing qualities of a ship largely depended. In the seventeenth century the tonnage figures were generally untrustworthy; the Sovereign was quoted by three different authorities as being of 1141, 1637, and 1556 tons burthen. In the eighteenth century tonnage and dimensions possessed greater comparative value. We confine ourselves to quoting the following table of typical dimensions, taken from Charnock, showing the gradual expansion which took place in the hundred years which have just been reviewed.
| Establishment | Length (gun-deck) | Keel | Breadth | Depth | Tonnage |
|---|---|---|---|---|---|
| 1706 } | 171′ 9″ | 139′ 7″ | 49′ 3″ | 19′ 6″ | 1809 |
| 1719 } 100-gun ships | 175′ 0″ | 140′ 7″ | 50′ 3″ | 20′ 1″ | 1883 |
| 1745 } | 178′ 0″ | 145′ 2″ | 52′ 0″ | 21′ 6″ | 2091 |
| Commerce de Marseille (120) | 208′ 4″ | 172′ 0″ | 54′ 9″ | 25′ ½″ | 2747 |
| Caledonia (120) | 205′ 0″ | 170′ 9″ | 53′ 8″ | 23′ 2″ | 2616 |
§
The slow progress of naval architecture up to the end of the eighteenth century, an advance the rate of which may be gauged from the fact that, except for sheathing and pumps, no important improvement was patented between the years 1618 and 1800, has been characterized as consisting mainly of approximations to the successive forms and arrangements of Italian, Portuguese, Spanish, and French ships, all of which had been in their turn superior to ours. Until the end of the eighteenth century the “bigotry of old practice” had effectually opposed any radical improvement, even though such improvement had been operating for years in foreign navies and were brought continually before the eyes of our professionals, embodied in captured prizes. In his Naval Development of the Century Sir Nathaniel Barnaby has drawn attention to the remarkable similarity which existed between the Caledonia of the early nineteenth, and the old Sovereign of the seventeenth century: “Almost the only things of note were the reduction in height above water, forward and aft, and a slight increase in dimensions. The proportion between length and breadth had undergone but little change. There was almost the same arrangement of decks and ports; the same thin boarding in front of the forecastle; the same mode of framing the stern, the same disposition of the outside planking in lines crossing the sheer of the ports; nearly the same rig; the same external rudder-head, with a hole in the stern to admit the tiller; and probably the same mode of framing the hull. For the ships of 1810 had no diagonal framing of wood or iron, but the old massive vertical riders; no shelf or waterway to connect the beams with the sides; no fillings above the floor-head; and no dowels in the frames. Ships were still moored by hempen cables, and still carried immense stores of water in wooden casks.”
To Sir Robert Seppings was due the series of innovations in constructional method which placed shipbuilding on a relatively scientific basis and thereby rendered it capable of meeting the increasing demands involved in the growing size and force of warships. His scheme, some elements of which had already been tested in H.M. ships, was described in a paper read before the Royal Society in 1814. In the briefest language we will attempt to explain it.
In the theory of structures, a jointed figure formed of four straight sides is known as a deficient frame, since it has not a sufficient number of members to keep it in stable equilibrium under any system of loading. A triangle, on the other hand, is a perfect frame, since it has enough, and not more than enough, members to keep it in equilibrium however it may be loaded.
The hull of a timber-built ship consisted of a number of rigidly jointed frames or cells, some lying in horizontal, some in vertical, and some in intermediate planes: the unit cell being a quadrilateral, whose sides were formed by the frames and vertical riders and by the planks, wales, and horizontal riders. Practically all the materials composing the fabric of a ship were disposed either in planes parallel to the plane of the keel or in planes at right angles to it. And up to the end of the Napoleonic wars our ships, without appreciable exception, were built on this primitive quadrilateral system. The system was essentially weak. All warships showed a tendency to arch or hog—to become convex upwards, in the direction of their length—owing to the fact that the support which they derived from the water was relatively greater amidships than in the neighbourhood of their extremities. In the old days when ships were short in length this tendency was small, or, if appreciable, a remedy was found in working into the structures additional longitudinal and transverse riders, until the holds were not infrequently clogged with timber. But as ships increased in length, the forces tending to “break the sheer” of a ship and arch its keel increased in greater ratio than the ship’s power of resistance to the distortion; and by the end of the eighteenth century, in spite of the aid of iron knees, stronger fastenings, and improved material generally, the essential weakness of our mode of construction had been gradually exposed. The Victory herself suffered from arching. The extremities of a 74-gun ship dropped six inches, sometimes, when she entered the water from the stocks. A similar tendency to hog took place also across the breadth of a ship, occasioned by the dead weight of her guns. When rolling in heavy weather the momentum of her top weights caused large racking stresses to be thrown on the joints between the frames and the deck-beams. The biographer of Admiral Symonds quotes Captain Brenton as follows: “I remember very well, when I was a midshipman in a 64-gun ship coming home from India, cracking nuts by the working of the ship. We put them in under the knees, as she rolled one way, and snatched them out as she rolled back again.”
DIAGRAM ILLUSTRATING DISTORTION OF FRAMES UNDER LOAD
From these remarks it will be clear that a new method of construction which, by substituting the triangle for the rectangle, prevented the distortion of a ship’s hull under the stresses of hogging and sagging, would constitute an important innovation: even more important if, in addition, the new method resulted in a large economy of material. Such a system Sir Robert Seppings introduced. Treating the hull as a girder liable to bend, he disposed the timbers to the best advantage to resist deformation. The rectangular system, wherein frames and riders formed rectangular cells with no other power of resisting distortion into rhomboids than that derived from the rigidity of the joints, had been proved inefficient; just as a common field gate would be inefficient, and would easily distort, if built up solely of vertical and horizontal timbers without any diagonal brace to make it a rigid figure. He solved the problem with the triangle. By bracing each quadrilateral cell with a diagonal timber he thereby divided it into two rigid and immovable triangles, and thus made the whole ship rigid. The quadrilateral, when braced, was known as a trussed frame. All the chief frames in the ships he trussed; and since all bending took place from the centre of the ship downwards to its ends, he made the trussed frames symmetrical about the centre: the diagonals sloped forward in the after body, and aft in the fore body, so as to resist the arching by extension. The truss frame was embodied, not only in the lower part of the vessel (where its effect in resisting longitudinal bending was comparatively small), but in the more nearly vertical planes, and even in the topsides between the gun-ports (where it was most effective). Its use was estimated to result in the saving of nearly two hundred oak trees in the building of a 74-gun ship.