Fig 8 2 Free screw-type propeller 



the distance it would advance in a solid 

 substance and the actual distance traveled, 

 expressed as a percentage of the former. 

 According to Taggert (6), the screw propeller 

 is the most efficient means of obtaining pro- 

 pulsive force, and when used at its designed 

 rpm and speed of advance its efficiency is 

 exceeded by no other device. Miller (7) states 

 the screw propeller's efficiency as 75 to 85 

 percent under ideal conditions and contrasts 

 this with the 40 percent efficiency of a cycloi- 

 dal propeller. Obviously, the submersible 

 community is in agreement, for only the 

 French vehicles SP-350 and SP-500, and 

 the U.S. Navy's MAKAKAI use other than 

 screw propellers for propulsion. NEKTOlK's 

 propeller is of the open screw variety, in that 

 the shroud around it is solely for protection, 

 not to improve its efficiency. 



Kort Nozzle Propellers 



By fitting a specially-designed shroud or 

 nozzle around a screw propeller, its effi- 

 ciency can be increased. The design shown in 

 Figure 8.3 was invented and patented by 

 Ludwig Kort in the U.S. in 1936. In cross 

 section the nozzle resembles an aircraft wing 

 with the outer side or face being practically 

 straight and the inner side being cambered. 



In practice the nozzle experiences a negative 

 pressure forward of the propeller's plane of 

 rotation and a positive pressure aft, which 

 results in forward thrust. According to refer- 

 ence (2), in a well-designed propeller-nozzle 

 combination the nozzle thrust is about half 

 of the total thrust applied. A further advan- 

 tage (8) is that the static shroud thrust is 

 maximum at zero speed; in other words, it is 

 most effective when the submersible begins 

 moving from a stationary position. Reference 

 is made by Taggert (3) to the Kort nozzle 

 being particularly advantageous when oper- 

 ating under high slip conditions (during ac- 

 celeration or when the vehicle is heavily 

 loaded) and about equal in performance to 

 the free propeller during low slip conditions. 

 The advantage of obtaining greater thrust 

 from a shrouded screw propeller at zero 

 speed has not been ignored by the submers- 

 ible builders — a large number of vehicles em- 

 ploy this feature. Equally attractive is the 

 fact that a shrouded propeller can be consid- 

 erably smaller than its "free" counterpart 

 and still deliver maximum efficiency. This 

 feature is desirable in that it decreases the 

 overall vehicle size envelope. 



Ducted Propellers 



Johnson and Barr (9) define a ducted pro- 

 peller as any scheme which takes water 



Fig 8 3 A Kort nozzle surrounds the Ihree-bladed ttiruster propeller of ALVIN 

 (WHOI) 



372 



