182 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
v=11 11 11 11 11 
v+ V=.7348 7479 . 7614 785 .8076 
e. h. p.=1160 1200 1260 1370 1580 
e, h. p. +E. H. P.=.3244 3357 3525 . 3832 442 
Zp (Fig. 6) = —.51 — 492 —.47 — .432 — .382 
K=1.175 1.2 1.23 1.27 1.31 
S. H. P.a=1716 1827 1970 2220 2569 
Act. Power =1750 1850 1980 2200 2700 
By comparison of the estimated and actual powers it will be seen that the only 
difference of serious amount is with the vessel having the heaviest nominal B. C., 
as was to be expected due to the excess fullness of the after body lines over those 
of an equally balanced ship of the same nominal B. C. This difference existing 
is, however, too great to be credited entirely to this cause. 
There is another explanation for the excess of actual power over estimated in 
the case of the fifth vessel, and this is, in all probability, the correct one. It will be 
noted that the value of K in this case is 1.31 while that of Z, is —.382. Taking 
.382—Log K as the value of Z, corresponding to the total work (load fraction) 
Ses Idle 0). i ii 
Pe corresponding to this is found 
being delivered by the propeller, the value 
from Fig. 6, to be .555. 
Plotting this point on Fig. 7, Plate 78, with vy + V = .8076as ordinate, it will 
fall below the curve of S=.2175 for vessels of type 2, slips of the second order. 
Now assume that the existence of a K loss produces earlier cavitation and that the 
propeller in the case in question is cavitating. The value of eRe: corresponding 
BeEeP? 
a = .8076, is approximately 
.525, and the value of Z, corresponding to this is .29, then the increase log factor 
due to cavitation is .29 —(.382 —.11727) = .02527 and the corrected value of 
I. H. P.,, taking cavitation into account, will be:— 
to the point on the curve S = .2175 whose ordinate is 
Log I. H. P.., = Log I. H. P., + .02527 = 3.43503 
I. H. P.,, = 2723, which corresponds very closely with the actual power, 2700. 
Now K undoubtedly does produce earlier cavitation; therefore, when de- 
e. eios 
IMalals) 2) 
point, corrected for K, will fall well above the basic curve of S for the vessel. 
The remainder of the vessels in the McEntee problem plot above their curve of 
S when the correction for K is made, and therefore are free from any cavitation. 
Assuming that the above is correct, as it undoubtedly is, the supposition that 
model tank trials give no indication of cavitation is incorrect, as in the case just 
discussed the model propeller is clearly indicating ‘‘Dispersal of the Thrust 
Column,’’ a term which will be explained later. 
ciding upon the design point on Fig. 7, it should be so located that the 
