through an inlet flush with the hull, adds 

 energy with a pump located within the hull 

 and discharges it through a nozzle flush with 

 the hull. Under this definition falls the 

 ducted propellers of DOWB (Fig. 8.4). These 

 ducts are located aft on DOWB and the pro- 

 peller screw shafts are canted 15 degrees 

 outboard. This arrangement provides the fol- 

 lowing advantages: The center of drag is 

 moved aft for hydrodynamic stability and it 

 reduces the beam while still maintaining a 

 large twist moment aft. Analyzing the per- 

 formance of ducted propellers, Johnson and 

 Barr state that the inlet and outlet designs 

 are extremely significant. Ideally, a well- 

 designed duct would utilize a bell mouth 

 inlet and a sharp exit extending a short 

 distance beyond the hull. Where the duct 

 exit is flush with the hull, jet entrainment 

 effects cause low pressure in the exit side of 

 the hull and reduce thrust where both ends 

 of the duct are sharp and flush with the hull. 

 In the case of DOWB, there could be a thrust 

 reduction of unknown magnitude resulting 

 from an eddy created near the hull by the 

 exit jet. To prevent the propellers from foul- 



Fig. 8-4 The Ducted Propellers on DOWB provide surge and yaw thrust. The two 



motors are in the deck plane of the vehicle and canted outboard at angles of 15 



degrees (NAVOCEANO) 



Fig. 8.5 Two pi-pitched cycloidal propellers attached to the exostructure of MAKA- 

 KAl provide Its entire propulsion (U.S.M.C. Air Station, Hawaii) 



ing with ropes or cables while submerged, 

 DOWB's operators installed fine wire screens 

 at both the inlet and exit ends. Such screens 

 also reduce thrust by restricting the flow of 

 water through the duct. This penalty, how- 

 ever, could be well worth the price in terms 

 of safety. Ducted propellers, of varying de- 

 signs, are found on quite a few vehicles — 

 mainly because they reduce the vehicle's en- 

 velope and chance of entanglement and pro- 

 peller damage. 



Cycloidal Propellers 



The U.S. Navy's MAKAKAI is the only 

 submersible known to use cycloidal propul- 

 sion (Fig. 8.5), although its predecessor HI- 

 KINO was the first. The two units on MAKA- 

 KAI are Kirsten Boeing, pi-pitch*, cycloidal 

 propellers mounted on the vehicle's exostruc- 

 ture which operate similar to a paddle wheel: 

 The propeller disc rotates and moves the 

 blades through the water. Talkington and 

 Murphy (10) describe its operation as follows: 

 "T/ie cycloidal propeller is capable of 

 directing its thrust in any direction in 

 the propeller disc's plane of rotation. 

 The pirpitch propeller is used because 

 of its mechanical simplicity and be- 

 cause of the four degrees of freedom 

 propulsive control that can be ob- 

 tained by using two thrusters. Thrust is 



373 



