METHOD OF CONTROL IN DIVING TYPE BOATS
Horizontal rudder set down aft inclines the vessel down by the bow, in which condition, with only a small reserve of buoyancy, she will "dive." When she reaches the desired depth a lesser inclination of the diving rudder is supposed to reduce her angle of inclination sufficiently so that the pressure on the top of her hull will offset the tendency to rise due to her positive buoyancy. To be successful there must be no movable ballast, and variable stream line effect requires expert manipulation of the diving rudder.
Depth Control.—Practically all modern submarines use hydroplanes with a horizontal rudder for the control of depth when under way. Hydroplanes might be said to correspond to the side fins of a fish. They are substantially flat vanes that extend from either side of the vessel. They are set on shafts that may be partially rotated by mechanism in control of a man within the vessel. They readily control the depth of the vessel with a certain amount of either positive or negative buoyancy. For instance, submarines are usually submerged with a small amount of positive buoyancy. If a vessel has positive buoyancy she will float. We have seen that in a surface condition the five-hundred-ton submarine has about one hundred and twenty-five tons of positive buoyancy.
METHOD OF CONTROLLING HYDROPLANE BOATS
Showing a proper arrangement of hydroplanes and horizontal rudders. C B represents the centre of buoyancy of the vessel when submerged. G represents centre of gravity, which lies directly beneath centre of buoyancy. Now if hydroplanes are located at equal distances fore and aft their up or down pull is always balanced and does not cause the vessel to dive or broach, but holds her to a level keel. If stream line pull tends to upset this level keel, horizontal rudders may be used to correct it.
Now to prepare the vessel for a submerged run, we admit, say, one hundred and twenty-four tons of water; the positive buoyancy is then reduced to one ton. Now if the forward edges of the hydroplanes are inclined downward (see diagram), and the vessel is given headway, the pressure of the water on top of the inclined hydroplanes, combined with the tendency for a vacuum to form under the planes, will overcome the one ton of positive buoyancy and will pull the vessel bodily under the water. When the desired depth is reached the operator sets the inclination of the hydroplanes so as to just balance the upward pull of the one ton of positive buoyancy, and the vessel proceeds at the desired depth. On modern boats the control of depth is most remarkable; it is very common for submarines to make continuous runs of several hours' duration without varying their depth more than a couple of feet. When the headway or motive force of the submarine is stopped, if she has reserved some positive buoyancy she will come to the surface. If she has negative buoyancy she will sink, but while under way with as much as a ton of positive or negative buoyancy the hydroplanes will absolutely control the depth of the vessel.
HOW HYDROPLANES CONTROL DEPTH OF SUBMERSION
The vessel being "under way" in the course of the arrow, the water contacting against the upper surface of the hydroplanes, as in the upper view, its course is thus diverted and adds weight to the upper surface of the planes. There is also a tendency to form a vacuum under the plane. Both these forces tend to overcome the positive buoyancy of the boat and force her under water and on a level keel if these forces are properly distributed fore and aft of the centre of buoyancy and gravity of the vessel.