The earliest type of the modern floating dock is that known as the box dock. It consists of a pontoon divided into cells or compartments, and having on either side a large wall also divided into compartments arranged in tiers, the ends of the structure between the walls being open. The earliest of these docks were made of wood, and compared with those of later date were of small dimensions. One of the most noteworthy wooden docks was that at Rangoon, launched in February 1866, and having a length of 300 feet, with a breadth of 90 feet, and an inside breadth of 70 feet, and able to take vessels drawing from 15 to 16 feet of water. There is also at Altona a wooden floating dock built in 1868 and still in active use; it is 138 feet in length, and can lift vessels up to 420 tons register. The early floating docks were usually in transverse section like the capital letter U, and followed fairly closely the form of the round-bottomed ships of the time. As the girder principle, however, became introduced in shipbuilding it was recognised that floating docks must be constructed approximating to that shape, and modern floating docks are now built rectangular in transverse section, though in constructional details this form is a modification of the U shape.
Floating docks themselves are in occasional need of repair, and when it was found that they could be constructed of a greater size than any then existing dry dock, it being customary to dry dock them for repair, the necessity arose of devising a means whereby the repairs could be made without taking the floating dock out of the water. Sometimes a dock can be tilted endways or sideways as occasion requires, for a portion of its under-water surface to be exposed, but there is obviously a limit to this operation and to the effectiveness with which work under these conditions can be carried out. This difficulty was met by constructing docks on the sectional principle, whereby any two sections of a floating dock constructed in three sections can lift the other one; while with off-shore docks, which are usually built in two sections, either can lift the other. An attempt to careen the old U-shaped Bermuda dock nearly capsized her altogether.
One of the earliest—if indeed not the earliest—of self-docking double-sided docks is that associated with the name of Mr. Rennie, and now generally known as the Rennie type, or, in an attempt made at uniform classification of self-docking docks by Messrs. Clark and Standfield, who probably have had greater experience of floating-dock designing than any other firm in the world, the “sectional pontoon” dock. This is an extremely simple form of dock, consisting of a series of similar pontoons connected together into a whole by the walls or side girders, which run along each side on top of the pontoon, to which they are attached by bolts. In self-docking, any particular pontoon can be unbolted from underneath the walls, allowed to sink slightly, and then be drawn out sideways, turned half round, and lifted on the rest of the dock. The type is also very suitable for erection abroad, for the pontoons can be built and launched separately, and, being but light structures, require no expensive launching slips, whilst the side walls can be erected on top of the pontoons after they are afloat.[100]
[100] “Modern Floating Docks,” by Lyonel Edwin Clark, M.I.N.A.
The first Bermuda Dock, launched at North Woolwich by Messrs. Campbell, Johnstone and Co., in September 1868, was the largest built up to that time, and was ordered by the Admiralty for the use of British ships in the West Indian Squadron. It was 381 feet in length, 123 feet 9 inches in extreme breadth, and had a total depth of 74 feet 5 inches. Caissons enclosed a dock space of 333 feet by 83 feet 9 inches in width, capable of receiving a vessel of 3000 tons. The section of the dock is of U form throughout, though for convenience of towing, a tapered bow of wood was added, and remained until it rotted off at Bermuda. The dock was designed by Mr. Campbell. The sides consisted of a cellular space 20 feet in width, and midway between the inner and outer skin was a water-tight bulkhead, running the whole length of the structure. Each side was subdivided by longitudinal bulkheads into three compartments, named from the bottom, the “air,” “balance” and “load” chambers, and was further subdivided into twenty-four water-tight cells. The dock was fitted with four steam engines and pumps on each side. Hitherto all floating docks had been built in sections, shipped to their destinations and erected there. The Bermuda dock, however, was towed there, experimentally, and so successfully was the work accomplished that the towing of floating docks across the ocean has become the rule, and some wonderful feats of towing have been performed. This dock, becoming unequal to the requirements of modern shipping, gave place to the present dock built at Wallsend in 1902.
Model of the Bermuda Dock.
The length of the present Bermuda dock is 545 feet over the keel blocks, its width of entrance 100 feet, and it is capable of normally taking vessels drawing 33 feet of water over keel blocks 4 feet high. The walls themselves are 53 feet 3 inches high, and 435 feet in length, and they form girders of enormous strength. Three pontoons, secured to the lower portions of the walls by fish-plate joints, lugs, and taper-pins, form the bottom or deck of the dock. The middle pontoon is a rectangle 96 feet by 300 feet; the end pontoons, each 120 feet long, taper for 49 feet towards their outer extremities to facilitate towing.
At this immersion the walls have a freeboard of 3 feet 6 inches, which in urgent cases might be safely reduced by a foot or more in order to increase the depth of water over the blocks. Its lifting power up to pontoon-deck level is 15,500 tons, but by utilising the “pound” formed by the bulwark surrounding the pontoon decks, additional lifting power up to 17,500 tons can be gained. The dock, without its machinery, weighs 6500 tons. When called upon to perform its maximum lift the dock is sunk until the summit of its walls is but 2 feet 6 inches above sea-level. Water is admitted into the three pontoons and the two side walls, and from them removed by eight 16-inch centrifugal pumps at a rate sufficient to lift an ironclad of 15,000 tons in three and a half hours. In order that the dock may not tilt as it rises, the whole is divided into fifty-six divisions, each of which is separately connected with the pumps. By turning off cocks, water can be left in any desired divisions, and the dock forced to incline in any direction for purposes of cleaning and repairs. When undergoing its official tests the Bermuda dock lifted H.M.S. Sans Pareil over 11,000 tons, and after its arrival at Bermuda it received and raised completely out of the water H.M.S. Dominion, when that vessel was badly damaged through stranding and was so down in the water as to displace nearly 17,000 tons.
It is specially important that a structure of this kind should be self-docking, that is, able to lift any part of itself clear of the water. To expose the bottom of one side the dock is first lowered to a depth of 20 to 21 feet, the water inside the wall compartments being brought to the same level as that of the water outside. The dock is then raised by emptying the pontoons, and when these are exhausted the water is released from the side to be exposed until the outer corner is two feet or more clear. The pontoons are lifted in turn by withdrawing the pins of one, and allowing it to float, while the rest of the dock sinks. The pontoon is then made fast to the walls at its floating level, and the dock emptied, so exposing the whole of the bottom of the raised pontoon. The two end sections can be treated simultaneously, and floated if required on to the central portion, but the latter must be moved only when the other pontoons are in position. Electric lights and hauling machinery are distributed over the dock. A crane capable of lifting five tons runs along each wall from end to end.