Sec. 70.12 



SCREW-PROPELLER DESIGN 



599 



the magnitude of wake velocity around the cir- 

 cumference at any radius is highly irregular. This 

 is the case in the disc position of a single screw 

 carried abaft a centerline skeg on a ship of normal 

 form. 



(Ill) A possibility or probability of cavitation, or 

 perhaps a certainty of it if the effective angles of 

 attack of the blade elements at each radius are 

 not kept within certain limiting values. Although 

 cavitation is not normally to be expected in the 

 upper blade positions of the propeller of a single- 

 screw ship, it can and probably does occur if the 

 vessel is fast and if it is driven hard in a loading 

 condition where the at-rest tip submergence is 

 small, approaching zero. 



Good propeller design to meet situation (I) 

 unquestionably calls for a radial variation in 

 pitch corresponding generally to the radial varia- 

 tion in wake velocity, so that a given distribution 

 of thrust with radius fraction is achieved. It has 

 been the aim of many inventors and ship designers 

 to incorporate a stern bulb around a single-screw 

 propeller axis which would produce a high average 

 wake velocity as well as one which was reasonably 

 uniform around a circumference at each radius. 

 However, because of the predominantly upward 

 component of flow under the sterns of most ships, 

 this desirable end has so far not been attained. 



L. C. Burrill and C. S. Yang made a compre- 

 hensive theoretical study of many situations, 

 involving both uniform and non-uniform wake 

 velocities over the propeller disc, for a series of 

 screw propellers having a great many types of 

 variation of pitch with radius [INA, 1953, pp. 

 437-460]. In fact, they cover analytically most 

 of the situations that would be encountered in a 

 wide variety of ship designs. They arrived at 

 certain general conclusions concerning radial 

 pitch variation which appear to cover most 

 phases of situations (II) and (III) preceding that 

 a ship designer might be likely to encounter. 

 Items (1) through (4), listed hereunder, are 

 quoted verbatim from page 446 of the Burrill 

 and Yang reference: 



"(1) From the point of view of overall efficiency, and apart 

 from any consideration of cavitation or flow breakdown, 

 there appears to be no material advantage to be gained 

 from the adoption of a radial variation of pitch, both in a 

 uniform stream and in a variable wake-stream 

 "(2) In particular, it seems that there is no special advan- 

 tage to be gained from the application of the various 

 alternative methods of design, based on the principle of 

 minimum-energy loss, which have been examined, as any 



gain which might be achieved in tlie ideal or hydrodynamic 

 efficiency by the adoption of these procedures is very 

 small, and is overshadowed by the effects of blade-efficiency 

 (i.e., section-drag losses, etc.) introduced by the changes 

 in angles of incidence where the wake concentration is high 

 "(3) On the other hand, the above results suggest that no 

 special loss in efficiency is to be expected from the adoption 

 of moderate pitch variations which are favourable from 

 the point of view of cavitation or flow breakdown, and 

 this leaves the designer considerable freedom in the 

 matter of adopting such alternative pitch-variation lines 

 from root to tip of the blades as might be considered 

 desirable from this point of view 



"(4) It appears that the quantity designated relative- 

 rotative-efficiency has a real meaning, in terms of the 

 methods of analysis usually adopted, and its value can be 

 estimated by calculation, in the manner described in 

 the paper. The numerical values obtained agree reasonably 

 well with the experimental data." 



In the example of propeller design by the circu- 

 lation theory carried through in detail in Sees. 

 70.21 through 70.38 of this chapter a propeller is 

 worked up by the Lerbs short method for the 

 single-screw transom-stern design of the ABC ship. 

 It may be argued that, by the Burrill-Yang 

 criteria, the variations in wake velocity and 

 wake fraction for this case, indicated by Figs. 

 60.M and 60. N, are not sufficient to justify the 

 design of a wake-adapted propeller. Furthermore, 

 cavitation is not expected to be a problem in the 

 propulsion of this vessel. Nevertheless, the Lerbs 

 1954 design procedure is carried through on the 

 basis that both of these features do require 

 special attention. This renders the design solution 

 more general in character and makes it fully 

 applicable, as described, for a situation where 

 radial wake variation and the possibiUty of 

 cavitation should definitely be taken into account. 



If large ship propellers could be and were pur- 

 chased from stock, there might be some reason 

 for omitting a propeller-design calculation that 

 requires more than one or two man-days. For a 

 large ship, expensive to run as well as to build, 

 requiring a custom wheel, so to speak, there is 

 every reason why a propeller-design procedure 

 should take account of all the possibiUties and 

 should take advantage of all the latest develop- 

 ments in the art. 



70.12 Choice of Number of Blades. The 

 choice of the number of blades is one of the first 

 decisions to be made in the design of a screw 

 propeller. As an aid in reaching this decision, two 

 or more preliminary designs may be worked up 

 as a sort of series, each with a different number of 

 blades. The final selection is usually based on a 



