THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 175 
same gentleman also made the statement that he was able to determine by in- 
spection the proper diameter and pitch of propeller for any given hull. 
It will now be necessary to say a few words concerning the resistance to be 
overcome by the propeller in driving the hull through the water. 
RESISTANCE—-HULL AND APPENDAGES. 
The question of hull resistances being so fully understood, it is only neces- 
sary to state that, for propeller work in developing a system for design purposes, 
only such vessels should be used as have had their resistances determined by 
towing the model of the hull in the Model Basin. 
It does not suffice to tow the model of the bare hull, but this model should 
be fitted with all external submerged attachments in order that the true resis- 
tance of the hull may be arrived at. 
Fig. 2, Plate 73, shows curves of appendage resistance in percentage of bare 
hull resistance, erected on values of speed of vessel+ VL. L. W. L., as prepared 
by the writer from reports of trials of models in the model tank at Washington. 
These curves are rough approximations, as the actual appendage resistance varies 
with the form of hull and the amount of power carried by the hull and also with 
the revolutions at which the propellers are to run, these latter two factors fixing 
the size of shafting, external lengths of shafting and size of propeller struts. 
PROPULSION LOSSES DUE TO TYPE OF HULL AND TO LOCATION OF PROPELLER IN 
RESPECT TO THE HULL. 
The Power Augment Factor ““K.” 
It is a little premature to speak of these losses at this stage of the investi- 
gation as they really cannot be studied until the primary design curves have been 
arrived at, and this result can only be reached by selecting for the early investi- 
gations such vessels as, from the high values of the propulsive coefficients ob- 
tained, are decided upon as having all such losses reduced to the smallest possible 
minimum. 
The losses, other than those due to poor workmanship in the propeller itself, 
arise from the reduction in pressure in the area in which the propeller operates, 
and also from the obliquity of flow of water to the propeller caused by the fullness 
of lines of the hull interfering with the flow. There may also be serious losses due 
to eddying around improperly formed stern and rudder posts, around struts of 
improper section and arrangement, around and between strut bosses and propeller 
hubs, around the hub itself, and to excessive roughness of surface of the propeller 
blades and excessive variation of pitch between different blades of the propeller 
or throughout each blade. Of course these latter named losses are due to poor 
workmanship and can easily be avoided by care in casting, finishing and setting 
the propeller blades. 
The loss factor, or ‘““power augment,” is given the letter ‘““K’”’ to denote it. 
In previous works of the author this letter ‘““K’’ has been spoken of as the thrust 
