were calculated using the resistance of the model as measured during the self-propulsion 

 tests, in association with the corresponding values of torque and rpm. This is a correct 

 measure of propulsive efficiency since all the quantities apply to the model in the actual 

 condition of test. However, for a variety of reasons, the actual resistance of the model at 

 that time may not agree exactly with that measured during the original resistance tests, and 

 on which the ehp values are based. The model has a keel piece, rudder, and propeller hub 

 installed, and the surface and shape of the wax model may have changed slightly. The dhp 

 values deduced straight from the torque and rpm will therefore correspond to a different 

 resistance from that used to calculate the ehp values. For this reason, the dhp values have 

 been calculated from the ehp values, using the propulsive efficiencies measured during the 

 propulsion experiments but ignoring the actual torque and rpm values, i.e., 



ehp 

 dhp = 



Propulsive efficiency 



In this way there is no inconsistency between the dhp values and the ehp values previously 

 given from the resistance experim.ents and made when the models were new and in the bare- 

 hull condition with no appendages and a new, clean surface. 



The choice of a 600-ft ship to illustrate the propulsion tests was made principally 

 because it was considered more representative of modern ships than the 400 ft chosen for 

 the resistance presentation. This latter is made in coefficient form and may be corrected 

 quite easily to any other desired ship length; most of the resistance data published else- 

 where are on the 400-ft basis. The propulsion data, on the other hand, cannot be so corrected, J 

 and must be completely recalculated for any other length. Moreover, unless the model has 

 been run at a num.ber of loadings, it is possible to make such correction only for a small 

 change in length. 



The results are presented in detail in Tables 27 through 31. The change in wake 

 fraction with movement in LCB affects the optimum pitch ratio for the highest propulsive 

 efficiency, but series chart calculations show that this effect does not amount to more than 

 1 or 2 percent. Cross curves of dhp similar to those of Tc^have been drawn for the same 

 four chosen speeds. The data are tabulated in Table 32 and the cross curves are shown in 

 Figure 38. On these have been drawn the loci of optimum LCB location to give minimum dhp. 

 Those already derived from the resistance data to give minimum ehp are also shown for purposes; 

 of comparison. 



In general, the minimum dhp is not so well defined as the minimum ehp, but, within 

 practical limits, the dhp and ehp results agree in defining the same optimum LCB loci for 

 each set of models except the fullest - that with Cg = 0.80. For this set, the dhp results 

 indicate reducing power the further forward the LCB , even beyond the extreme position of 

 3.51 percent used in the experiments. The ehp results indicate an optimum location at about 

 2.50 percent of the length forward of midships, and this is a more practical answer - any 



VII-4 



