350 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 59.17 



x'= R/RMax 



Fig. 59. N Variation op Modified Torque Coeffi- 

 cient OF A Screw Propeller With Dimensionless 

 Radius, on a Basis of Dimensionless Radius, at 

 Four Angular Positions 



or by all the blades of a screw propeller can be 

 predicted in the design stage. Its cost, for each 

 new ship or for the first of a class, would be 

 insignificant compared to breaking a shaft on a 

 single-screw vessel, towing the ship home, and 

 replacing both propeller and shaft. 



It is explained in Sec. 17.6 of Volume I that 

 the thrust and torque variations in question arise 

 principally from non-uniform wake and irregular 

 inflow velocity at the propeller. Techniques are 

 available in many model basins whereby the 

 actual velocities can be measured across propeller- 

 disc positions (without the propeller working) 

 and the wake velocities and fractions can be 

 calculated. 



At the present time there is no known precise 

 method for calculating the thrust and torque 

 variations directly from the wake-survey diagram. 

 However, A. J. Tachmindji of the David Taylor 

 Model Basin staff has calculated the vibratory 

 forces from two wake-survey diagrams derived 

 from a model of the Victory ship U.S.N.S. Lt. 

 James E. Robinson. 



These diagrams, reproduced in slightly modified 

 form in Figs. 60.1 and 60.J, are different from the 

 others described and illustrated in Sees. 11.6, 

 11.7, and 60.6 in that they carry three sets of 

 numerals at each observation point. The first set 

 indicates the wake fraction, derived from the 



longitudinal component of velocity approaching 

 the propeller-disc position. The second indicates 

 the radial component of velocity, and the third 

 the tangential component, all in fractions of the 

 ship speed V. Since it is customary, in non-axial 

 flow, to resolve the actual inflow velocity in a 

 direction normal to the blade axis, indicated in 

 diagram 2 of Fig. 17.C of Sec. 17.7 of Volume I, 

 the radial component can be ignored in this 

 analysis. However, the radial component is not 

 to be confused with the transverse component of 

 Figs. ll.E and 60.L. The latter corresponds to the 

 vectorial addition of the radial and the tangential 

 components. 



In the referenced analysis and calculations, 

 Tachmindji made use of the following assump- 

 tions: 



(a) The instantaneous forces on a blade element 

 are those of the steady state for the same velocity 

 conditions 



(b) The forces on a blade are not influenced by 



Thrust Eccentricit\j 



Thrust Position 

 Blade is VerticQ 

 ot Zero decrees 



Thrust 



Blade Vertical 



20 

 Blade- 



40 

 Position 



60 

 Anqle, 



Fig. 59.0 Thrust Variation for Part op a Pro- 

 peller Revolution on a Victory Ship, U.S.N.S. 

 Lt. James E. Robinson, A = 8,268 Tons 



