Читать книгу On Naval Timber and Arboriculture - Patrick Matthew - Страница 6

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Timbers, as before stated, are the ribs of the vessel, spreading out and upward (excepting at the bow and stern) at right angles to the keel and keelson, two large straight logs which form a double spinal support or backbone. The ribs or compass timbers in great public building establishments are sometimes bent by machinery, after being softened by steam or hot liquids8; and for this purpose the {15} cleanest straightest wood is requisite. We, however, do not believe that pieces of great diameter, bent artificially, can have equal strength and resilience as when grown bent—the fibre must in some degree be crippled. We admit that timbers and frames may be built of separate bended pieces of no great thickness, and have all the strength and resilience of natural bend: the strongest and most elastic mode of forming vessels would be to compose them of different layers of plank over each other in diagonal fashion, or at an angle 60°, but the labour and inconveniency of these modes would be great. We will not admit that an experiment between the strength of a piece of coarse cross-grained timber, half naturally bent, half cut out of the solid, and that of a piece of clean timber artificially bent, is any proof on the subject. Let us produce a clean natural bend, exactly fitted to its place, without any section of fibre, and make experiment with it. But at any rate, as this plan (bending of timbers) has never been adopted to any extent in our private building-yards, we must doubt its economy—either {16} that the practice is of no considerable advantage, or that the requisite machinery is too expensive for private establishments, and conclude that fine bent timber still continues a necessary in the formation of at least our mercantile marine.

Of the very ingenious innovations in the structure of vessels contrived by Sir R. SEPPINGS, by which knees and crooked timber might nearly be superseded, we can only say, the practice is not followed, and, at least in private building-yards, not likely to be so;—that the demand for fine crooked timber, comparatively, is, and will continue to be, as great as ever. Should our war navy, from the introduction of steam impulse and bomb cannon, be reduced to fleets of strong gun-boats, the demand for crooked timber, instead of lessening, will greatly increase—the building of frames of straight timber being more expensive, and less suitable, in small than in large vessels; and should war occur, in the hurry of the formation of a new war navy under a different principle, the speediest and simplest mode of construction will be followed.

Nearly two-thirds of the timbers of a vessel consist of the curves and bends a, b, c, d, e, f; the other third is of straighter timber, and easily obtained. {17} All timbers require to be straight in one way—in the plane of their side, and the sides generally to be square. The under measures embrace timbers of appropriate size for vessels from 50 to 500 tons register; it is seldom that merchantmen are required under or above this size. Of course, large war-vessels require timbers of larger dimension. The corresponding timbers of vessels of different size are nearly similar figures, and the length of their respective lines not far from being in the ratio of the cube root of the tonnage—a little deeper and thicker in the smaller vessels. When timbers are formed of larch or pine, they require to be a little more in diameter than when of oak. {18}

Fig. a, Flat floor, from 9 ¹⁄₂ to 18 feet long (that is, 9 ¹⁄₂ for a vessel of 50 tons, and 18 for one of 500), and from 9 to 16 inches deep at middle; thickness ¹⁄₄th less than depth, the diameter increasing in proportion to the length. When fillings such as s are used, flat floors are cut from straight logs.

b, Rising floor shorter, and same depth and thickness as former.

c, c, High rising floors, from 4 to 8 feet in length of wing, and a little deeper, and same thickness as former. From the difficulty of procuring this bend, the wings are often used of unequal length, according as the timber turns out, the shorter wing to exceed 3 feet, and more when of considerable diameter. Floors are of every rise from a to c, being flattest at midships, and rising gradually as they approach the bow and stern. In all timbers, it is necessary, for strength, that the fibre of the wood extend from one end to the other without much cross grain. See lines on high rising floor, c.

d, First foot-hook, from 7 to 13 feet long, and from 7 to 14 inches deep; thickness ¹⁄₅th less than depth.

e, Second foot-hook, from 6 to 10 feet long, and from 6 to 13 inches deep, thickness ¹⁄₆th less than depth. This curve, when of great size, is valuable as, breast-hooks—curved timbers stretching horizontally within and at right angles to the bow-timbers, to support the bow.

f, f, f, Knees, the one wing nearly at right angles to the other; from 2 to 9 feet in length of wing; depth at middle as much as possible; thickness from 4 to 12 inches—generally required about 3 ¹⁄₂ feet in length of wing, and from 6 to 8 inches thick. Knees, when large, suit for high rising floors.

Fig. h is a valuable piece, and easily procured by bending the young plant; when cut, it forms two second foot-hooks.

