Contours of constant values of w^, w^, and Wy, are shown in Figures 72 through 77 for the 

 five parent models and the two stern variations of the 0.70 Cg design. 



In designing a propeller for a ship, we are interested in the fore and aft and transverse 

 components of the wake. The total transverse wake w^^, compounded of the values of w and 

 Wf^, is shown for the seven models in Figures 78 through 84. 



Figure 75 shows that the wake in the fore and aft direction has the same general pat- 

 tern for all block coefficients; there is a steady increase in the wake values with increase 

 in block coefficient, and the same is generally true for the transverse wakes in Figures 81 

 through 84. 



One rather important feature of the transverse wake pattern for the three different 

 stern designs. for the 0.70 Cg model should be noted in Figures 78, 79, and 80. In the V-stern 

 model, there is a strong upward component over most of the disk except for an inward and 

 downward component near the centerline immediately above the propeller. For the parent 

 form, intermediate between V and U, there is an indication of a definite rotation in the wake 

 below the propeller centerline (Figure 78) and with the more pronounced U-stern, this rotation 

 seems to be definitely established (Figure 79). Flow tests carried out in the circulating 

 water channel at the Taylor Model Basin have shown that when a model has excessively U 

 stern sections, a definite vortex may leave the bilge line some distance ahead of the propeller 

 and extend aft right through the disk. In such cases, this downward flow ahead of and into 

 the propeller may cause cavitation with consequent noise and vibration. It is therefore wise 

 to avoid a very hard bilge radius aft when using U sections, and it would be good practice to 

 carry out flow tests before deciding on the final shape of the aft end sections of fuller ships. 



The principal.uses of the wake data are in the design of the propeller and the calcu- 

 lation of the variation in thrust and torque on the blades. 



The average circumferential wake around a circle of any particular radius within the 

 propeller disk can be found from such diagrams, and from this the appropriate pitch and blade 

 section can be determined. Fowever, in actual operation, the propeller section at that radius 

 will meet constantly changing velocity conditions in the course of a revolution and so experi- 

 ence constantly changing thrust and torque forces. Integrating such forces over the blade will 

 give the variation of thrust and torquQ on that blade during a complete revolution, and summing 

 these forces for all blades will give the variation in total thrust and torque on the whole 

 propeller. While the forces on a single blade will vary over 360 deg, the pattern for the 

 whole f)ropeiler will repeat itself as the blades successively reach the same position. Thus 

 for a 4-bladed propeller, the pattern of thrust and torque variation will repeat every 90 deg; 

 in other words, at blade frequency. The importance of these variations in thrust and torque 

 is that they are one of the causes of hull and machinery vibration; the varying pressures 

 around the blades cause varying pressures on the neighboring hull structure, and the varying 

 force and torque are also transmitted through the shaft and stern bearings to the hull and 



XI-17 



