Chapter I

In 1916 we began to look to Germany to produce something very unpleasant in the way of submarines. We were certain she would follow the obvious course indicated by the lessons all belligerents were learning, and produce the big U-cruiser. Very fortunately for us, she produced nothing of the sort until well into 1918, when one small U-cruiser did us a great deal of damage. The point was this--We were worrying and chasing U-boats with trawlers, motor-boats, destroyers, and numbers of other comparatively small and weakly-armed craft. If a U-cruiser, armed with, say, four 6-inch guns, and armoured along her top-strakes, had risen to fight her tormentors--well, it is clear that our small patrol-vessel service would have become suddenly very expensive. Each convoy would have required cruiser protection, and we had not enough cruisers to provide this. In 1917, by our constructor's reckonings, there was no reason why a German submarine could not have been produced which could proceed safely to the East Indies (round the Cape), and repeat (on a bigger scale) the exploits of the _Emden_. Well, the Germans didn't do it; they produced U-cruisers with two 5·9" guns apiece in 1918, but the type was unsatisfactory and unstable. It is still a puzzle to us that the idea came to them so slowly. We had K-class boats with the fleet in 1916 of 2600 tons, and had shown that a big submarine was a working proposition. (The K boat, of course, is not a cruiser-submarine; she is a fast and lightly-gunned type for use in battle only, and she does not leave the fleet except when detached for watching patrols.) We produced the M-class in 1917, and--for obvious reasons--we kept the type as secret as possible until the Armistice. The M boat is rather smaller than the K, and is of only seventeen knots speed, but she has far better under-water capabilities than her big companion. She carries, besides her torpedo armament, a 12-inch gun of the normal battleship type. This gun can be carried loaded submerged by the use of a watertight tampion and breech. The boat can rise in the wake of an enemy, fire as she "breaks surface," and submerge again--all in a matter of seconds.

The type was extremely successful, and one can only be thankful that such boats were not on the enemy's side. They would have been the very devil to deal with on the trade routes, and would have caused us to reconsider very hurriedly our whole system of anti-submarine defence. Of course, four 6-inch would be better than one 12-inch from a German point of view, especially as a destroyer would be likely to attack unscathed under the fire of one big gun, but our type was intended for use against such things as enemy cruisers, and not for sinking merchant ships. By the end of the war the enemy had arrived at the stage of submarine design where one says, "We've got a type that works--let us stick to it, and just add improvements." We passed that stage before the war and are now in the confident (with reservations!) state of feeling that we can turn out anything required to combine the properties of under-water and surface craft. If submarines continue as weapons of war, they will improve very considerably, and the range of possibility of future types is so great that any prophecy now would be rash. It must be remembered, however, that the depth of water in which a boat is intended to operate limits her size. It is not only that the distance from her keel to periscope is (in the case of a big boat) some 50 feet, but also that the great length of a big boat's hull means that even a slight fore-and-aft inclination as she dives will add enormously to her draught; a very long boat in twenty fathoms of water (the North Sea average) would have to be careful not to get off an even keel, as it might happen that in the presence of the enemy her bow or stern would touch bottom, with the result of causing the whole boat to bounce up to the surface. A submarine submerged is in a state of equilibrium, in that she has little or no tendency to rise or sink if her motors are stopped and the boat left to herself. I am afraid in this history that I continue to speak of submarines as if everybody knew a good deal about them. I use technical expressions and words as if I was dealing with things like motor-cars. I will try and explain more clearly what I mean by "bouncing to the surface," and will do so in the idea that there are probably readers who know as much about diving boats as I do of bimetallism.

A submarine is a surface ship which can be submerged and driven ahead at a steady depth-line. She is built strongly in order to resist the water-pressure when she is deep down. She is propelled on the surface by (usually) heavy-oil Diesel engines. When submerged she cannot use these, as they consume air, so she has an electric battery and motors for use under water. The battery is charged when on the surface at her convenience or by favour of the enemy. The Diesel engines are used for this purpose, and they recharge the batteries through the motors, using the latter as dynamos.

In old pictures of projected submarines of the seventeenth century one sees that the principle of "trimming down" for submersion was known, and that our present-day system of tanks was intended to be substituted by pig-skins which fitted into cylindrical hollows in the hull. These skins were emptied by screwing out a ram from inside (on the idea of old-fashioned printing presses) which squashed the skins. Nowadays a boat is all tanks along her lower half, the upper half being living space and battery room, etc. These tanks are flooded through valves in order to destroy the boat's buoyancy. The water is ejected from them either by pumps or by the use of compressed air, the latter taking the place of the old seventeenth-century screw-press idea. When the tanks have flooded until the boat's buoyancy is all gone--_i.e._, until you could press her down or lift her up with one hand--she is "trimmed," and by going ahead and working the bow and stern hydroplanes you can keep whatever depth-line is required. When submerged, a look-out is kept through a periscope--a tube about thirty feet long which has lenses all the way up, is watertight, and has an eyepiece like an ordinary telescope at its lower end.

To dive, a boat opens her vents, puts "dive" helm on, and goes under with her motors running. The flooding valves are kept open to save time; in surface trim a boat is "hanging on the vents"--_i.e._, if you open the vents (upper valves of the tanks) the water enters and she goes down; until the vents are open the water cannot enter beyond a certain point; when they do open the air in the tanks can get away and the tanks fill up with a rush.

During a trip the "trim" of the boat alters continually. She is using fuel, ammunition, food, and water, and calculation is necessary to allow for this. Certain tanks are used for this compensating, so that on all occasions when a rapid dive is necessary, there is nothing to do but flood the big external tanks, and yet know that the boat will be in hand when under. If mistakes are made, they will show at once. If too much has been put into the "internals" to compensate, the boat will run on down to the bottom in spite of "up helm" and full speed. If too little, you have to flood internals according to an estimate of what is needed as she ploughs along half-submerged; the latter case is one to be avoided, as you may be killed by the enemy while flooding. The usual war practice is to compensate on the "heavy" side--_i.e._, let her go with a rush and blow tanks so as to catch her and hold her at sixty feet; then you can bring her up to patrol depth at your leisure.

It can be seen, then, that the description of a boat as "bouncing" is not incorrect. When going to the bottom for the night it is a common occurrence, if too rough a "landing" is made, to proceed like a tennis-ball along the sand for a couple of hundred yards. It is a curious thing that both in the Cattegat and the Sea of Marmora, boats have been able to lie for the night with motors stopped at depths of from thirty to seventy feet. In the Marmora the junction depth of the salt and fresh water is about seventy. A boat trimmed with about two hundred pounds of negative buoyancy out there will, if she stops her motors, sink slowly through the upper layer of fresh (or brackish) water, till she meets the denser salt below; on reaching this she will be in a state of "positive" buoyancy, and after a little bouncing up and down to find her "zero" stratum, she will settle at a steady depth. The same sort of thing--a blessing in the Marmora--is a nuisance in the Bight. A boat crossing the mouths of the German rivers may be at one moment diving comfortably with zero helm on the hydroplanes--the next, she meets a layer of fresh water from the Jahde ebb and is bumping on the bottom with "hard-up" helm and the pumps working on the tanks.