The following information was collected by Leon Karff, a German spy, and was luckily intercepted by Raymond and Jacox who were working together to isolate and disrupt the workings of the German spy agencies.
"Report by Leon Karff, late foreman-fitter at Kiel Dockyard, on Submarine 'F 2,' now building in Shed No. 4, Portsmouth Dockyard.
"This boat would appear to me to be of about 700 tons displacement when complete, possibly rather over. She is, as far as I am able to measure, about 180 feet long with an extreme beam a little forward of amidships of 20 feet. She is fitted with three propeller shafts with three small four-bladed propellers on each. As she is provided with what appear to me to be some kind of turbine engines, I imagine that the centre shaft is for going astern only. The propellers on this shaft seem to be attached in such a way that they could be 'feathered' by suitable gearing on board so as not to retard the vessel's way when going ahead. The engines of this boat are of a type which I have never before seen. I imagine that they are a combination of the new 'gas-producer' engine and the turbine system, the explosion of the combined gas and air being split up and passing into the turbine through a number of different channels simultaneously. This would be a very economical system if the necessary power can be obtained, and would be much safer for use below than petrol engines.
"The boat is evidently intended to operate a good deal in an 'awash' position, for there is fairly thick armour-plating over the greater part of the upper side of the bow, while the fore end of the superstructure is made of two 6-inch Krupp steel plates meeting at an acute angle, and so forming a kind of stem when the boat is moving in this way. The space enclosed between these two plates is evidently intended to be used as the conning-tower. Here there are a periscope, steering-wheel, voice-tubes, and everything necessary for the control of the vessel. There are two horizontal propellers or fans, which seem to be driven by electricity derived from an installation of accumulators, and which are certainly intended to secure horizontal immersion, so the vessel will not plunge or dive, but immerse herself horizontally by means of these propellers, which, by the way, work in vertical shafts running completely through the boat, one forward and the other aft, as was the case in the Nordenfeldt, Waddington, and other early submarines.
"Forward there is an air-lock and diving-chamber, as in the 'Lake' boats, so that divers can get in and out of the vessel whilst under water. It would also afford a means of escape for the crew in the case of accident. This is further provided for by a detachable boat or caisson at the after end of the superstructure capable of holding ten men, I should say, or possibly a dozen. There are also appliances which I suppose are telephone buoys for communicating with the surface. There are six torpedo tubes fitted, one forward, one aft, and the others two on either broadside. And there seems to be provision for six other torpedoes of the 18-inch type.
"There is a long rudder for ordinary steering, and four horizontal ones or planes which are placed abreast the horizontal screws and which, I imagine, act automatically in conjunction with them, as they seem to gear up with the shafts for these propellers. There is a big safety detachable weight which fits loosely into a recess amidships, and four broad wheels with ball bearings which do not fold up as in the 'Lake' boats, but always protrude nearly half their diameter. After all they would not obstruct her way when water-borne more than a keel—or very little more. They are quite independent and unconnected with the interior of the vessel, which while resting on them would receive forward impetus from her propellers. In the 'awash' position she would offer a very small and almost invulnerable target."
Chapter 2 The Secret of the Silent Submarine, Spies of the Kaiser-Plotting the Downfall of England, William Le Queux