The earliest submarines had a circular tower amidships which projected above the hull, glass lights around its circumference serving for the navigation of the vessel. The submarines "porpoised" to allow for frequent observations. Later an optical tube, fitted with inclined mirrors top and bottom, was added giving deeper immersion of the hull, if not better observa tion. The periscope was introduced about 1902. It consists of a tube with a window and a glass prism at the top, and an eye piece and prism at the bottom, various lenses being fitted in the tube to obtain the best image at the eyepiece. The periscope tube is about 6 inches diameter, with 5 or 6 feet of the upper portion reduced to 2 or 3 inches diameter, about 3 feet being above water when the periscope is fully raised, the submarine then being at "periscope depth." The field of view is restricted to about 15 degrees, but two handles allow the observer to ro tate it and sweep the horizon, the angle in azimuth being pro jected on the image. The length of the periscope has gradually increased, and 3o to 35 feet is now not unusual, the pressure hull of the vessel being 25 to 3o feet below the sea level. Two periscopes, one before and one abaft the conning tower, are fitted in the majority of submarines, but the largest types have three. The normal view is a horizontal one, but aircraft have necessitated an addition by which the top prism can be rotated, and an almost vertical view obtained.
Towards the middle of the 19th century designers turned to steam engines and screw propellers for both surface and submerged propulsion, but a funnel and an air duct extending above water when submerged had to be fitted. In 186o, Brun and Bourgeous in the "Plongeur" fitted an air engine, the compressed air for its operation being stored in large cylinders, the vessel being completely closed in when submerged. In 1863 the American, Alstitt, used steam for surface propulsion, and an electric motor and battery when submerged, the first in stance of the dual methods of propulsion. The "Gymnote" and the "Gustave Zede" were fitted with electric motors for both surface and submerged propulsion, and had to return to their base for recharging when the batteries were exhausted. The "Narval" had a petrol engine for surface, and an electric motor and battery for submerged navigation, and the "Holland" was similarly equipped. The internal combustion engine possesses marked advantages for the surface propulsion of submarines, and the evolution of the latter has closely followed that of the former. Petrol engines were replaced by Diesel engines when the latter had sufficiently developed (1905), the first British sub marine so fitted being "A.13" completed June, 1908. In the mod ern submarine twin screws are fitted, the propeller shafts being driven either by the Diesel engines, or by the electric motors placed abaft the engines. For submerged propulsion the motors drive the propellers. The motors are generally used for surface propulsion when navigating in narrow waters or coming alongside ships or quays, being more easily operated than the engines.
The Diesel engines fitted in submarines are of a special type running at from 38o to 450 revolutions per minute when develop ing full power. In 1914 the b.h.p. of the largest available subma rine Diesel unit was from Boo to goo in 6 to 8 cylinders. At
present the most powerful unit develops 3,00o b.h.p., but the diameter of hull for two such engines is such, that the size of the submarine is too large for ordinary naval purposes, and con sequently less powerful engines are being fitted, ranging from 1,200 to 2,400 b.h.p. per unit. The displacement of submarines has gradually increased with the power of the available engines. In 1914 the largest submarines had surface displacements of from 600 to Boo tons, it is now 2,800 tons in a few special cases, but in the majority of new vessels ranges from i,000 to 1,800 tons. The surface speed in 1914 varied from 15 to 18 knots, it is now from 17 to 20. With the 3,00o b.h.p. engine it is estimated that no more than 22 to 23 knots surface speed can be attained, the displacement reaching about 3,00o tons.
Steam was employed in some of the earlier classes of French submarines, and in the British "K" Class. In the former the Diesel engines were unreliable and had to be replaced by steam, whilst in the "K" Class the speed required (24 knots) was be yond the possibility of the Diesel engine. Turbines of io,000 b.h.p. were therefore installed, and an auxiliary Diesel engine driving a generator which supplied current to the main motors on the propelling shafts was also fitted for low speeds. The number of large openings for funnels and boiler air supply, the greater consumption of fuel per unit power with the consequent limited endurance, and the heat in the engines and boilers after submergence, are objections to steam and strong reasons for the preference for Diesel engines. At present the British "K.26" is the only steam propelled submarine in commission.
The underwater propulsion and navigation of a sub marine depend upon the batteries which supply current to the propelling, rudder and hydroplane motors. These batteries are of the lead pasted plate type, the details varying with the makers. In small submarines, two batteries of 55 to 6o cells are installed, and in the larger, two or three of iio to 120 cells each. For conven ience in replacement and transfer, the cell containers have standardised dimensions, those in British submarines being such that the cell weighs about 900 lb. The power for submerged propulsion varies with the cube of the speed, and the higher the latter the sooner the battery is exhausted, necessitating recharging by the engines on the surface. The cruising underwater speed is from 2 to 3 knots, the battery capacity being sufficient to main tain this for 3o hours, and also meeting the auxiliary requirements. At full speed the battery is exhausted after 1 to 1 4 hours. The capacity of a typical cell is such that a current of 2,300 amperes can be maintained for one hour or 225 for 20 hours when fully charged. These currents, with the appropriate voltages, corre spond to about 5.5 h.p. and .5 h.p. per cell. In the smaller types the cells stand on the crown of the ballast tanks, portable wooden beams and airtight covers closing the space, and in the larger a steel deck is fitted over the battery room, the cells at the sides standing at different levels to suit the section. When the battery is being charged hydrogen gas is evolved, and to obviate explo sions, exhaust fans and trunks discharge this into the open. A battery switchboard is fitted in or close to the control room.