Sec. 70.3 



SCREW-PROPELLER DESIGN 



583 



avoids major duplication and appreciable overlaps 

 with published material in books, papers, and 

 reports, especially those readily available to the 

 average marine architect. The major portion of 

 the chapter is devoted to a description of the 

 short method of Dr. H. W. E. Lerbs for the 

 design of a screw propeller, based on the circulation 

 theory. Accompanying a step-by-step description 

 of this method, a sample calculation is carried 

 along for a propeller to be used with the transom- 

 stern design of ABC ship, the hull of which is laid 

 out in Chaps. GG, 67, and 68. This description, inci- 

 dentally, is believed to be among the first based 

 on this theory by which all the elements in the 

 design are derived by a continuous, straight- 

 forward procedure. This makes it suitable for a 

 designer mth Uttle or no background or experi- 

 ence, save the knowledge of flow and circulation 

 and its apphcation to the ship and screw-propeller 

 combination, to be found in Chaps. 14-17 and 

 32-34 of Volume I of this book. 



The symbols, terms, and definitions employed 

 in this chapter conform to those listed in Appendix 

 1 of this volume. They are described in SNAME 

 Technical and Research Bulletin 1-13, containing 

 "Explanatory Notes for Resistance and Pro- 

 pulsion Data Sheets," July, 1953, and illustrated 

 in Figs. 32.F, 32.G, and 32.H of Sees. 32.8 and 

 32.9. 



70.2 Design Requirements for a Screw Pro- 

 peller. It is possible that one reason for the 

 shortcomings in the numerous design methods 

 and procedures, including those discussed in 

 Sec. 70.3, is a partial lack of appreciation of the 

 basic requirements to be met and of the practical 

 needs of the ship owner and operator. Perhaps 

 even more basic are what might be termed 

 the practical needs of the ship itself. 



For example, as long ago as 1938 a propeller 

 designer, F. McAlister, when discussing the 

 results of a symposium on marine propellers 

 [NECI, 1937-1938, Vol. LIV, pp. D141-D142], 

 declared that, as reported in SBSR, 6 October 

 1938, page 415, "none of the papers in the sym- 

 posium tabulated the qualities required (the italics 

 are those of the present author) for full-sized 

 propellers for ships under service conditions. He 

 suggested that from the purchaser's point of 

 view these requirements were: 



"(I) That the (new) propeller must be of the highest 

 possible efficiency — say 10 per cent (or more) higher 

 efficiency than the average existing propeller 

 "(II) Must not sing or be unduly noisy 



"(III) Must not vibrate or must eliminate whatever 

 vibration may be due to the existing propeller 

 "(IV) Must not erode, or at least must be better in this 

 respect than the existing propeller 



"(V) Must be of sufficient strength and first-class work- 

 manship to ensure a long life free from trouble. 



"The information given in the papers gave no 

 guidance in these directions. It might be the 

 case that these 'purchaser's' requirements can be 

 met in exceptional cases, but it is surely a serious 

 reflection on ordinary practice to suggest that 

 propellers as now fitted are, on the average, 

 10 per cent less efficient than they might be." 



70.3 Comments on Available Design Methods 

 and Procedures. It is expected, when many 

 minds work on a problem in the atmosphere of a 

 democratic way of fife, there will be many fines 

 of attack and many partial or complete answers. 

 This is as it should be, because different kinds of 

 answers are required for different situations. 

 Furthermore, the problem — screw-propeller de- 

 sign in particular — is so complex that no single 

 line of attack can do more than make a rather 

 narrow path through the entire region to be 

 covered. 



The procedures now in use among naval 

 architects and marine engineers vary from the 

 heavily theoretical to the intensely practical, but 

 fortunately each is useful in some particular 

 situation. An engineer may select a propeller 

 diameter and pitch by some simple formula or 

 nomogram and order a propeller out of a catalog, 

 or he and his assistants may toil for several 

 months, calculating a propeller design for which 

 no ready rules or precedents are available. The 

 day is past, or nearly so, when the marine engineer 

 sketched his propeller freehand in the foundry- 

 man's notebook, penciling in the few principal 

 dimensions. 



It is well to recognize, therefore, that a group 

 of several of these methods has its place in the 

 scheme of things, even in a so-called advanced age. 

 For example, in the design of the underwater 

 hull of the ABC ship, begun in Chap. 66, one of 

 the principal aims is to swing as large a propeller 

 as possible. Since the limit of ideal efficiency 

 described in Sec. 34.2 increases as the thrust 

 loading decreases, and the latter decreases as 

 the propeller diameter increases, it works out 

 that the larger the propeUer, the greater the 

 propeller efficiency, aU other things being favor- 

 able. The tentative diameter of 20 ft, on a 26-ft 

 draft, is based on a propeUer somewhat larger 



