330 P. Mandel 
submerged Aluminaut about 20 degrees against her metacentric stability. As mentioned 
earlier, the trim system is currently sized primarily to accommodate the trim unbalance 
imposed by the loss of buoyancy in the stern capsule at depth and the movement of person- 
nel within the boat. However this system could be readily enlarged after completion of the 
boat if it is evident that additional capacity is needed to augment pitch angle control. 
Stern control surfaces are also employed partially. to improve directional stability in the 
vertical plane, but also to serve as trim tabs to assist in maintaining horizontal flight with- 
out constant use of the vertical propeller. It is possible that they may not be able to fulfill 
even the latter limited mission in the event that the critical speed (see Section 7 of Ref. 2) 
falls within the operating speeds of the Aluminaut. At this speed no combination of hull 
pitch angle and stern plane angle can simultaneously balance out both the hydrodynamic 
forces and the combination of hydrodynamic and hydrostatic moments that act on any sub- 
marine. However, at speeds just slightly removed from the critical value the stern planes 
will be effective as trim tabs even though they may have to be used in a sense opposite to 
that which intuition would dictate. 
Even with the preceding systems for controlling pitch angle and depth, difficulty may 
be encountered in maneuvering along an irregular bottom. For this purpose use of a trail 
rope such as is used by balloonists for maintaining constant distance above an irregular 
land terrain may be helpful. However, there has been little or no experience with this 
device with submarines and the forces involved may be too small to be effective. In the 
event that difficulties are experienced after the boat is in operation, the alternatives remain 
of refining the controls of the vertical propeller or increasing the capacity and refining the 
controls of the hydrostatic trim system. 
Main Propulsion and Control in the Horizontal Plane 
A 15-horsepower dc motor located in the stern capsule and connected via bevel gears 
to a four-foot-diameter propeller is used for main propulsion. This motor (as well as the 
topside motor for the vertical propeller) has hydrodynamic problems of its own since it must 
operate in an environment of very high pressure oil. To improve motor efficiency, both the 
rotating as well as the fixed elements must be especially streamlined to minimize hydro- 
dynamic drag. Even with these measures a motor efficiency of only about 50 percent is 
assumed. It is expected, however, that the motor can be substantially overloaded because 
of the excellent heat dissipation into the surrounding medium. As shown in Fig. 13, a maxi- 
mum horizontal speed of about 4.7 knots is expected with 15 horsepower. 
Stability and control in the horizontal plane is effected by large fixed fins, balanced 
rudders (each 6 square feet in area) and provision to swivel the main propulsion propeller 
through a total arc of 120 degrees. Both the rudders and propeller pivot on a common verti- 
cal shaft as indicated in Figs. 1 and 2. The swiveling propeller by itself should assure 
excellent control at any speed in the horizontal plane. 
DIRECTIONAL STABILITY IN TOW TESTS 
Full-scale operations with the Aluminaut doubtlessly, will at some time require that 
she be towed on the surface to the scene of diving operations. Experience in the past with 
vessels of similar shape to the Aluminaut has revealed that in some conditions, the behavior 
