186 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
fraction aa is less than unity will be spoken of as ‘“‘cavitation of’’ or “dispersal 
of the thrust column” of the propeller, while for those phenomena met with where 
the load fraction exceeds unity the term ‘‘cavitation of the suction column”’ of the 
propeller will be employed. 
Dispersal of the Thrust Column. 
It has already been stated that vessels can be divided into types according to 
the magnitude of their basic slips and that vessels of type 2 exhibit the variations 
due to changes in this magnitude. 
Turn now to Fig. 6. Without cavitation, it has already been pointed out that 
to obtain the value of the apparent slip for any other than basic conditions the 
equation is:— 
ILE <<) Or 
ee me 
or Log s = Log S + Log K + Z; — Z,, where Z; is taken from curve I. 
With hulls of type 2, at any ae value of load fraction there will be a corre- 
sponding value of speed fraction —, at which the quality of the wake will change 
a 
materially; in the cases of vessels of type 2, having basic apparent slips of the 
first order, two changes will occur and there will be two corresponding values of 
y + V. The first change with such vessels will be from a basic slip of the first 
order to one of the second order, while the second change will be from slips of the 
second order to cavitating slips. The first change will occur without loss of effi- 
ciency of propulsion when the value of K equals unity. When the value of K is 
greater than unity, the change of slips will be accompanied by an increase in value 
of K and all the phenomena of cavitation will occur. When K equals unity the 
second change will be accompanied by a rapid loss in efficiency of propulsion. 
When the first change occurs, if it were known what the new basic S were to 
be, it could be inserted in the equation for apparent slip in place of the original 
basic S, and the values of Z; be taken from curve 1, Fig. 6, as before. In place of 
making this change of basic slip, available data ‘hese given the curve marked ‘‘2” 
from which, using the value of the original basic S, the new values of Z,; are oe 
For this curve of Zs, the values of Zs; vary as 3.09 Log (ee 
Now assuming that when the actual apparent slip at any gross load fraction 
has reached the value of the basic S, apparent slip conditions are on the verge of 
change, 
Log s = Log S+ Z; —Z, + Log K 
Sa S 
95 Ag, ae yore VRC A 
