To comprehend thoroughly the safety with which this is accomplished, it is necessary to appreciate the almost insuperable difficulty of discovering an object like a submarine vessel when once sunk beneath the surface of the water. There are many sunken ships containing valuable treasures and cargoes that lie along our coast, and in most of the harbors of the world, that have been known to have sunk within a radius of less than a mile from some given point, but which have never been located. Some of these vessels have been searched for for years and never have been found. Dozens of vessels have been sunk in the waters of the North and East Rivers and never have been located. Some of the British and French submarines have been lost in localities well known, but it has been impossible to locate them.
During several years of experimental work with submarine investigating bottom conditions I have travelled many miles in the Chesapeake and Sandy Hook bays, along the Atlantic coast and Long Island Sound, and later in the Gulf of Finland and the Baltic Sea; and it is a fact that cannot be successfully disputed, technically, by any one, that a submarine of the type recommended by the United States Army Board may be taken into any harbor in the world entirely unseen, and remain there, if necessary, for a month at a time, destroying shipping, docks, and war craft deliberately and leisurely, and yet defy discovery.
My method of entering harbors or through mine fields consists principally in providing submarine vessels with bottom wheels and other component undisclosed details. When submerged, the vessel is given sufficient negative buoyancy so that she will not be drifted off her course by the currents when resting on the bottom. The vessel is what may be termed a submarine automobile, and it may be navigated over the bottom as readily as an automobile runs on the surface of the earth. The submarine automobile has one great advantage over the surface type in its ability to mount steep grades or go over obstructions, because the vessel is so nearly buoyant that she will mount any obstruction she can get her bow over.
My early experience proved to me that a submarine could not be satisfactorily navigated submerged in shallow, rough water by the same method of control as was found to be practical in deeper water, for the reason that the vessel would pump up and down with the rise and fall of the sea. Neither could the vessel lie at rest on the bottom, as the lift of the ground swell in bad weather was sufficient, even with a considerable negative buoyancy, to cause the vessel to pound so badly that the storage battery plates would be destroyed in a few minutes. I therefore suspended the wheels on swinging arms and applied a cushioning cylinder. The hull of the vessel was then free to move up and down, synchronizing with the lift of the ground swell, and at the same time the weight of the wheels kept the submarine close to the bottom and able to keep her position while at rest or to be navigated over the bottom at any speed desired.
Most of our Atlantic coast, Long Island, and Chesapeake Bay water-beds are comparatively uniform as to depths. In other countries I have navigated over rocky bottoms filled with giant boulders. A rough bottom limits the speed at which it is advisable to travel, but I have never seen a bottom so rough that it could not be readily navigated.
"Lake" boats, fitted with bottom wheels, have, in a competitive test abroad, entered landlocked and fortified harbors without discovery, where the entrance from the sea has been through a tortuous channel. All other vessels, except the one fitted with bottom wheels, were discovered long before reaching the outer fortifications, because it was necessary for them to show their periscopes to sight their way. They struck the sides of the dredged channel, which caused them to broach and be discovered, because they had to maintain a comparatively high speed to be kept under control. In tests carried out in Russia the boat fitted with bottom wheels simply wheeled along in the channel at slow speed and stopped and backed to change course at will. The revolutions of the bottom wheels gave the distance travelled, the manometer gave the depth, and the compass the proper direction; consequently, with a correct chart as to courses and depths, navigation on the bottom in entering harbors is very much easier than on the surface, unless the channels are well buoyed.
Most mines, as at present installed, are either of the observation or contact type; the observation mines are fired usually from shore stations when the enemy is seen to be over them, while the contact mine is anchored a few feet beneath the surface and is either exploded by contact with the surface of the vessel's bottom or by the agitation caused by the rush of water due to the swiftly passing vessel. The European belligerents have put out contact mines to protect their capital ships from the submarines. The dread of these mines is holding the submarines outside of the mined areas, and the mines are therefore effective. None of the British vessels are fitted with bottom wheels and diving compartments, and they must be navigated at such speed to keep submerged control that they would explode a contact mine if either the mine or its anchor rope were touched. This also applies to some of my later boats, as the bottom wheels have been omitted to meet the demand for greater speed on the surface and submerged.
I am inclined to the belief that this has been more or less of a mistake, because the bottom-wheeled submarine can go to and dig the enemy out of its base in addition to hunting the big surface craft of the enemy on the high seas.
With the bottom wheels, navigation can be conducted so carefully over the bottom that inspection of the course can be made, if desired, foot by foot, as progress is made, and all mines can be avoided.