tests on the parent models with the propellers of standard diameter (= 0.7 H) were run over a 

 complete speed range, and those with the larger and smaller diameters were made only from 

 speeds 10 percent below the service speed to 10 percent above the trial speed. 



The three parent models of 0.60, 0.70, and 0.80 Cg were run at two lighter conditions, 

 with the standard propellers of diameter equal to 0.7W only. The conditions chosen were 60 

 and 80 percent of the load displacement. With the models in the lighter of these conditions, 

 the propellers were just submerged at a speed about 10 percent below the service speed; this 

 was considered essential if reliable wake data were to be obtained. In addition to tests on 

 even keel, the models were also run at 80 percent of load displacement with a trim of 1 per- 

 cent of the LBP by the stern and at 60 percent of load displacement with a trim of 2.5 percent 

 by the stern. These were chosen after reference to much data in the records of the Maritime 

 Administration. The results of the propulsion tests in these conditions are shown in Figures 

 51 through 53. 



Figure 54 shows the variation with diameter in the values of the propulsive coefficient 

 and its various components for the five parent models. The trial and service speeds used 

 throughout the presentation of these propulsion experiments are those derived on the Alexander 

 basis given in Equation (1), page V-14, The wake fraction shown is the Taylor wake fraction 

 calculated on the basis of thrust identity in open and behind the model. Having obtained 

 actual wake fractions from these model experiments, estimates were made from the Troost 

 design charts for Troost-type propellers for all the different conditions in which stock pro- 

 pellers had been used. These showed that any increase in propeller efficiency which would 

 result from such a change was quite small— on the average less that 0.5 percent, the maximum 

 being 1.1 percent. 



Figure 55 shoWs the propulsive efficiency factors plotted against block coefficient. 

 Figure 55b for the standard propellers represents actual test data. The results given in 

 Figures 55a and 55c are for the smaller and larger diameter propellers, respectively, modified 

 to suit the variation of diameter with block coefficient shown in Figure 56. 



Similar curves for the 80- and 60-percent displacement conditions are shown in Fig- 

 ure 57. In the 60-percent condition for the 0.60 C g model, even keel, there was some indi- 

 cation that air was being drawn into the propeller, and it is significant that the wake curve 

 for this model appears to be inconsistent with the other data. 



The principal reasons for running the experiments described in this section were to 

 compare the propulsive performance of the parents with existing modern designs of ships 

 and to give the practicing naval architect guidance on the general effects on propulsive 

 efficiency of changes in propeller diameter, in ship displacement, and in trim. 



As to the first of these, comparisons were made between models of the SCHUYLER 

 OTIS BLAND and PENNSYLVANIA and their Series 60 counterparts. The corresponding 

 pairs of models were run under as nearly similar conditions as possible. Thus the Series 60 

 sterns were modified to give the same aperture and rudder arrangements as in the actual 



(Text continued on page X-18.) 



X-11 



