Hull Structure And Planking

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The building of big wooden hulls with load-bearing frames reached a stage in development at the end of the sixteenth century in which all structural elements were already present more or less as in nineteenth century ships. So refinements were slow in the seventeenth and eighteenth centuries. Building methods varied from country to country only in the details. All ships, or at least all square rigged sailing ships which were built in European shipyards from Trieste to Königsberg, were essentially the same from a structural point of view.

Eighteenth century English shipbuilding is fairly well documented, first by a number of original contemporary models made by the dockyards which had built the ships. Then there is a considerable amount of modern, high-level, research. What is missing are systematic, detailed works by contemporary writers during the early eighteenth century. These were only produced at the end of that century.

Since the plans of Royal Caroline provide many indisputable elements, we can hope that the hull structure described below is, if not identical, at least very close to the original.

She was certainly built like the other ships of the time. Correspondence shows that she received greater attention than was usual for a small vessel of the Sixth Rate. She was a small ship and many structural elements which had to be made up from several timbers on large ships could easily be made from a single plank or tree-trunk for her.

The longitudinal structure, formed of keel, false keel, keelson, sternpost and stem, is clear from the drawings. Here we would observe that the keel was built of elm in several pieces joined by vertical scarphs, while the oak keelson and false keel were fashioned from several pieces joined by horizontal scarphs. A minimum of six or eight metal fastenings joined the two elements at each scarph joint and long bolts ran upwards from the lower face of the keel, through the individual ribs to the upper face of the keelson, where they were clinched over roves. These bolts did not pass through the false keel, which was joined to the keel by staples and by nails hammered in upwards from below. In this way the whole framework did not need to be dismantled to replace a damaged false keel. If the ship ran aground, the false keel broke easily because of its weak joint with the keel and in a certain sense acted as a 'shock absorber' to the body of the ship.

The metal bolts, which were distinct from nails, ran through the whole thickness of the pieces to be joined and were clinched at the opposite end from the head. They were round bars, up to 2 inches thick on large ships, with a truncated conic head which was countersunk in or, sometimes a rounded head which protruded from, the wood. The craftsmen who did the job of inserting these bolts or rods through the wooden skeleton were highly specialized, and this was their sole occupation. It seems incredible today that, using only hand drills, they could cut a passage through a solid wood bow or stern more than 9 feet thick with minimal deviation from true.

Ordinarily the bolts were made of iron or a copper alloy. Iron, under moist conditions, reacted in contact with tannic acid (in which oak is particularly rich) and caused the metal to corrode rapidly and the wood to decay. Nails and bolts soon ceased to be watertight and the joints loosened. So there was a tendency to make them in a copper alloy, which was less resilient than iron but much more durable in seawater. This became the rule when the practice of coppering the hull was adopted in England between 1770 and 1780.

Bolts subject to particular tensile stresses (for example those which fixed the knees at the ribs and beams), rather than being clinched over a rove, were fixed with a key or metal spike called a 'gib' which fitted into a hole running diametrically through the bolt itself, resting against a rove. The entire skeleton of the ship was held together by these bolts, while nails were used only to join pieces less subject to stress.

The athwartship structure consisted of frames usually formed by a pair of ribs bolted together. Every third pair is shown in the Admiralty plan. The frames were composed of different pieces since it was not possible to find wood of a suitable length and curvature naturally. In three-deckers, each rib might have been formed of seven or nine pieces. In Royal Caroline, as can be seen from the main frame taken directly from the original plans, it was formed of only five parts, a floor timber and two futtocks per side. Naturally, the joints between these elements were in different places in the two ribs which formed the frame so that when the two ribs were joined by the bolts, the result was a fairly rigid whole. Drawing B1 shows the English method of building frames around 1750.

The 'room' or distance between the timbers was very narrow in ships of the line. The tendency was for it to be less than the calibre of enemy shot, at least up to the gun deck. Higher up, as the section of the futtocks reduced, the space widened. Light craft were built according to less expensive criteria, leaving wider spaces between the frames. The original Royal Caroline plans show that the room was more or less the section of a frame.

