A submarine of the first kind seldom has rough weather to contend with, and so she need not be built as strong as one that is designed for ocean going. She also has a slightly different shape from the latter, in that she has a round cross-section, as shown in [Fig. 23].
FIG. 23. CROSS-SECTION OF COAST-GOING SUBMARINE.
By looking at [Fig. 24] you will see that the ocean-going submarine is more nearly half-round; this tends to prevent her from rolling unduly. She also has a double hull which is divided into watertight compartments to protect her from sinking should an enemy ship ram or fire on her. The compartments are used as stowage tanks for fuel, etc., and so the space between the inner and outer skins, as the hulls are called, is not wasted.
A large number of fittings are fixed to the hull, such as the diving and steering rudders (all of which will be described later), while the superstructure is built on top of it.
What the Superstructure Is.—The superstructure consists of (1) the deck and (2) the conning tower. In the Holland submarine the deck is only about one-third as long as the boat and this allows it to dive easily.
FIG. 24. CROSS-SECTION OF OCEAN-GOING SUBMARINE.
The Lake boat is decked over nearly the whole length of the hull. In it are fitted rapid-fire guns which disappear, and there are watertight ventilators which let in fresh air when the boat is traveling on the surface. Further there is a hatch—that is, an opening in the deck with a watertight door—through which the torpedoes can be lowered into the hold of the submarine.