542 Edward V. Lewis and John P. Breslin 
Design Problems 
It should be noted that the control of a semisubmarine involves more than a problem of 
stability in the usual sense. Suppose the heading of a stable ship is disturbed downward; 
if the craft is stable, it will steady down on a new course that will take it deeper and deeper. 
Similarly, if it is displaced upward, it will steady down on a new course upward so that it 
will eventually break surface. The ship should have a tendency to come back of its own 
accord to the original course and depth. This quality is here referred to as depth stability. 
It will be shown later that this can best be achieved by means of control surfaces with auto- 
matic control, which can keep the ship running level. This problem appears to be roughly 
the same difficulty as the control of completely submerged hydrofoils on a hydrofoil boat. It 
will be discussed in more detail in the next section. 
If there is a large surface-piercing fin, or a smaller fin near each end of the craft, it may 
be expected that the increase or decrease of buoyancy associated with changes in immersion 
would assist in the depth control of the body. However, with a single minimum fin, the buoy- 
ancy effect appears to be small and reliance must be placed on dynamic control. A small 
amount of positive overall buoyancy would be desirable in the submerged condition for safety 
in case of power or control failure. 
The next problem, after the consideration of maintaining a level course under a calm 
sea, is the behavior of the ship in a rough sea. Here we find that the head-sea case is no 
problem. The runs on the analog computer indicated that this particular design had a pitch- 
ing period of about 20 seconds, or at least when it was disturbed it oscillated in a period of 
about 20 seconds, which is a sort of dynamic, natural pitching period. It signifies a very 
low response to outside disturbances, so that the high frequencies encountered in head seas 
will not cause motions of any consequence in the craft. However, in following seas we may 
expect that it will run into the critical zones of Fig. 1 of Ref. 7. The problem of providing 
vertical control so that the ship will run level or approximately so will be discussed further 
in the next section. In long waves it may be desirable for the ship to contour the surface, 
moving up and down, just as a hydrofoil craft must do. 
Another problem to be investigated is that of lateral control and turning. A fore and aft 
location for the strut must be determined which, in combination with other fins, will provide 
satisfactory directional stability in a horizontal plane and at the same time will not lead to 
serious heeling in a turn. This problem can easily be investigated by the use of conven- 
tional model tests on a rotating arm. Recent work by Strumpf suggests that an arrangement 
of two fins as shown in Fig. 10 might be the most satisfactory solution in combination with 
the surface-piercing strut. 
In addition to the purely hydrodynamic problems discussed, there are many other de- 
sign problems to be solved, including the following: 
1. Compact, lightweight air-breathing power plant, 
2. Air supply and exhaust, 
3. Structure of surface-piercing strut, 
4. Static transverse stability, 
5. Minimization of resistance by selection of optimum hull proportions and obtaining the 
best balance between depth and strut drag, 
6. Automation of operation to minimize crew requirements. 
