632 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.41 



(a) Reducing the number of blades and the blade 

 overlap (when viewed generally normal to the 

 blade surface) to a minimum 



(b) Keeping the slip ratio low, with a reasonably 

 low angle of attack on each blade 



(c) Increasing the pitch-diameter ratio, to in- 

 crease the gap between blades. 



The slip ratio can only be held down by loading 

 the propeller lightly. This may be achieved by 

 increasing the disc area or the expanded blade 

 area, but is best accomplished in the case of the 

 supercavitating propeller by reducing the ship 

 resistance and the propeller thrust to the lowest 

 possible values. At the high speeds in question, 

 this is only possible with planing craft in which 

 the resistance varies as some power of the speed 

 less than the square, possibly even less than the 

 first power; see Fig. 53. D. 



Not more than three blades, and not too wide 

 blades at that, should be used on a propeller 

 working for the most part in the heavily cavitat- 

 ing or supercavitating range. Two-bladed pro- 

 pellers are preferred. Pitch-diameter ratios should 

 probably exceed 1.4, and may run as high as 2.0 

 or more. 



If it is known that a propeller will cavitate 

 fully throughout the running range, its blade 

 sections may be of triangular shape, with blunt 

 or square trailing edges. The blade speed is so 

 extremely high, and the static pressure usually 

 so low that the water can not possibly close in 

 behind even a fair blade section. Wedge-shaped 

 propeller blade sections for supercavitating pro- 

 pellers are discussed by G. Rabbeno [Ann. Rep. 

 Rome Model Basin, 1938, Vol. VII, p. 91]. The 

 stiffness — and strength — of the blade may be 

 concentrated in the metal near the trailing edge, 

 enabling the leading edge and the blade section 

 to be considerably finer than normal. Comments 

 on supercavitating flow past foils and struts, 

 applicable to the propeller-design problem, are 

 given by M. P. TuUn ["Cavitation in Hydro- 

 dynamics," NPL, Oct 1955, paper 16; SBSR, 

 3 Nov 1955, pp. 570-571]. 



For the ultra-high-speed screw propeller which 

 provides the dynamic lift for holding up the stern 

 of a very fast planing craft, with the propeller 

 shaft, the struts, and the propeller hub normally 

 out of water, it is important that there be a 

 stabiUzing influence to hold the stern of the boat 

 at its proper level. One solution is to set up a 

 compensating downward vertical force known as 



the "antiUft." This force exceeds the dynamic 

 upward lift created by the lower blades if the 

 propeller rides too high but it is less than the 

 upward lift if the propeller rides too low. Means 

 of incorporating this feature in a propeller design 

 are described in considerable detail by E. C. B. 

 Corlett [The Motor Boat and Yachting, Sep 1954, 

 pp. 387-388]. 



70.41 Design of Bow Propellers, Coupled and 

 Free-Riuining. Bow propellers are either driven 

 by independent engines, at a speed suitable to 

 the needs of the moment, or they are, in the case 

 of many double-ended ferryboats, coupled to the 

 engine and the stern propeller by a straight- 

 through shaft. Icebreakers with bow propellers 

 are in the first category, along with the larger 

 ferryboats, where fuel economy is important. 



Icebreakers and other vessels with bow pro- 

 pellers are usually required to back hard upon 

 occasion, or to run in the opposite direction. The 

 bow propeller then becomes the stern one. Under 

 these conditions symmetrical sections are em- 

 ployed, with straight meanlines. Actually, since 

 the propellers rotate in opposite directions at 

 different times, the sections are elliptical or 

 lens-shaped, symmetrical on each side of the 

 midchord position, similar to those of Fig. 70. A 

 [S and P, 1943, Fig. 153, p. 132, Type 3]. What 

 might be termed double-symmetrical blades of 

 this kind, running in the open, give identical per- 

 formance when rotating one way or the other. 



70.42 Open- Water and Self-Propelled Model 

 Tests. No existing propeller-design procedure is 

 sufficiently comprehensive and reliable to give an 

 accurate prediction of the open-water performance 

 of a screw propeller built to a particular design, 

 either on model or full scale. When the new pro- 

 peller design is only slightly different from that 

 of a propeller which has already been tested it 

 would appear that the designer could be reason- 

 ably certain of predicting its performance. 

 Nevertheless, minor changes which seem in- 

 significant often produce appreciable differences 

 in performance. One never knows when this will 

 happen. 



It seems wise, therefore, in the case of a new 

 propeller design, to build a model and to test it, 

 (1) in open water, (2) in a variable-pressure water 

 tunnel, and (3) in a self-propelled model of the 

 ship for which it is designed. Unfortunately, it 

 was not possible to do this for the wake-adapted 

 propeller designed in Sees. 70.21 through 70.37, 

 nor to include the test data in Chap. 78. 