Figs. a, b, c, d, e, are suitable, though the part cut off by the dotted line be awanting. In good work, this plan is often followed, and a cross-chock put on. (Vid. s, left side of the cross-section of a vessel thus timbered, page 20). By this {19} mode of building, vessels can be constructed from much straighter timber, and the vessels are superior, from being more elastic; but from the nicety and expense of the work and waste of timber, the practice is not much in use.

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Cross-section of a Vessel at midships—knees not inserted.

A first foot-hook alternates with each floor, and second foot-hook, alongside, extending from o to q; and a top-timber, or third foot-hook, alternates alongside of each second foot-hook, and chock extending from q to a. These timbers are bolted together, and constitute a frame or double rib; and the skeleton is composed of a series of double ribs (several inches separate, of course wider above than lower down, as the timbers decrease in thickness), to within a little of the bow and stern, where the timbers are usually placed singly, without framing. {21} In large vessels a fourth futtock is used; thence straighter timber is suitable.

The knees occupy the position at x, stretching horizontally along the inside of the vessel and end of the beams.

Of British trees, timbers are formed of oak, Spanish chestnut, larch, red-wood pine, red-wood willow (the stags-head ozier, Salix fragilis), and sometimes the broad-leafed elm (Ulmus montana) under water.

In Britain, crooked oak for timbers is found chiefly in hedge-rows and open forests, where the winds, casual injury, or overhanging superior branches, have thrown the tree, while young, from its natural balance; or, by the tree, from open situation, or excision of lower branches, parting early into several leaders, which, in receding from each other, form curves and angular bends. On the Continent of Europe, in the natural forest, it is chiefly the tops of old lofty trees which afford the crooks; in consequence, those we import are, for the most part, of a free, light, insufficient quality9. {22}

To procure a sufficiency of excellent crooks, every person who has the charge of young plantations of timber intended for naval purposes, ought, in the more exposed situations not favourable to the growth of plank timber, or timber for bending, when the plants are from 3 to 15 feet high, to mark out the most healthy, suitably formed plants, sufficiently close to fill the ground when of the proper size, say 6 yards apart, and to bend these, as the under figures will illustrate. The dotted portion is the growth after being bent. {23}

The bend of floors requiring to be at the middle, and of angular bend, see Fig. f, young trees of one-half the required length, should have the earth removed from the bulb of the root, from one or both sides, according to circumstances, and the tree and stool partially upset to windward, that is, generally south-west; (the operator, in effecting this, may be assisted by a strong pronged instrument); then fixed in this inclined position, and the earth filled in. This inclination may be given at planting, when the plants are tall.

The best mode of securing the larger plants in their bent position, is by rods, forked or hooked at one end, the other end nailed to a ground-stake;—the upper end, if forked, firmly tied to the bent plant by mat or straw rope. Smaller plants may be secured to the notched tops of stakes by ligatures; and the smallest, particularly larch, pinned down by small stakes with hooked tops. Advantage may also be taken of an adjacent tree of small value, and which would ultimately be required to be thinned out, to tie the bended standard down to the most convenient part of its top or stem, lopping off all above the ligature, if it interfere with the standard, and barking it near the ground, to prevent much future growth. When the workmen comprehend {24} the required bends, they will fall upon methods of fixing the plants in the most suitable position, better adapted to the locality than any directions can teach. The plants will require to be fixed down at least two years, and bent a little more than what is requisite, as in their after-growth they have generally a tendency to become straighter, from depositing the thickest layers in the hollow of the bend. A fine regular curve may be obtained by bending the plant for several successive years, a little lower every year; this gradual lowering does not so much check the growth of the leader, nor tend so much to cause the feeders upon the upper side to push as leaders. When oaks are bent, great attention must be paid to cut away any ground-shoots, and to cut off or twist down any strong feeders that stand perpendicular on the upper side of the tree; and also for several years afterwards, to look over the trees twice a-year, correcting any exuberant feeder, and destroying root-shoots. The forester ought to keep in mind that his pupils are proverbially pliant, and that, should his growing timber not be of the most valuable and most appropriate figure, he must rank either with the negligent or the incapable.

