Another application of hydraulics is the operation of submarines. Inside a submarine, between the outer skin and the pressure hull, are several tanks of various design and purpose. These tanks control the total weight of the ship, allowing it to submerge or surface. They also control the trim or balance, fore and aft, of the submarine. The main ballast tanks have the primary function of either destroying or restoring positive buoyancy to the submarine. Allowing air to escape through hydraulically operated vents at the top of the tanks lets seawater enter through the flood ports at the bottom to replace the air. For the sub to regain positive buoyancy, the tanks are "blown" free of seawater with

compressed air. Sufficient air is left trapped in the tanks to prevent the seawater from reentering.

We use other tanks, such as variable ballast tanks and special ballast tanks (for example, the negative tank, safety tank, and bow buoyancy tank), either for controlling trim or stability or for emergency weight-compensating purposes. The variable ballast tanks have no direct connection to the sea. Therefore, we must pump water into or out of them. The negative tank and the safety tank can open to the sea through large flood valves. These valves, as well as the vent valves for the main ballast tanks and those for the safety and negative tanks, are all hydraulically operated.

The vents and flood valves are outside the pressure hull, so some means of remote control is needed to open and close them from within the submarine. We use hydraulic pumps, lines, and rams for this purpose. Oil pumped through tubing running through the pressure hull actuates the valves operating mechanisms by exerting pressure on and moving a piston in a hydraulic cylinder. Operating the valves by a hydraulic system from a control room is easier and simpler than doing so by a mechanical system of gears, shafts, and levers. The hydraulic lines can be readily led around corners and obstructions, and a minimum of moving parts is required.

Figure 10-13 is a schematic sketch of the safety tank-one of the special ballast tanks in a submarine. The main vent and the flood valves of this tank operate

hydraulically by remote control, although in an emergency they may operate manually.

Hydraulics are used in many other ways aboard submarines. They are used to raise and lower the periscope. The submarines are steered and the bow and stern planes are controlled by hydraulic systems. The windlass and capstan system, used in mooring the submarine, is hydraulically operated. You will find many more applications of hydraulics aboard the submarine.

In some hydraulic systems, oil is kept under pressure in a container known as an accumulator. As shown in figure 11-14, the accumulator is a large cylinder; oil is pumped into it from the top. A free piston divides the cylinder into two parts. Compressed air is forced into the cylinder below the piston at a pressure of 600 psi. Oil is then forced into it on top of the piston. As the pressure above it increases, the piston is forced down, squeezing the air into a smaller space. Air is elastic; you can compress it under pressure, and it will expand as soon as the pressure is reduced. When oil pressure is reduced, large quantities of oil under working pressure are instantly available to operate hydraulic rams or motors any place on the submarine.

The Navy uses many devices whose operation depends on the hydrostatic principle. You should remember three points about the operation of these devices:

Pressure in a liquid is exerted equally in all directions.

Hydrostatic pressure refers to pressure at any depth in a liquid that is not flowing.

Pressure depends upon both depth and density.

The working principle of all hydraulic mechanisms is simple enough. Whenever you find an application that seems hard to understand, keep these points in mind:

    Liquids are incompressible. They cannot be squeezed into spaces smaller than they originally occupied.

    A force applied on any area of a confined liquid transmits equally to every part of that liquid.

    In hydraulic cylinders, the relation between the force exerted by the large piston to the force applied on the smaller piston is the same as the relationship between the area of the larger piston and the area of the smaller piston.

    Some of the advantages of hydraulic machines are:

    We use tubing to transmit forces, and tubing can readily transmit forces around corners.

        Tubing requires little space.

        Few moving parts are required.