So when the order is given, the diving rudders are set at only ½ a degree from the horizontal, as shown in [Fig. 28]. The craft must move through the water at about 5 knots, which is the proper diving speed. When the conning tower is one-fourth submerged the angle of the rudders is increased to 1¼ degrees.
This angle is held until the conning tower is half submerged; and then the angle is changed to 2 degrees, and it is held there until the conning tower is three-quarters submerged. As soon as this takes place, the angle is decreased to 1¼ degrees again, and the rudders remain at this angle until the dive is completed.
Why a Shallow Dive Is Made.—The reason such a shallow dive must be made is the result of having to let water into the ballast tanks to bring the craft to the awash condition before diving.
If the angle of the dive were to be suddenly increased to 10 or 15 degrees the tilting of the boat would throw all the water forward in the tanks and this would seriously upset her balance, and might even make her settle nose downward to the bottom of the sea.
How the Boat Is Kept Submerged.—When a dive is to be made the diving rudders are set at the angles just mentioned, but water is not let into the ballast tanks at the same time, for this would also tend to destroy the balance of the boat.
But when the captain wants to keep his submarine at a certain depth below the surface of the water after the dive is made he has water admitted to the ballast tanks and this keeps her at that level. When he wants to return to the surface, or “break water,” the water is pumped out and then the diving rudders are set and the boat makes an upward glide.
The Time It Takes for a Dive.—The time needed for a submarine to get ready to dive is about 2 minutes; but this is often long enough for it to become a target if a submarine chaser is on the little war-dog’s trail. Should she be hard pressed she might dive at a steeper angle—say, 5 degrees, but never more, and under ordinary conditions she will never dive at more than 2 degrees’ inclination.
The reason it takes time for a submarine to get ready to dive is because the wireless masts have to be folded in, the machine guns disappeared, and the hatches fastened down. Finally, it must not be forgotten that a submarine can dive only when she is pushing ahead under power. If she is at rest and the captain wants her to sink he must either start her engines or else be content to simply submerge her on an even keel.
The Ballast Pumps and What They Do.—The pumps which pump the water from the ballast tanks are driven by electric motors. They must be powerful pumps, for they not only have to pump the water out of the tanks quickly, but they have to force it out against the pressure of water in which the submarine is submerged; this pressure increases the deeper the boat sinks, and it is often 80 pounds or more to the square inch. The pumps are controlled from the conning tower, and also from the navigating compartment.