Ballast, in the form of lead slabs, was placed in the interstices between the floor timbers. Normally there was little or no space between the floor timbers of warships and the ballast, in the form of cast iron slabs, old cannon balls or simply gravel, was laid above the internal planking. The system used on board Royal Caroline left more free space in the hold and this, as can be seen from the longitudinal section of the ship showing the internal divisions, was exploited down to its last cubic metre. Stowing the ballast in the lowest possible position made for greater stability, and also made it easier to keep the bilges clean and prevent an unpleasant odour from fouling the air below decks. This was, however, a peculiarity of this ship, not to be taken as a general rule. Among other things, a centre of gravity very much below the centre of the hull due to the special lead ballast gave the ship much greater stability, but after being heeled over by a gust of wind or a wave, her righting was much faster and sharper. The masts were stressed more and even the hull structure felt the effects. Hutchinson (Practical Seamanship, 1777) dwells at length on this subject with illuminating examples. Cargoes that were too heavy and compressed were not to be stored below the estimated hull centre to avoid the risk of the topmasts breaking off as a result of rolling too quickly. In Royal Caroline this drawback could easily be corrected by adjusting the amount of ballast so as to give a stable ship without excessive 'stiffness', and an uncluttered hold for personnel and equipment. Hutchinson's work makes interesting reading because he displays his very fine intuitive appreciation of the forces governing the movements of a hull in water.

With regard to materials, the entrenched Royal Navy opinion was that British oak was by far the best type of wood. Hard, compact, durable in seawater as long as it was well seasoned, it was excellent timber. But some of the various species of Mediterranean oak were just as good and the magnificent woods which the Spaniards used in their shipyards of Manila and Havana were even better - alas, only as long as the forests lasted!

All Royal Caroline's planking (decks included) was of English oak. This gave the hull the warm, yellowish-brown colour seen in many marine artists' paintings. The planking consisted of strakes 10 to 12 inches wide, about 3 inches thick for a small ship (7 to 8cm). Planks a little narrower tended to be used for the decks as they were less subject to structural strain, and thus to splintering. The planking was fixed to the hull frame using nails, bolts and treenails. There is a certain amount of confusion on this point due to the inaccuracy of certain authors, who lump periods, national traditions and types all together. The practice in England around the middle of the eighteenth century was to use treenails for a good part of these fastenings. This combined the advantages of econony (compared to iron and copper alloys), lightness and durability. They neither oxidized nor corroded the wood into which they were driven, and lasted practically as long. It is obvious that at the beginning the resistance of an iron nail or bolt was much greater than a treenail, but after two or three years the situation was the opposite.

The following is one possible method for fixing the planking: two treenails for each strake on every frame, and two metal bolts running through and clinched on a rove at every butt joint between planks in the same strake. A metal nail was interposed in every two or three frames. When the width of the strake was less than 9 inches (20cm), a single treenail was used on each frame. Metal nails only were used on the wales and they were bigger and more numerous than those on normal planking. They were often round-headed instead of flat.

The deck planking was fixed to the beams by treenails alone. Since there is often discussion on this point we would say that these treenails must not be confused with the dowels which were (and still are) used to cover the heads of bolts fixing deck planking to the iron frame of composite hulls, or the heads of spikes which attached planks to wooden beams where this was the practice. The grain of the dowels ran parallel to the deck, both for aesthetic reasons and because the deck had to remain uniformly smooth in use. These problems obviously did not arise with treenails. There were two treenails for each beam and ledge per plank. The diameter of the head was IVi to 2 inches (3 to 4cm). The deck was fashioned from planking running parallel to the keel for the whole length. The habit of tapering planks at the extremities of the decks was not unknown in the Royal Navy but the above rule applied more frequently.

The planks were always jointed regularly on a beam, with an interval of three or four strakes between joints on the same beam. Caulking between the strakes was done with oakum and pitch which gave the seams between the planks, about one centimetre wide, a dark grey or blackish colour.

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