It is estimated that between thirty and forty submarines operate at a time in the waters surrounding the British Islands and French Coast. At least one is now known to be on White Sea trade lanes.
29. Best anti-submarine weapons:
One of the most efficient weapons now used by all destroyers and patrol craft against submarines is the so-called "Depth Charge," sample and drawings of which have been forwarded by our Naval Attaché. These are merely explosive charges designed to explode at a certain depth, formerly eighty feet, now about one hundred feet. They are dropped overboard where a submarine that has submerged is assumed to be and are counted upon to badly shake up and demoralize if they do not actually cause serious damage.
Howitzers and Bomb-throwers of large calibre are under construction, designed to throw similar depth charges to distances of about 2,000 yards. Details will be forwarded.
30. Torpedo Protection:
This subject may be summed up by the statement of the Captain of a British Dreadnought who said in effect that after a year's experience he did not fear being sunk by a torpedo. Unless struck by several the worst to be anticipated is damage to shafts or rudder, thus necessitating towing. Cruisers have often been struck and been able to reach port. Vital water-tight doors are kept continuously closed at sea.
Destroyer officers have been heard to express the curious opinion that the enemy ships were more or less unsinkable. This is probably to be explained by the fact that they carry very few supplies; that they have their storage spaces compartmented or filled with wood or other water-excluding material; and that when in port, they quarter their crews in barracks, and when leaving for a cruise carry the minimum amount of berthing and supply facilities. These points, however, are not positively known.
On the contrary, all vessels of the British Fleet must be kept fully supplied and fuelled at all times for extended cruising. This is particularly true of Battle-cruisers and Cruisers.
31. All officers of rank and actual experience consulted are convinced that the enemy have no unusual methods of protection, or in fact any "surprises" in ordnance or other fighting equipment.
32. All are agreed that the best protection against torpedoes is SPEED and ZIGZAGGING.