THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 223 
MAIS SS PyAuia : 
eh. p.= eat = .10593 TEA .328 From Fig. 8. 
ey benpe i 
ue .0057804 IPC, = (05 
E.H.P. = 18.3255 (2 propellers) ih. p. = 27.557 
a = 5 iny(ofeyh | T.S. = 7250 
21 
v = —=— = 3.8341 I—S = .865 
V 30 i 
= Le 7 — 19-75 
V = 21.32 P= .65835 Fs 
= BOs25 60833’ 
PAN! 
E. T., x DA. => 3.3360 
As the model propeller performance can be analyzed by the laws of comparison 
and as the model propeller can be designed by the use of these laws, it appears that the 
laws of similitude not only apply to the design charts but also to the propeller itself, 
and that if such conformity on the part of the charts is to be taken as the proof of 
their correctness, then judgment in their favor must be given. 
When the propeller is working in the range of ‘‘cavitation of the thrust column,” 
the estimate of power and revolutions of the actual propeller without cavitation must 
first be obtained by comparison with the model propeller and the corrections for cavi- 
tation applied to these results to obtain the final results; this is necessary as the curves 
of cavitating thrusts on the chart are not extended down to such low load and speed 
fractions as those under which the model operates. 
Mr. Smith in his discussion expressed the hope that in time the true theory of 
the propeller may be developed from these charts and a composite method, built up 
from the work of Admiral Taylor and the author, be produced. The author does not 
hope for this but is of the opinion that his work combined with that of Commander 
McEntee forms a complete system of design and checkage as they stand today, the pro- 
pellers designed by the author’s method to be checked in the model tank by the use 
of the ship’s model and the model of the propeller resulting from reduction in size 
according to the laws of comparison. This combination fills a long-felt want as in 
the checkage, the model trials give absolute assurance as to the performance of the 
actual propeller, and enables errors in estimate of K and of S to be corrected, something 
which has not been had in the past until the actual ship with its propeller has been tried. 
In reply to the questions put by Mr. Stevens, the questions will be taken up in 
order, as follows: 
1. Is it to be understood that, in determining the slip B. C. of single-screw ships 
of all three types, no correction for variation of midship section coefficient is to be made? 
And does this also apply to twin-screw ships of types 1 and 2? 
Answer: No correction is made for type 3 nor for either of types 1 and 2. 
2. Is there no correction to be made for variation of midship-section coefficient 
in arriving at the K block coefficient? 
