(See Nos. 142 and 143.)
Civil, ENGINEER A. C. CUNNINGHAM, U. S. Navy.—A Review of Preceding Discussions.—Those who have had to do with the self-docking of floating dry docks will most fully appreciate the importance of simple and safe methods. In some methods careful calculations must be made beforehand of the flotation of the separated parts, and the interior water levels must be adjusted accordingly in order that there may be no shearing of bolts and pins as the connections are opened. A teach self-docking these calculations must be repeated as the interior water levels will vary with the accumulations of growth and sediment on the interior and exterior of the dock. In other methods the submergence of decks of separated parts, acting as water planes, will cause surging that is troublesome and may be dangerous. In the design under discussion all of this has been over come. A pontoon being disconnected, the balance of the dock is slightly pumped up and the pontoon floats free, after which it is regularly docked on the balance of the dock. There are no preliminary calculations for flotation and no submergence of separated decks to cause surging. The process of self-docking a pontoon is illustrated in the accompanying figures. While simple and safe methods of self-docking a floating dry dock will appeal most strongly to those who have had to do with this operation, the value and importance of the time required will appeal with equal force to everyone. It is of the very greatest importance both from a commercial and a military standpoint that self-docking should take as short a period as possible. If self-docking requires a long time it may result in the interruption of business and the loss of work, or it may mean the temporary or permanent loss of a battleship, or the lowering of the efficiency of a fleet from loss of speed due to lack of cleaning foul bottoms. Most methods of self-docking require from two to three months at the least for cleaning and painting alone. This is due to the fact that the entire dock is extensively separated into parts at one time and reconnected for self-docking in an abnormal condition. In the method proposed only a small fraction of the dock is separated from the main part at one time and the self-docking of this small part is a normal operation, the same as the regular docking of a ship. To separate and self-dock a pontoon by the proposed method would require only a few hours; to replace the pontoon would require only a few more hours, so that this method of self-docking is a matter of days where it is ordinarily a matter of months. The continual readiness of a floating dock for use is very desirable. With the proposed type this is secured by having a spare pontoon. When a pontoon is detached for self-docking, the extra pontoon is put in its place and the dock is then ready for its full function. With a spare pontoon the dock need not be out of working order more than six hours at the most. Those who have been directly concerned with floating dry docks fully appreciate that continuous and careful preservation is of the utmost importance if the dock is to be kept up to its original strength and a long life insured, for when deterioration once begins it is difficult to overcome and stop. The proposed design of dock is particularly adapted to continuous and easy preservation. The pontoons may be docked periodically in rotation and when docked are in the best possible position for being conveniently worked on. Their entirely safe position and the safe condition of the balance of the dock does away with any necessity of undue haste. The ease and quickness of self-docking a pontoon also makes it possible to self-dock for inspection alone, an operation which is prohibitive with a dock that requires from one to two months to self-dock. The design of this proposed dock as a weight-carrying structure is perfectly simple and definite and is the equivalent of a plate girder bridge with its floor beams. The longitudinal stresses are definitely carried by the side walls and the transverse stresses by the pontoons. In designing a floating dry dock it is frequently the case that some longitudinal stiffness is credited to the pontoon portion; as the most of the weight is received by the central bulkhead and as this is a girder in the general proportion of six hundred feet of length to twenty feet of depth, it is evident that without excessive weight in the central bulkhead its resistance to bending is slight, especially in the early stages of deflection. The entire absence of longitudinal stiffness in the original sectional docks that had no rigid connections whatever between the sections has left a prejudice against joints in a side wall. It is, however, perfectly evident that a joint can be designed for a side wall that will be stronger than the body of the side wall itself, and this with no great excess of material. Where a joint in a side wall must be opened every time a self-docking is undertaken, there is naturally a tendency to make the connections as few as possible consistent with safety. In the proposed design the side wall joints are not opened in regular self-docking and from that point of view might be omitted entirely. The joints are to permit the separation and extension of the dock when separation or extension may become desirable, and being for this purpose alone may readily be made stronger than the body of the side walls without in any manner retarding the self-docking process. By making the side wall joints at least as strong as the body of the side wall it makes no difference whether a joint comes at the center of the dock or else where during the docking of a ship. The importance of being able to expand a floating dock in dimensions, and especially in lifting power alone, is best illustrated by reference to the government floating docks at New Orleans and in the Philippines. The New Orleans dock is rated at fifteen thousand tons, and the Philippine dock at sixteen thousand tons capacity. Within six and twelve years of their completion both of these docks have become too small in lifting capacity for the modern ships of the navy. In length, width, and submergence capacity they might still be used, but their lifting power has been greatly exceeded by modern war vessels and there is no way of increasing it. In the proposed type the increase of lifting power by substituting deeper or longer pontoons is as simple and economical a way as could be devised, and the actual substitution would take no longer than a self-docking. The lifting capacity of a floating dry dock cannot be increased by placing low bulkheads across the ends and covering the openings in the deck to form a shallow pound to be pumped out, without straining the dock beyond what it was designed for. If the dock is designed originally for such a pound, it would not be an increase of lifting capacity. There are practical objections to such a pound, also, one of which would be the surging of the large unconfined body of water as the pound was coming above the surface. The Havana floating dry dock, later known as the Pensacola dock, was actually designed and constructed for a pound on the deck, and was called a combined floating and graving dry dock. Instead of bulkheads at the ends, which would have to be higher than the blocking to be of any use, it had low caissons that were to be seated as the dock came up. It was found in practice that the rush of water off the deck as the dock came up made it impossible to seat the caissons in their grooves and they were never used. Expansibility of a floating dock in dimensions is of more importance from a commercial point of view than from a military point. For a full return on the investment a commercial dock must be generally worked to nearly its full capacity. This means that it should not be designed for much beyond the average ship that visits its port. If, however, the dock is so designed that its dimensions can be increased, the gradual increase of the average ship is readily taken care of. It may also be possible that capital is available for a dock of small dimensions, but not for a large one; if the small dock is of an expansible type it may be increased in size with its own earnings. The maximum possible ship that may visit a port cannot be taken into account by the ordinary commercial dock unless the ship is a regular patron; two average expansible docks-of the same type, however, can be readily coupled up to handle such cases to advantage, while ordinarily the two docks are both in use for the average business. In an expansible dock it is an advantage to have each section fitted with its own pumping machinery. When so fitted the operating power is always correctly proportioned when the dock is either separated or expanded. Furthermore, it makes the power elastic and the chances of complete failure of operation from a break-down are greatly reduced. In this proposed type of self-docking steel floating dry dock, the peculiarities and deficiencies of preceding types have been considered and avoided as much as possible, and the object has been to secure the following results: A dock that can be continuously preserved with the least time, trouble, danger and expense. One that can be readily, quickly and economically expanded through long periods to meet seen and unforseen conditions. One that admits of separation before or after expansion. One that can be quickly and economically built in any location. One that can be towed from one locality to another without extraordinary preparations or facilities.