Free Surface Effects tn Hull Propeller Interactton 
cations invariably caused by viscous effects behind the hull, we first 
compared the calculated and measured wake in reverse” motion, see 
Figure 6. The measured values were obtained from calibrated wake 
wheels directly as circumferential averages at ten discrete radii. The 
calculated values based on thin ship theory (see Appendix B5, espe- 
cially Equation (B56)) were available pointwise in the propeller plane 
and were numerically averaged along the circumference at various 
radii for the ease of comparison with measurements. It is encourag- 
ing to observe in Figure 6 the fair agreement between theory and ex- 
periment, the discrepancy being nowhere larger than 0.03. In par- 
ticular, both the mean effect of Froude number and the general varia- 
tion with radius are correctly predicted by theory. However, the mea- 
sured wake shows some erratic oscillations of unclarified origin at 
the outer radii. 
Figure 7 shows an analogous comparison of calculated and 
measured wake in forward motion, Here we cannot expect direct 
agreement between experiment and theory since the former contains 
a substantial viscous component not included in the latter. However, 
if we subtract the calculated from the measured wake, we notice that 
the remainder is relatively insensitive to Froude number (see Fig. 7) 
as we would expect of the true viscous component. This may be inter- 
preted as indirect evidence that wave effects actually present in the 
measured total wake are of the same order of magnitude as predicted 
by thin ship theory. This is quite encouraging, especially in view of 
the relatively poor agreement between calculated and measured values 
of wavemaking resistance. 
For the sake of completeness the conventional ''potential"' or 
zero Froude number component of wake as calculated by theory (Ap- 
pendix B.5, Equation (B54)) is also plotted in Figure 7. It is by de- 
finition independent of Froude number. In view of the foregoing, the 
trichotomy of nominal wake in potential, wave and viscous compo- 
nents as displayed in Figure 7 can be regarded as quite meaningful. 
Evidently, the wave effects are by no means negligible as commonly 
assumed, 
* Incidentally, by virtue of the longitudinal symmetry of our hull the 
"stern'' wake in the propeller plane x =x, in reverse motion is equi- 
valent to the ''bow'' wake in the reflected propeller plane x = -Xp in 
forward motion, 
1855 
