Holed below the water-line, a wreck must be patched before it can be salvaged. How to fix the patch most expeditiously over that yawning hole? Engineering ingenuity indicated how this task could be accomplished under the sea, and also produced the means. This wartime device, no longer secret, consists of a “gun” whose charge of explosive is powerful enough to drive inch-and-a-half steel nails through two half-inch steel plates so that these are held together in the closest possible contact, almost as effectively as though riveted in the ordinary way.
The salvaging of ships sunk in Antwerp approaches has been expedited in this manner. A steel plate patch with sufficiently large overlap is lowered into position and held temporarily to the ship's hull, sometimes by means of a magnet. Around the edge of the plate the nail guns are fixed at intervals, and when the explosive charge is touched off the skittle-shaped nails are driven home violently through the plate and into the vessel's hull.
The great heat which is generated at the moment of impact serves to reinforce the join between the plate-edges and the hull, and there can be no question of the patch shifting or developing a leak. Owing to its peculiar nature the nail gun is essentially an underwater tool, but peacetime requirements may lead to even more useful developments in the manner and nature of its employment.