612 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.23 



cent was expected. That this estimate was fairly 

 accurate is shown by the recorded rpm of 109.7 at 

 20.5 kt in the model self-propulsion test. However, 

 using the resistance of the ship with appendages, 

 obtained from the resistance test of TMB model 

 4505, and a thrust-deduction fraction of 0.07 at 

 20.5 kt, obtained from the self-propulsion test 

 with TMB model propeller 2294, a revised thrust 

 is calculated. 



V = 20.5 kt; Pe = 10,078 horses, from model- 

 resistance test; thrust-deduction fraction t = 0.07 



550Pb 550(10,078) 



Rq 



1.6889(20.5) 



= 160,098 lb 



T = 



R, 



1 - t 



160,098 

 0.93 



= 172,148 lb. 



The predicted thrust required to drive the ship 

 is much less than the estimated thrust. This 

 means that a new combination of P/D ratio and 

 rate of rotation n might give a higher propeller 

 efficiency than the combination used in selecting 

 the model stock propeller, based on the higher 

 thrust. 



Consulting Prohaska's logarithmic charts for 

 Wageningen Series B.4.40 and B.4.55 model pro- 

 pellers, one of which is reproduced as Fig. 70. B, 

 and entering with the thrust-load coefficient 

 Ctl of 0.709, based on the lower thrust value and 

 an effective wake fraction of 0.195 from the self- 

 propulsion test, the following optimum charac- 

 teristics are determined: 



P/D =1.2 J = 0.86 

 n = 1.620 rps or 97.2 ipm r?o = 0.72 

 The open-water propeller efficiency obtained from 

 the self-propulsion test with the stock propeller 

 at the designed speed was roughly 68 per cent. 

 This means that there is a possible gain of about 

 4 per cent in propeller efficiency, with a cor- 

 responding increase in the propulsive coefficient. 

 A new propeller design for the ABC ship is thus 

 definitely indicated. 



70.23 Choice of the Number of Blades for the 

 ABC Design. General comments concerning the 

 number of blades to be used in a screw-propeller 

 design are embodied in Sec. 70.12. Those in this 

 section are limited to the final design of propeller 

 for the transom-stern ABC ship. 



This vessel, with the afterbody profile of Fig. 

 66. Q, supplemented by Fig. 67.U, is designed with 

 what is known as a clear-water stern. As might be 

 expected, and as is revealed in Fig. 60.M, there is 

 a high-wake-velocity region near the top of the 



propeller circle, where the propeller passes the 

 skeg ending and cuts through the boundary layer 

 beneath the transom. Cutting back the lower 

 portion of the skeg should have reduced the 

 magnitude of the wake velocities in the lower 

 half of the propeller circle. Nevertheless a localized 

 region of moderate positive wake velocities 

 remains in the neighborhood of the 6 o'clock 

 position. With such a wake configuration, it is 

 advisable to use a propeller with an even number 

 of blades, to minimize the unbalance in thrust 

 between the blades in the 12 o'clock and 6 o'clock 

 positions and to reduce the periodic bending of 

 the propeller shaft in a vertical plane. A 4-bladed 

 propeller is the logical choice, as has been found 

 from long experience with single-skeg single-screw 

 ships, unless a subsequent analysis indicates that 

 a 6-bladed propeller is needed to place the blade 

 frequency (n times Z) in a certain range. 



Ample edge clearance is allowed in the propeller 

 aperture of the ABC transom-stern design, be- 

 tween the end of the skeg and the propeller 

 sweep line, to keep vibratory forces to a minimum. 



70.24 Determination of Rake for the ABC 

 Propeller. The propeller aperture and stern 

 arrangement of the transom-stern ABC ship are 

 designed so that no rake is required. There is 

 some contraction in the inflow jet, to be sure, as 

 for any screw propeller, but this is not augmented 

 greatly at the disc position because the lines of 

 the skeg ending ahead of it are deliberately 

 made fine. The rake angle is set at deg for the 

 design carried through here but it could have 

 been set at any angle up to about 5 deg if the 

 designer wished to take advantage of the inflow 

 contraction. 



70.25 Propeller-Disc and Hub Diameters. 

 The transom stern of the ABC ship was designed 

 to accommodate the largest practicable propeller 

 diameter on the given draft of 26 ft. This was 

 done to obtain the greatest possible propulsive 

 coefficient, on the basis that the machinery could 

 be designed to produce the required shaft power 

 at whatever rate of rotation appeared best for 

 the propeller. The maximum propeller-disc diam- 

 eter resulting from this procedure was 20 ft. 

 The diameter of the propeller is thus considered 

 fixed at the outset of the design. The rate of 

 rotation n in rpm and the P/D ratio are now to be 

 chosen to give the maximum propeller and pro- 

 pulsive efficiency. 



For a design situation where the diameter is 

 not fixed, it is necessary to determine the optimum 



