Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/498

Rh 470 HARBOUES [DOCKS. pontoon lias been brought about the tide-level, the water is allowed to escape, when there is sufficient floating power to admit of the whole being removed to any place where the repairs can be conveniently made. The pontoons, which can accommodate vessels of a length of 350 feet, are about 320 feet long and 59| feet broad. &quot;The power of the hydraulic lift is 6400 tons. The largest pontoon will carry a dead load of 3200 tons in addition to its own weight.&quot; Slips are the contrivance of the late Mr Thomas Morton of Leith, and consist of a carriage or cradle working on an inclined railway, falling generally at the rate of about 1 in 17, and extending above high water to a sufficient distance for the class of vessels which are expected to use them to several feet below the level of low water, and a truck or carriage which moves on it. When the carriage is let down unier the water, the vessel is floated above the place, and the carriage is drawn up till the vessel catches it forwards. When the ship is placed truly above the line of the carriage, a powerful crab-purchase at the top of the slip, which is generally worked by steam, is set in motion, and raises the truck and ship out of the water. The gridiron is a simple framework of timber placed at a level sufficient to admit of vessels being floated above it during the flood-tide, and grounded upon it during the ebb, and when thus left high and dry the vessel s bottom can be examined to ascertain if it be necessary to take her into the graving dock, and trifling repairs can also be made. The gridirons at Liverpool vary from 25 feet to 36 feet 3 inches in breadth, and from 228 feet 3 inches to 31 3| feet in length. 1 The hydraulic and screw docks used in America are chambers into which vessels are floated during the flood- tide, above a cradle which is drawn up above the high water level, either by means of Bramah s press worked by a steam- engine, or by a powerful apparatus of screws. The patent slip possesses the following advantages over the graving dock. (1) The cost of its construction is less. (2) When the fall of tide is languid, a vessel can generally be more quickly laid dry. (3) When so laid dry she can be more easily examined, and, from the duration of the day light being greater than in a deep graving dock, the time in which work can be done is considerably extended during winter. (4) There is more perfect ventilation, by which the vessel s sides are sooner dried, which is of some moment with an iron ship. (5) A vessel can be hauled up long before high water, and the repairs can be begun at once ; whereas with a dock the pumping occasions considerable delay. (6) While the upper part of the slip is occupied, an additional vessel rmy be taken up for a shorter time than its predeces sor, without interrupting the workmen. The advantages afforded by a graving dock, on the other hand, are these. (1) Although its construction is more costly, it is nevertheless, if properly built, unquestionably a more durable structure the rails, rollers, carriages, and chains connected with the slip being liable to derangement, which entails occasional repair, (2) The management of the graving dock is simple, and involves comparatively little superintendence ; whereas that of a slip is intricate, and requires more than mere nautical skill. (3) The working of a graving dock is equally simple for large or small vessels, while it is undeniable that the raising of a large vessel on a slip is a delicate operation, and should be attempted only under the direction of persons thoroughly versed in such matters, and having ample mechanical resources at command. (4) The graving dock possesses the advantage, which is sometimes important, of affording the means of more easily filling a vessel with water, so as to detect leaks which may not be discoverable by other means. For this purpose a 1 &quot; Historical and Descriptive Sketch of the Mersey Docks and Harbour,&quot; by J. J. Rinckel, in the Artizan for 1864. nozzle to receive a flexible tube should be fixed into the dock-gates. (5) Where double gates are provided the water contained in the dock affords a certain limited power of scouring the forebay and entrance, an advantage which is of course not possessed by a slip. (6) In any rapid land current, or strong tideway, it is a much easier process to dock a vessel t han to land her safely on the cradle of a slip an operation which, when incautiously gone about, has been in some cases attended with serious consequences even in sheltered situations. (7) The graving dock need not interfere with the set of the currents, whereas a slip which projects a long way seaward of low water may deflect them and produce shoals in the channel. (8) Mr Mallet has remarked that the strains on a ship s timbers are more direct than when she is on a slip, especially when she is leaving the cradle. The late Mr J. M. Balfour suggested, in order to meet this objection, that the cradle for a slip might be made of a wedge shape, so that its upper surface shall be parallel with the horizon, or that the back end should even be tilted slightly, so as to give a bite on the vessel and prevent her from slipping. The relative advantages of the other contrivances for the repair of ships already described may be judged of by com paring them with each other in a similar manner. Mr P. W. Barlow has given the following formulae for Dock the strength of dock-gates : Formula for Straight Gates. If = horizontal angle (or &quot;sally&quot;) between pointing sill and line joining heel-posts of the two leaves; W = pressure on the length of the gate with any head and for a given depth of the gate ; and 8 = hole transverse strain at angle &amp;lt;p, then From this Mr Barlow has deduced that the salient angle, where the strain is the minimum, is 24 54, but as the length of the gate increases with the secant, the strength will not at this angle be the greatest with a given section of timber. The &quot;sally&quot; or angle which gives the greatest strength, with a given section of timber, is stated by him as 19 25. Formula for Curved Gates. When 6 is the salient angle, or camber of the beam, formed by a chord line drawn from the heel to the mitre-post, with the tangent to the curve of the gate There is great difference of opinion among engineers as to the strain to which dock gates are subjected, and the reader is referred for further information to the 18th and 31st volumes of the Minutes of Proceedings of the Institution of Civil Engineers. In these dis cussions Mr Brown pointed out an error in Mr P. W. Barlow s paper, which stated that the line of pressure at the mitre-posts would always be a tangent to the curve of the separate gates, whereas that line must always be at right angles to the centre line of the lock, and could only be a tangent to the curve when the two gates formed a segment of a circle, or, as Mr Bramwell says, at all events when their junction at the mitre-posts formed at that point part of a continuous curve. Mr Brown gives elaborate formulae suited to meet a yielding or deflexion of the structure, which he alleges must always take place. Mr Bramwell states, and we think justly, that, when the gates form when closed a segment of a circle, they cannot be subject to transverse strain, and that the whole of the gates would be subjected simply to compression. Mr I!. P. Brereton very properly suggests that when the gates are of malleable iron the boiler-plate should never be less than inch thick, what ever the formula may indicate. Where I represents the length of one-half of a straight or cambered malleable iron gate, w the distributed pressure over the length of the leaf taken on a given element of the gate, bounded by two horizontal planes 1 foot apart, t the thickness of framework of gate or distance between the two skins, s the transverse strain in middle of gate, 6 half of the mitring angle i.e., the angle formed by meeting of gates all the dimensions being in feet, and- weight in tons, then ._*?. 4t &amp;gt; s = sectional area of metal on compressed side in inches ; i s do. do. on extended side do. ;
 * ?c tan = compressive strain produced by other leaf of gate.