Sec. 70.26 



SCREW-PROPELLER DESIGN 



61- 



value of D by the use of existing propcllcr-design 

 charts, subject to the comments given earlier in 

 Sec. 70.9, or by model experiment. 



On the ABC ship, no shaft calculations were 

 made for the transom-stern design, so the hub 

 diameter was assumed as 0.18Z), the same as 

 for the stock model propeller. The hub is faired 

 into the rudder horn as shown in Figs. 66. Q, 

 67.U, and 74.K. 



70.26 Calculating the Thrust-Load Factors 

 and the Advance Coefficients. With some of the 

 primary characteristics fixed it is possible to 

 start the design of the wake-adapted propeller by 

 Lerbs' short method. For the ABC propeller- 

 design problem only the thrust T, the ship speed 

 V, and the maximum propeller diameter D are 

 fixed. The rate of rotation n is to be chosen to 

 give maximum efficiency. In many design cases 

 there will be limitations on rpm as well, due to 

 restrictions on the size of reduction gears or by a 

 requirement for a given rate of rotation at a 

 given power with a direct-drive diesel engine. In 

 these cases, the designer accepts the limitations 

 and attempts to attain the maximum propeller 

 efficiency possible, even though it is less than the 

 optimum. 



For the ABC ship a rate of rotation to give 

 maximum propeller efficiency was chosen in 

 Sec. 70.22, on the basis of the Wageningen Series 

 propeller data as laid down on Prohaska's loga- 

 rithmic charts. These indicated an t/o of 0.72 for 

 an n of L620 rps or 97.2 rpm. 



For Lerbs' short method it is best to work on a 

 thrust basis. Because of the assumptions involved 

 in this method, described in Sec. 70.21, correspond- 

 ing formulas on a power basis do not yield equally 

 good results. When using the rigorous method 

 described by Lerbs in his referenced paper on 

 moderately loaded propellers, either a thrust 

 basis or a power basis can be employed. Thus if 

 the designer is limited to a given power plant, he 

 must convert the shaft power to effective power 

 Pe and this Pe to thrust. In the design of the 

 ABC ship, there is no maximum limitation on 

 shaft power, so the design is started by using the 

 thrust obtained from the model resistance test 

 with appendages, calculated in Sec. 70.22. 



The following data are known: 



Diameter, D^a. = 20 ft Radius Rm.. = 10 ft 



Ao = 7ri?Ma. = 314.16 ft' 

 Designed ship speed V = 20.5 kt = 34.622 ft per 



Pe = 10,078 horses, from model resistance test 



Thrust T = 172,148 lb 



Thrust-deduction fraction I = 0.07; 1 — / = 0.93; 



from model self-propulsion test for 20.5 kt 

 Effective wake fraction w) = 0.195; I — w = 0.805; 



from model self-propulsion test for 20.5 kt 

 Rate of rotation n = 1.620 rps or 97.2 rpm; 



from Sec. 70.22 

 Number of blades Z = 4 

 Average local wake fraction at the various radii, 



w^. , from the wake survey on TMB model 4505, 



derived by the method described subsequently 



in this section 

 p = 1.9905 slugs per ft^ for salt water at 59 deg F 

 0.5p = 0.99525 slugs per ft'. 



The next step is to calculate the thrust-load 

 coefficients and advance coefficients. The initial 

 calculations are all based on non-viscous flow. 

 It is therefore necessary to convert the thrust 

 calculated from the model test to a non-viscous 

 thrust. A good approximation for this is given 

 by the following relationship, which was deter- 

 mined by considering the viscous forces on a 

 blade element: 



Tj = 1.03T 



where T is the customary thrust in viscous flow 

 and Ti is the thrust in non-viscous flow. 



Ti = 1.03(172,148) = 177,312 lb 



Speed of advance Fa = F(l - w) = 34.622(0.805) 

 = 27.871 ft per sec 

 Thrust-load coefficient 



C TL — 



Tr 



Tr 



i0.5p)AoVl (0.5p)7r(fiMa.)'Fl 



= 177,312 ^7 



0.99525(3 14. 16) (27. 871)' 



The thrust-load coefficient based on ship speed is 



('• TLjS — 



{0.5p)AoV- 



177,312 



0.99525(314. 16)(34.622) 

 The absolute advance coefficient J^b, or 



= 0.4731 



X = 



27.871 



V, 



wnD (3. 14) (1.620) (20) 



= 0.2738 



The absolute advance coefficient based on ship 

 speed 



V 



34.622 



wnD (3.14)(1.620)(20) 



0.3401 



