British Official Photograph
It often happened, however, that neither one nor the other of these things occurred, but that the submarine's delicate electrical machinery was thrown out of order by the violence of a depth-charge explosion, even when a considerable distance away. With the electric engines used for submerged propulsion no longer available, and possibly the interior of the vessel in darkness, there were only two courses open. She could either rise to the surface and endeavour to fight it out with the aid of her powerful deck guns, or else sink to the bottom and trust to luck that other depth charges would not be dropped close enough to seriously damage her hull. In the open sea, however, the latter chance was denied because of the depth of water. Three hundred feet may be taken as the greatest depth to which an ordinarily constructed fighting submarine can safely descend without running a grave risk of having her plates crushed in by the great water pressure. Even at this depth the weight on every square foot of hull surface exceeds 8¾ tons.
If the damaged submarine rose to the surface the guns of her pursuers were ready and could generally be relied upon to place her at least hors de combat before the hatches of the under-water vessel could be opened and her own guns brought into action.
Fig. 9.—Diagram showing how depth charges are carried on the stern of a motor launch. AA. Depth charges, each containing 300 lb. of high explosive. B. Hydrostatic device by means of which the charge can be made to explode when it has sunk from the surface to a depth of 40 or 80 feet, and by which it is rendered comparatively safe while on deck. C. Slings holding charges in place on inclined platform. D. Greased bolts which, on being pulled out, allow wire slings to fly free and depth charge to roll into the sea. Depth charges can only be released from vessels under way, otherwise the explosion which occurs a few seconds after release damages surface vessel.
In shallow water where there was a fairly smooth bottom it generally happened that a submarine damaged by depth charges elected to sink to the sea-bed and trust to luck. This was also frequently resorted to as a means of eluding pursuit even when the U-boat was not damaged by the first few charges dropped. It was then that the hydrophones carried by the surface ships were again brought into use to ascertain if the submarine was still under way. When no sound was heard those on the surface knew that "Fritz" was lying doggo, or else that he had escaped. If a number of ships were available a few waited over the spot where it was considered the U-boat was lying, while the others scoured the surrounding seas in circles trying to pick up the sound of the runaway's engines if she had escaped in the mêlée. When nothing further was heard they returned to the scene and set about the work of systematically bombing the surrounding sea-bed.
Fig. 10.—Diagram illustrating a depth charge attack on a submerged submarine. A. Motor launch, which has dropped a depth charge to destroy a submarine B travelling at a depth of 90 feet below the surface. C is the depth charge sinking as the M.L. steams away from the danger area. D is the point (80 feet below the surface) at which it will explode, and E indicates the danger area for the submarine B.
As many as one hundred depth charges were dropped in quite a small area of sea and yet a submarine known to have been lying "doggo" in the locality was not damaged. In cases such as this other means, which will be described in a succeeding chapter, were then resorted to.