nature of its containment, all submersibles 

 use electric motors to initiate and maintain 

 motion. The motor may be directly coupled to 

 a rotating shaft, or it may move the shaft 

 indirectly through an intermediary hy- 

 draulic pump. The nature of such motors (or 

 propulsive power) depends, in part, upon the 

 vehicle's size and configuration and the de- 

 sired speed. Neglecting motor horsepower 

 requirements for the moment (dealt with in 

 the following section), let's turn to an exami- 

 nation of the design and electrical current 

 options available to the submersible de- 

 signer. 



Alternating Versus Direct Current 

 Motors 



The choice between a propulsion motor 

 operating on alternating or direct current 

 has been almost unanimously in favor of DC. 

 Only 9 of the lOO-l- submersibles use AC 

 propulsion motors; the reason is one of eco- 

 nomics. While AC motors are simpler in de- 

 sign and construction, and require less main- 

 tenance, an inverter must be supplied to 

 change DC to AC. This adds weight and will 

 take away from pressure hull space if 

 mounted therein. More importantly, the in- 

 verter adds to both the vehicle's cost and 

 complexity. 



DC motors, on the other hand, can be oper- 

 ated directly from the battery, they have 

 better speed control than AC, and produce 

 higher torque. Because DC motors use com- 

 mutator bars and brushes they must be pro- 

 tected from seawater; additionally, they re- 

 quire more frequent maintenance, which 

 may be every 40 to 50 hours of service. This 

 latter feature has not been a great disadvan- 

 tage because most vehicles are taken out of 

 the water after each dive when such mainte- 

 nance may be performed. 



Before examining the design of present 

 electric motors, a basic problem should be 

 identified. A screw-type propeller turns on a 

 shaft which in turn is rotated by an electric 

 motor. Somewhere in this scheme the compo- 

 nents of the motor must be protected from 

 seawater. The most obvious solution is to 

 place the motor inside the pressure hull or 

 inside a pressure-resistant case, but the fact 

 that the shaft must both penetrate the case 



and rotate within this penetration presents 

 severe problems. From earlier chapters it 

 was seen that thru-hull shaft penetrations 

 which are watertight and pressure resistant 

 are common, but when the shaft must main- 

 tain these two features and rotate at the 

 same time a new set of problems is con- 

 fronted. In essence, a dynamic seal capable 

 of limiting the leakage of seawater into a 1- 

 atmosphere motor container is not available 

 for great depths. In some shallow vehicles, 

 e.g., the NEKTON series and the Perry vehi- 

 cles, the pressure at their operating depth 

 can be overcome by a pseudo-packing gland. 

 In these vehicles the motor is contained in a 

 separate, pressure-resistant compartment 

 either outside or inside the hull. The only 

 relatively deep submersible in which the 

 drive shaft penetrates the pressure hull and 

 the motor is not sealed off from the hull in a 

 separate compartment is the 2,500-foot AU- 

 GUSTS PICCARD. The water-tightness of 

 AUGUSTS PICCARD's propeller shaft is ob- 

 tained by a graphite joint inside of which is a 

 packing gland. An inflatable rubber ring 

 serves as a security fitting between shaft 

 bearing and the hull. A variety of contact 

 seals of this nature is presented in reference 

 (14), and this report concludes that all con- 

 tact-type seal configurations are, in effect, 

 bearings whose generated pressure and 

 clearance are utilized to restrict leakage. 

 Design of such seals is, according to Sasdelli 

 and Spargo (ibid), a trade-off between wear 

 and leakage, and in the military submarine 

 where large, powerful propulsion plants are 

 required and must be protected from both 

 nature and man, the problem is severly com- 

 plicated. In manned submersibles, propul- 

 sion power requirements are minute in com- 

 parison and nature, through formidable is 

 the only adversary. 



A state-of-the-art summary of propulsion 

 motors for submersibles was presented by 

 Mr. L. A. Thomas of Franklin Electric Com- 

 pany in 1968 (15). Though modifications have 

 taken place since this summary, the motor 

 categorizations and the design principles 

 Thomas outlined are still applicable. 



Thomas presented three different design 

 concepts in electric motors for outboard (vs. 

 in-hull) propulsion in seawater: Open- 



389 



