The latest type of pumping machinery is the electrically driven submersible. It has been tried under all conditions and found more efficient than any of the older types. It can be placed aboard in boats and driven from the dynamos of the salvage ship lying off the wreck. There is no necessity to secure it, as it works tically without vibration and can be used from the derricks and lowered down into the water. Its great advantage is that while the ordinary salvage pump has only a lifting capacity of Soft., the submersible is able to raise the water to a height of 8oft.; no priming is necessary, and no particular attention need be paid to the discharge hose as far as making it air and watertight. Former electric salvage pumps that were tried were fitted with watertight cases, and proved entirely for this particular kind of work. In the submersible pump, water is allowed free access to the electrical parts.
Lifting Operations.—With this somewhat brief description of salvage plant used the particulars of actual salvage operations will be explained, commencing with the raising of sunken vessels from deep water by means of lifting barges dependent on the rise and fall of tide.
The method of procedure is to place lifting vessels of sufficient buoyancy over the wreck and pass a number of gin. steel wire ropes under the vessel, bringing the ends of the wires up and making them fast to the lifting ships at low water; then as the tide rises. if the calculations have been correct, the vessel is raised from the bottom the height of the rise of tide, and carried into shallower water where the wreck is grounded, and the opera tions are continued each tide until her decks are above water (see figs. I and 2). The fractures are then patched by the divers, salvage pumps installed and the vessel pumped out and floated. The dimensions of lifting vessels that have been em ployed in a large number of cases are : length 165ft.; breadth 36ft., and each with a lifting capacity of 1,50o tons maximum.
One of the principal difficulties with this method is the placing of the heavy wire cables under the bottom of the wreck, especially if she has sunk in sand, mud or clay. If not too deeply embedded this is generally accomplished by towing them under with two vessel§, one on each side, and the ends of the cables attached to them. Where this is not found to be possible, owing to the wreck having become too deeply embedded, a system of hydraulic boring at high pressure is used.
In cases where there is little or no rise of tide submersible pontoons are used. These are really steel cylinders with a lifting capacity ranging from so tons each to 25o tons or more. They are divided into three water-tight compartments and fitted with low pressure air connections for expelling the water, and with high pressure air for opening the valves. The ends of the cylinders used at the lifting of H.M.S. "Gladiator" were dome-shaped, and covered with collision mats to protect them against chafing. Strong channel bars were rivetted around the pontoons at intervals to carry the wire cables, and the whole of the cylinders between the channel bars were covered with Sin. fir planking for protec tion against obstructions when lifting. Double 6in. special extra
flexible wire ropes were fitted in the required position around the pontoons, and to these were attached plate shackles to which the gin. lifting wires were fastened. In this case the pontoons were used to make the vessel upright, as she lay over on her beam ends at an angle of 113 degrees.
The gin. wire cables were swept under the wreck and the out side ends attached to the pontoons. These were filled with water and sunk alongside the ship; the other ends of the wires were then hove in as tightly as possible and made fast to massive steel bollards fastened to the upper side of the ship. The cylinders or pontoons were then emptied of water by means of compressed air and the vessel uprighted, assistance being also given by com pressed air and pumping from some of the sound compartments. In this case pontoons were only used on one side, but in order to raise a vessel that is sunk and lying upright on the bottom two sets of cylinders are necessary, and the plan of lifting is as f ol lows : A sufficient number of gin. wire cables are placed under the ship, the pontoons are attached to the ends of the wires on one side of the ship and filled with water, and by heaving in on the opposite cables they are hauled down into the required positions alongside the wreck. A special form of hydraulic gripping ma chines are attached to the pontoons on the other side of the wreck and the wire cables rove through these hydraulic grips. When everything is ready the pontoons are filled and allowed to slide down the wire cables to the bottom. The grips are then closed from the surface vessel and compressed air pumped down to all the pontoons, which expels the water, and if sufficient buoyancy has been given by the pontoons the vessel is brought to the surface.
In the method of lifting by compressed air all openings are closed by the divers, and compressed air pumped down to con nections made to all the different compartments from air com pressors of the salvage ships. The water is then expelled through the fractures and the vessel rises to the surface. This method entails a very large amount of diving work, as funnels, etc., have to be cut away and the openings closed and made air-tight. Decks have to be strongly shored up and supported.
The conditions of stranding are very varied. Assuming a large vessel stranded on a rocky bottom on an exposed coast and sustaining such damage that all compartments are filled with water and the vessel appears at first sight to be a hopeless wreck, a diver's survey of the position is made as soon as possible, and at the same time a careful survey is made of all compartments to ascertain whether the water falls in each at the same rate as the tide. Some compartments will generally be found to be hold ing water, that is to say the tide does not ebb and flow in them as it does outside. The amount of pumping plant required can thus be easily gauged for compartments under these conditions. When the tide ebbs and flows as it does outside it shows con siderable damage to the outer bottom, which necessitates extra pumping plant and the patching of the fractures by divers.
(F. Y.)