Sec. 59.16 



PROPULSION-DEVICE PERFORMANCE 



347 



course of a conversion project, to estimate the 

 screw-propeller thrust long before the propeller is 

 selected or designed. Only the proposed ship 

 speed may be known, or at most the power and 

 rate of rotation of the engine. 



A rule-of-thumb often used involves the dimen- 

 sional relationship T = kPs , where in English 

 units T is in lb, Ps is in horses, and k varies from 

 30 on a tug exerting bollard pull at zero speed to 

 a range of 10-15 for normal propulsion. For high- 

 speed racing motorboats it may drop to 3 or less 

 [Spencer, D. B., Pac. N. W. Sect., SNAME, 

 2 Feb 1951]. 



It is pointed out in Sec. 34.7 of Part 2 and Sec. 

 70.5 of Part 4 that there is a rather definite 

 relationship between the thrust T and the torque 

 Q developed by a screw propeller when working 

 imder any given set of relatively steady condi- 

 tions. The torque to be exerted by a propelling 

 plant is readily determined from assumed or 

 known values of the shaft power and the angular 

 rate of rotation n. The thrust may be estimated 

 with equal facility by one or the other of the 

 following 0-diml equations, depending upon the 

 information available at the time of the estimate: 



Thrust-torque factor 



TD 



—p^ = a value derived from propeller charts 



Kc 



from open-water model propeller 



data. 



The latter relationship is easily derived from 

 available plots, provided the pitch ratio and other 

 principal characteristics of the propeller approxi- 

 mate those which will probably be used in the 

 contemplated design. The TD/Q ratio varies only 

 slowly with the real-slip ratio Sg or advance 

 coefficient J. In any case the open-water data 

 give appropriate values for any desired real-slip 

 ratio or advance coefficient. 



At a later stage in a ship design the propeller 

 thrust is estimated from T = R/{1 — t), where 

 the hull resistance R and the thrust-deduction 

 fraction t are estimated as described in Sees. 

 57.4 and 60.9, respectively, or are found from 

 self-propelled model tests. 



Using the ABC ship as an example for the 

 successive stages of this estimate, the rule-of- 

 thumb method, with /c-values of 10 to 15 for 

 normal propulsion, and for an estimated shaft 

 power of 17,000 horses, gives a range of from 



(10) 17,000 = 170,0001b to (15) 17,000 = 255,000 

 lb thrust. 



Selecting a TD/Q value from the Prohaska 

 logarithmic propeller chart in Fig. 70. B, by the 

 method diagrammed in Fig. 70. A, gives the ratio 

 TD/Q = 5.8 for a P/D ratio of 1.00. Again 

 taking the power as 17,000 horses for this example, 

 and the rate of rotation from Sec. 70.6 as 109.2 

 rpm or 1.82 rps, the expected torque at the de- 

 signed speed is found by 



2im 



17,000(550) 

 6.2832(1.82) 



= 817,600 ft-lb. 



With a propeller diameter of 20 ft, the thrust 

 prediction works out as 



T = 5. 



7 817,600 

 V 20 



= 237,104 lb. 



From the open-water test data of the stock 

 model propeller selected for the transom-stern, 

 TMB 2294, presented in Fig. 78.Mc, the value of 

 the fraction K^/Kq at a real-slip ratio of about 

 0.25, corresponding to a J-value of 0.735, is 

 found to be 



TD 

 Q 



0.148 

 0.0265 



= 5.58 



Hence 



T = 5.58 



817,600 

 20 



= 228,110 1b. 



From Sec. 70.6 the predicted propeller thrust, 

 worked out at a later stage of the ABC ship 

 design, is only about 193,500 lb. The thrust 

 derived from the self-propelled model test, taken 

 from Fig. 78. Nb for the 20.5-kt designed speed, 

 is only 172,170 lb. The range of prediction covered 

 by the preceding examples is rather large, but at 

 least the estimated values are conservative. 



59.16 Relation Between Thrust at the Pro- 

 peller and at the Thrust Bearing. For a propeller- 

 thrust estimate of the type described in the pre- 

 ceding section, it is usually assumed that the 

 thrust bearing takes the whole propeller thrust. 

 Actually, the thrust-bearing load equals the 

 propeller thrust only when the friction effects in 

 the bearings between the propeller and the forward 

 end of all elements attached to and working with 

 the shaft are neglected, and when the shaft 

 declivity in the running condition is zero. Other- 

 wise the thrust-bearing load equals the propeller 

 thrust plus or minus an axial component of the 

 weight of the propeller, shaft, and all engine 



