Then the sea-going, steam salvage-dock _Vulcan_ reached the scene and brought the _U-3_ to the surface in three hours.
“The _Vulcan_ is a double-hulled vessel, 230 feet in length with a lifting capacity of 500 tons. The width between the two hulls is sufficient to admit with good clearance the largest submarines. At a suitable height a shelf is formed along each wall of the interior opening, and upon this rests the removable floor of the dock. The two hulls of the ship are each built with water-tight compartments of large capacity, similar to those which are found in the side walls of the ordinary floating dock. When a sunken submarine is to be raised, the _Vulcan_ steams to the wreck and is moored securely in position above it. Spanning the well between the two hulls are two massive gantry cranes, each provided with heavy lifting tackle driven by electric motors. The first operation is to fill the compartments until the vessel has sunk to the required depth. The floor of the dock is then moved clear of the well. The lifting tackles are now lowered and made fast, either to chains which have been slung around the body of the submarine, or to two massive eyebolts which are permanently riveted into the submarine’s hull. At the order to hoist away, the submarine is lifted free from the mud and drawn up within the well, until its bottom is clear of the supporting shelves on the inner faces of the two hulls, above referred to. The dock floor is then placed in position on the shelves, the water is pumped out of the two hulls, and the _Vulcan_ rises, lifting the submarine and the dock floor clear of the water.”[18]
A similar vessel was built by the French government as a result of public indignation over the delay in raising the sunken _Pluviôse_. Great Britain has a salvage dock with a lifting capacity of 1000 tons. But the most remarkable craft of this kind belongs to Italy and was designed by the famous engineer Major Cesare Laurenti, technical director of the Fiat-San Giorgio works, builders of some of the world’s best submarines. She is a twin-hulled vessel, fitted not only to pick a sunken submarine from the sea bottom, but to care for it in every way, for she is also a floating dry-dock, capable of repairing two of the largest submarines, besides being a fully equipped mother-ship for a flotilla of six. With the ends of her central tunnel closed by a false stem and stern, and propelled by twin screws driven by powerful Diesel engines, she is a fast and seaworthy vessel, capable of keeping company with her flotilla on a surface cruise. She carries a sufficient armament of quick-firing guns to beat off a hostile destroyer. But the most noteworthy feature of the Laurenti dock is a long steel cylinder, capable of enduring great pressure from within, that is used to test the resisting strength of new submarines. A new boat, or a section of a proposed new type, is placed in this tube, which is filled with water that is then compressed by pumps, reproducing the effect of submergence to any desired depth.