The first problem, then, which confronts the designer of a submarine boat is to find the most suitable method of sinking it to the depth at which it is intended to navigate.
The most fundamental law of hydrostatics, which applies to all floating bodies, and is equally true of wholly submerged vessels floating at any depth, as of ships of ordinary form, floating on the surface, having only a portion of their volume immersed, is that a ship floating freely and at rest in still water must displace a volume of water having a weight equal to her own weight.
The “displacement” of a vessel is defined as the weight of water displaced, which is equal to the weight of the vessel and that of her lading. A ship floating on the surface “displaces” a certain weight of water; in order to force her beneath the surface two methods are open.
In the first place, her weight is increased by the introduction of water ballast; thus her “displacement” is altered and she sinks until her weight is again equal to the volume of water displaced.
In the second, the weight of the boat remains constant, but the displacement is altered by the drawing in of “cylinders” or “drums;” thus she sinks until her displacement again equals her weight.
The first inventor to employ the latter method was André Constantin, who built a vessel during the siege of Paris, which was furnished with pistons working in two cylinders; on these being drawn in from the interior the boat sank to the required depth. The actual trials were, however, not satisfactory. The Nautilus, of Messrs. Campbell & Ash, which underwent some trials in Tilbury Docks in 1888, depended also on the pulling in of cylinders (ten were employed, five on each side of the vessel), for her submersion; the results were equally discouraging, and some eminent men nearly lost their lives owing to the erratic behaviour of this craft.
THE SUBMARINE OF ANDRÉ CONSTANTIN.
(1874.)
No serious ship-constructor would nowadays think of adopting this method of submersion, and we may therefore pass on to consider those which are brought to the submerged condition by the admission of water into special reservoirs or tanks.
Submarine boats so far as their immersion is concerned may be divided into two classes.