176 THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 
deduction factor, but this does not convey all that ‘“K’”’ represents, which, in 
fact, are the losses which have just been given. 
The K factor has the minimum values for hulls of type 1, but these values 
for this type increase very rapidly as the tip clearance diminishes after a certain 
limit of tip clearance between the hull and the circumference of the propeller disc 
has been reached, this distance, so far as the author can determine, being de- 
pendent upon the draught of the vessel and the height of this minimum distance 
above the base line. The formula for obtaining this mean tip clearance is expressed 
H+10H* 
py UL 2 C= 
where H™ (wing screws) = (Actual tip clearance X 10)+ 
_ (Actual height of hub center above base line), and H= (actual immersion of upper 
blade tip in feet X 14) + (actual height of tip above base line, in feet.) 
Hulls of type 3 stand next in order of loss, while those of type 2 are the heav- 
iest losers, single-screw ships of type 3 having the same loss factor as twin-screw 
ships of type 2. 
Values of K for the different classes of hulls are shown on Fig. 3, Plate 74, the 
ordinates being mean referred tip clearances for type 1, while the abscissa values 
are values of K block coefficient, K. B. C., which are obtained from Fig. 1 by 
taking the intersection of the line through the plotted point Nominal B.C. — 
B+L. L. W. L. and the abscissa point of unity, with X, W or Z, according to 
whether the vessel is a wing or single-screw ship. The B. C. value of this inter- 
section is the K. B. C. for the vessel. The values given by Fig. 3 hold very closely 
for all vessels of what may be considered normal dimensions. For vessels having 
extremely great ratios of length of after body to draught, they may possibly de- 
crease to some extent, but there are no data of sufficient accuracy to enable this to 
be stated definitely. 
THE SCREW PROPELLER. 
The vessels which are selected to be used must all have propellers having the 
same radial distribution of projected area but of several different projected area 
ratios. The vessels selected by the author were of types 1 and 3, but of different 
B. C.’s and ranged from a S. B. C. of .375 to .60, from a destroyer to a dread- 
naught, while the projected area ratios, measured outside of a circle of two- 
tenths the diameters of the propellers, varied from .6 to .328. 
The blades were oval with the widest part of the projection at two-thirds the 
radius from the center, while the blade peripheries outside the widest part, meas- 
ured on the arc, were well rounded. 
The ships, at trial, were just out of drydock, thoroughly cleaned and painted, 
and were run over the measured mile in good weather. Each point of the trial 
curves was obtained by at least three runs at the same or approximately the same 
revolutions, two runs in one direction and one in the opposite. The highest point 
of the performance curves was obtained by five runs. 
The propellers of all the ships were of manganese bronze, machined to a true 
