In the bow on either side were balanced rudders on one and the same axle, always maintained in the horizontal position. The crew consisted of three men, and when the boat was closed up there was sufficient air to supply three men for six hours without causing discomfort, and this was not supplemented by any storage of compressed air or restorative chemicals. The depth below the surface at which the boat travelled could be varied in two ways; either by varying the speed of the vertical propellers, or by reducing the speed of the engines driving them by an automatic valve controlling the steam supply. On the surface the boat was driven by working the boiler in the usual manner, and the temperature of the water in the cisterns was kept up to a degree corresponding to a steam pressure of 150 lbs. When it was desired to descend, the ashpit and fire door were closed, as also the funnel inside the boat, and the vertical propellers were started. For sub-surface travelling there was available, as propelling power, the steam given off by the heated water (about eight tons), and this was found sufficient for a distance of 14 knots; on one occasion, when the boat was opened up, there was still over 20 lbs. pressure in the boiler.
Nordenfelt II.
Mr. Nordenfelt recognised that for the defence of open coasts and for operations where it might be necessary to keep the sea for days together without being able to seek the shelter of inlets or the mouths of rivers, other and larger proportions than those of his first 64 foot-boat would be desirable.
He accordingly constructed a boat on such larger lines, the details of which are as follows:—
Length 100 feet, beam 12 feet, displacement 160 tons, speed on measured mile 12 knots, distance travelled without re-coaling 900 miles, depth to which descent could safely be made, about 50 feet. Engines, surface condensing compound type, with two cylinders, and cranks at 90°, and at a pressure of 100 lbs. of steam indicating 250 h.p. Boiler, of the ordinary marine return tube type, having two furnaces; about 750 square feet of heating surface. Hot-water cistern, rhomboidal in body with spherical ends. Both boiler and cistern made for a working pressure of 150 lbs. per square inch. Armament, two fish torpedoes, 14 feet long, carried outside on the bow and discharged mechanically. Two Nordenfelt quick-firing machine guns consisting of 1–inch calibre. Sinking apparatus, two vertical propellers, driven by two engines, each indicating 6 h.p.; these propellers were placed in the fore and aft line. This was an improvement on the earlier boat whose screws were fitted in side sponsons. The mere arrest of these propellers sufficed to bring the boat to the surface, as it had a reserve buoyancy. Bow fins, whose action was both automatic and controllable, maintained the boat in the horizontal position. The main propeller was placed abaft the rudder. Two main cold-water cisterns placed at each end, and containing 15 tons of water each, also one in centre of boat for regulating buoyancy containing 7 tons; coal bunkers on the side of boiler; 8 tons of coal carried at the side of hot-water cistern and in middle of boat. Crew, three men in a watch: two watches carried. With coal in the bunkers only, this boat could keep the sea for five days or more. No attempt was made to purify the air when submerged. When descending, the boat was perfectly horizontal, and was invariably kept so when moving under water by means of the bow rudders operated by a plumb weight.
“NORDENFELT II.” RUNNING AWASH.
Nordenfelt II. had two distinct conditions of existence as a torpedo craft—that of a surface boat and a submarine one. The sinking operations were as follows: the furnaces were hermetically closed, upon which combustion was soon brought to an end. The piece of funnel connecting the boiler with the outward portion was then removed and the doors placed in position. Whilst these changes were being effected, water was allowed to run into the ballast tanks to reduce the buoyancy to its proper limit, and this arrived at, nothing remained but to close up the conning tower and to set in motion the vertically acting screws to place the boat quite out of sight.
In a paper which he read before the Royal United Service Institution, on February 5, 1886, Major-General Sir Andrew Clarke in the chair, Mr. Nordenfelt after mentioning previous under-water vessels, gave his views as to the reason of their failure.
First of all he said they were always built too small and too weak. The longest was 45 feet, and their small dimensions and weak plates made them useless in bad weather and dangerous for submersion; the small air space available forced the crew to use chemical means to obtain pure air. Secondly, they were never made for firing a fish torpedo; consequently they had to endeavour to fix a mine to the bottom of a vessel, a feat which Mr. Nordenfelt considered impracticable, owing to the risk of contact with the vessel, which, especially if it were pitching or moving, might easily destroy the boat. Thirdly, in all the early boats, the mines were charged with only black powder, the effect of which was less destructive than that of the gun-cotton or dynamite in the fish torpedoes. The effect of the explosion, again, against a wooden ship, was nothing like as serious as against the thin bottom plates of an ironclad. Fourthly, all the boats hitherto in use were propelled by hand power; this gave too much hard work to the crew, who could not take the boat any distance on the surface previous to the actual attack, and made it quite impossible for it to face any rough weather. In the Nordenfelt boat the use of steam diminished the number of men, and they had so little to do when below the surface that the temperature, lower than in modern stokeholes, was no detriment. Fifthly, all previous boats had most unreliable means of descending and ascending. The descent by steering downwards in the American boats of the Civil War period was quite as dangerous as the attempts before and after that time to lower and raise the boats and to keep them steady at any desired depth, by means of increasing and decreasing the weight of the boats by more or less water-ballast or by altering their displacement.