Ship timbers being generally required of greater depth than thickness, that is, broadest in the plane {25} of the curve, hedge-row is better adapted to growing them than the forest, especially when the trees are close in the row. The bend generally takes place across the row; and the bole of the tree acquires a greater diameter in that direction than in the line of the row. If the figure of the top of a tree be very elliptical in the horizontal plane, the cross section of the bole, instead of being circular, will also be elliptical (cake-grown). The lateral spread of the roots in thick planted rows being greater than the longitudinal, also tends to give elliptic bole, the stem swelling most on the sides where the strongest roots enter, which, of course, always occurs on the sides affording most nourishment. Forests intended for ship timbers might be planted and kept in rows a considerable distance apart, with the plants close in the row, and thus acquire the elliptic bole. This would also facilitate the bending; by being turned a little right and left alternately, they would spontaneously, from the weight of the top, and their inclination to avoid the shade of each other, increase the original bias. Were forests planted in close double rows, the plants thick in the row, with wide avenues or glades between, many of the trees would acquire crooked boles, and the crooked might be retained when thinning. Avenues of this description {26} would form agreeable diversity from the monotonous irregularity of the forest, and be highly picturesque.

Were close triple rows planted with wide glades between, having spruce, larch, birch, or other trees of more rapid growth than the oak in the mid row, and oak in the side rows, the greater part of the oak would be thrown out into fine curves by the overshadowing top of the superior tree. After the oak had received a sufficient side bias, the central row, which of those kinds comes soon to be of value, might be removed.

The easiest way to procure good oak knees is to look out in hedge-row and open forest for plants which divide into two or four leaders, from 3 to 10 feet above ground; and should the leaders not diverge sufficiently, to train them as horizontally as possible for several feet, by rods stretching across the top, or by fixing them down by stakes; see following figures. Figs. a, b, f, are drawn to a smaller scale than c, d; of course, a stem, after dividing, never extends in length below the division.

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When grown, the main stem, either used whole, sawn in two, or quartered, will form one wing of the knee, and the bent branch the other; see figs. c, d. The dotted lines shew the saw section. Particular attention must be paid to prevent oaks from separating into more than four leaders, and also to train up these leaders a considerable height, without allowing them to divide again, retaining always numerous feeders; thus, when the tree acquires size, {28} many valuable crooks g, h, i, will be formed above the knees. It is necessary, however, to guard against training the branches to too great a height, as, when so, they run much risk of being twisted and torn by high winds.

Knees may also be obtained by cropping the top from plants that have side branches similar to f, and training these branches for leaders as above directed. In this case, the section, where the top is cut off, must not be too large, and the branches, either two or four, well knotted to the trunk, or the situation sheltered, otherwise the trunk at the section may be split down by the strain of the wind on the new leaders. Also, in healthy growing trees of considerable size, which have spreading tops, and which are not to be cut down for a considerable time, the forester, if he have a good eye, may, by lopping off a few branches here and there throughout the top, throw the greater part of the boughs into condition to become knees, or valuable crooks, when of size. This is of most material consequence to the ultimate value of half-grown oak trees, in open situations, and ought to be particularly studied by the superintendent, as, when allowed to run into very numerous stemmy branches, without direction or curtailment, the top, instead of being ultimately of {29} considerable value as timber, is of none. Directions in writing will scarcely suffice to teach a forester this part of his business; he must consider attentively the knee figures and bends we have furnished, fix them in his memory, and use every eligible means to obtain them. Knees, of all descriptions of oak timber are in the greatest request. We have known them purchased at 7s. per computed solid foot, which, from the plan of measuring, is as much as 10s. per real solid foot. The prevailing inattention to judicious training will continue to occasion the supply of knees to be short of the demand, and thence the price high, provided some change does not take place in the structure of vessels, or iron knees be adopted, which are now sometimes used, or vessels, with the exception of the deck and rigging, be formed of iron altogether, which we have seen do very well in inland navigation.

As crooked round oak timber of the natural length is extremely unmanageable, and its distant transport very expensive, it is desirable that it be squared and cut in lengths suited to its ultimate use, where grown. This requires a thorough knowledge of the necessary curves, to which the figs. p. 19, will afford considerable assistance. However, the superintendent of any extensive fall of naval timber either should be {30} a shipwright who has had practice in lining off timbers, or should have passed several months in a dock-yard during the timbering of vessels, observing every piece that is put to use.

As most part of the timbers of a vessel have their sides squared, the cutter cannot err much in hewing away the sides in the plane of, and at right angles to, the curves, at least as deep as the sap-wood reaches, thus leaving only a little sap-wood on the angles; the sap-wood, in all cases (except in those small craft used in carrying lime, which preserves from rot), being worse than useless; by its decay not only weakening the vessel from the want of entireness of the timbers, but also acting as a ferment to further corruption.

In our directions for obtaining curved and angular bent timbers, we may be thought to have been a little too minute with the dimensions and figures: under the hand of the shipwright, or person of skill, a tree of almost any possible bend cuts out to valuable purpose: what is wanted is crooked timber, free of large knots;—first and second foot-hooks and knees are, however, most in demand.