584 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.4 



than that given by the rule of thiunb, D < 0.7//, 

 equal in this case to 0.7(26) or 18.2 ft. 



When, at a later stage of the design, after the 

 resistance of the hull had been estimated, together 

 with the wake and thrust-deduction fractions, it 

 was possible to calculate the thrust-load factor 

 and to know that it was low, as desired. When the 

 shaft power was estimated, it was found from 

 several propeller-design charts that a 20-ft 

 propeller would absorb it when running at a 

 reasonable rate of rotation. 



At a still later stage, when it was necessary to 

 pick a suitable stock propeller for the model 

 self-propulsion test, the P/D ratio and other 

 principal characteristics were determined approxi- 

 mately by several established methods, called 

 chart methods, mentioned presently and de- 

 scribed in Sec. 70.5. 



For the latter part of the preliminary design 

 of the ABC ship, a final design of screw propeller 

 for a second series of model tests (not conducted) 

 was carried through by the Lerbs short method, 

 based upon the circulation theory, described in 

 Sees. 70.21 through 70.38. 



These several methods are mentioned to show 

 that the approximations, estimates, and calcu- 

 lations required at different stages of a ship and 

 propeller design call for different procedures. 

 Some call these 5-sec, 5-min, 5-hr, and 5-day 

 procedures, depending upon how soon the answer 

 is wanted. The precision of each is of the same 

 order as the time required. Others call them 

 the 1st, 2nd, 3rd, and 4th approximations. 

 The important fact is to realize that the first 

 approximation is based upon the application 

 of one single rule of thumb; the last one upon all 

 the scientific and engineering information avail- 

 able. All of them have their logical functions in 

 the design of any one propeller. 



In general, neglecting the thumb rules, the 

 various procedures fall into two groups: 



(1) Those based upon systematic experimental 

 data, derived from tests of model propellers in 

 methodical series, with uniformly varying param- 

 eters and characteristics. The data, when checked 

 and analyzed, are put in the form of graphs, 

 diagrams, or charts, whence the name chart design. 



(2) Those based upon the application of hydro- 

 dynamic theories and knowledge of flow, embody- 

 ing such gap-fillers and correction factors (derived 

 usually from experimental data) as are required 

 to compensate for lack of accurate and adequate 

 knowledge here and there. 



The first group labors under the disadvantage 

 that it applies only to propellers with the same 

 number of blades, blade shape, section shape, 

 and so on, as the propellers of the series tested. 

 The second is deficient in that factual knowledge 

 concerning the nature of the physical water flow 

 around a propeller and the applicable hydro- 

 dynamic theories, as well as the necessary con- 

 firmations of the latter, lag rather far behind 

 the necessity for knowledge to give the practical 

 answers. 



The chart method tells only what happens to 

 the overall forces and moments on a propeller 

 which someone has already fashioned, whether 

 it be well fashioned or not. The analytic method 

 leads gradually but surely to a better understand- 

 ing of the physics of the problem, which governs 

 the forces and moments, and hence to an indica- 

 tion of just how a screw propeller should be 

 fashioned to give the desired results. 



70.4 Requirements for, Availability of, and 

 Listing of Propeller-Series Charts. It should be 

 recognized at the outset that any chart or analytic 

 procedure may, and probably will take a different 

 form, depending upon the characteristics that 

 are given or fixed and those which are to be 

 derived. For the pvn-pose of this book, one pro- 

 cedure only of each kind is described, embodying 

 freedom of choice for the designer in that primary 

 characteristic which should, for the best perform- 

 ance, permit him the greatest leeway. For 

 example, in the hull design of the ABC ship, 

 roughed out in Chap. 66, the weights to be 

 carried by the ship are specified, as is the speed, 

 leaving the length free for selection of the optimum 

 dimension. 



For the propeller designs to be worked out as 

 examples in this chapter, the power to be ab- 

 sorbed is governed by that required for driving 

 the hull at the designed speed. It is anticipated 

 that the best screw propeller will be that having 

 the greatest practicable diameter, so the hull in 

 the vicinity of the propeller position is designed 

 with this in view. Strictly speaking, this means 

 that the propeller diameter is fixed at the be- 

 ginning of its design, but at a figure which should 

 produce a most efficient wheel. The rate of rota- 

 tion, the pitch-diameter ratio, and other factors 

 remain to be selected so as to give high propulsive 

 efficiency in service. In other circumstances the 

 propeller power and rate of rotation might be 

 fixed, with the best diameter to be found. 



Regardless of the primary characteristics given 



