Free Surface Effects tn Hull Propeller Interactton 
As already noted in Section 3.3 (see Fig.5) , the measured 
values fallconsiderably short of the calculated ones , presumably 
due to viscous effects at the stern which reduce the wavemaking effec - 
tiveness of the afterbody . An interesting point to observe is that in 
one case the hull-propeller interaction term is found to be several ti- 
mes larger than the wave resistance of the propeller itself . This can 
be understood by reference to Equation (B 32) which shows that the or- 
der of magnitude of the interaction term is intermediate between that 
of hull wave resistance Rwy and propeller wave resistance Rwp. 
Since the wave resistance associated with the propeller is an indirect 
measure of the loss of efficiency , it follows that the effect of propel- 
ler waves on the propulsive efficiency of the system hull and propeller 
can be significantly larger than one would expect from the observed 
loss of open water propeller efficiency at the same submergence and 
loading . 
III. 6. Interaction Analysis 
IlIl.6.1. Propulsive Efficiencies 
The first step in hull-propeller interaction analysis was the 
empirical determination of the conventional Froude propulsion factors 
by an analysis of the self-propulsion tests in conjunction with the re - 
sults of the hull resistance and propeller performance (open water) 
tests . Using standard definitions and symbols , the factors in question 
are : the hull efficiency 7, the equivalent open water efficiency 70 
and the relative rotative efficiency 1p Which combine to yield the 
propulsive efficiency = 
3 
i 
RV Fi 2nnQ., 
= yy ™R (15) 
In the present context , the factor of primary interest is the hull effi - 
ciency "H which combines the effect of thrust deduction fraction t 
and the effective wake fraction we. 
GA. a a eh ate Bae a (16) 
Unfortunately , the breakdown of propulsive efficiency into various 
factors is not unique (except for the fraction t ) , but depends on the 
somewhat arbitrary definition of an ''equivalent'' open water propeller 
condition . The common alternatives are the thrust identity and the 
torque identity methods . In order not to prejudice our results by the 
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