Sec. 69.3 



GENER/VL DESIGN OF PROPULSION DEVICES 



569 



". . . for best efficiency of propulsion the screw propeller 

 should be located in a high friction wake, a negative or 

 low positive streamline (potential-flow) wake and in the 

 crest of a wave." 



The condition that the propeller work in a 

 negative or low positive potential or streamline 

 wake is that which obtains generally in a normal 

 form of ship if the hull is assumed to be expanded 

 to include the displacement thickness 6* (delta 

 star) of the boundary layer. With run lines of 

 easy slope, such an expanded form has only a 

 moderate positive streamline wake fraction due 

 to potential flow. 



To achieve the highest practicable negative 

 streamline wake fraction, propellers could be 

 mounted on outriggers abreast the section of 

 maximum area, occupying about the same 

 position as paddlewheels. Here the speed of 

 advance becomes greater than the ship speed. 

 Because there is no ship forward of the side 

 propellers, the thrust-deduction fraction becomes 

 practically zero. High negative-wake fractions 

 are actually to be found inside Kort nozzles and 

 other types of fixed propeller shrouding. 



When some form of hydraulic jet propulsion is 

 employed, requiring large volumes of water to 

 be taken within the hull boundaries and discharged 

 from internal ducts, the positions and shapes of 

 the inflow and the outflow openings become almost 

 a part of the main hull design. These features, of 

 which relatively little is known, are indeed 

 worthy of more attention than is normally 

 devoted to the position of a screw propeller and 

 the shape of the hull in its vicinity. Following the 

 practice on jet-propelled aircraft, the ram action 

 of the water flowing toward the hull is utilized 

 to force water into the inlet and to keep it moving 

 against the friction and pressure resistance en- 

 countered within the internal ducts. This is 

 accomplished, where practicable, by an inlet 

 opening facing directly forward or forward and 

 outward. 



As for the limiting dimensions of propulsion 

 devices, it is obvious that there are practical 

 limits dictated by good all-around engineering, 

 good mechanical design, and economic operation 

 of the vessel which are in conflict with hydro- 

 dynamic requirements for the ultimate in pro- 

 pulsion-device efficiency. To increase the latter 

 means increasing the thrust-producing area and 

 lowering the thrust-load coefficient Ctl but at the 

 expense of increased volume occupied by and 

 increased weight of the propulsion device and the 



propelling machinery. It is probable that the 

 designer of ship-propulsion devices always has 

 been faced and always will be faced with the 

 fact that existing fabrication and shipping 

 facilities for these devices are too small. Never- 

 theless, larger facilities have always been built in 

 the past and they will continue to be made 

 available in the future. There is no reason why 

 the propulsion-device designer should be bound 

 by existing facilities if he can produce a better or 

 more efficient ship. 



Since most propulsion devices rotate, there 

 are problems of gearing and transmission, which 

 may govern the rate of rotation and through it 

 may influence the size of the propulsion device, 

 the thrust-load coefficient, and the hydrodynamic 

 efficiency. Furthermore, the necessity for me- 

 chanical protection of the propulsion device 

 against damage by external objects, and for 

 shielding it against air leakage, as is done for a 

 screw propeller by the main hull of a single-screw 

 tug, may call for a diameter smaller than would 

 otherwise be used. 



The required thrust-producing area Aa of any 

 type of propulsion device being considered is 

 readily calculated from the ship resistance Rt 

 and the speed V. After estimating or assuming 

 values of the wake and thrust-deduction fractions 

 w and t and selecting a thrust-load factor Ctl 

 which will give a good value of the real efficiency 

 (0.8?;/), the values are substituted in the formula 



T = 



whence 



i2j 



C, 



Cr 



AoVl 



Ao[F(l - w)f 



A„ = 



2Crz.(l - t) 

 pRrlVil - W)f 



E. Burtner gives a simple dimensional formula 

 for determining quickly the diameter of a screw 

 propeller in the course of a preliminary design 

 [ASNE, Aug 1953, pp. 545-548]. This formula 

 appears to give reasonable values for both 3- and 

 4-bladed wheels and for a rather wide range of 

 expanded-area ratio. It is 



D (in ft) 



[Ps (in horses)]" 

 (rpm)°" 



(69.i) 



On page 547 of the reference Burtner includes 

 a small-scale nomogram for finding any one of 

 the three values quickly when the other two are 

 known. For example, assuming for the ABC ship 



