THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 185 
Before leaving the subjects of estimation of power and of revolutions it should 
be of interest to note that none of the vessels covered by the above problems was 
used to any extent whatever, in the derivation of the curves of design and esti- 
mating. 
The processes of estimating under normal conditions having been explained, 
it is now necessary to take up the same subject under abnormal conditions of load, 
which abnormal conditions are treated under the general head of— 
Cavitation (Fig. 7, Plate 78). 
It has been shown how vessels can be divided into three general types accord- 
ing to the form of the after-body lines. They may also be divided into types 
according to the values of the basic slips, namely :— 
1. Vessels having basic slips of the first order, which include vessels having 
basic slips of about 13 per cent and lower. 
2. Vessels having basic slips of the second order, which include all vessels 
having basic slips exceeding about 13 per cent. 
These distinctions according to the basic slips are encountered with vessels 
of type 2, and, in order to explain them, attention is called to Figs. 6 and 7. 
Many years ago, on the trials of the British destroyer Daring, difficulty was 
experienced in obtaining the contract speed, due to failure of the propellers, such 
failure being accompanied by heavy vibration. The difficulty was finally over- 
come by increasing the projected area of the propellers to quite a large extent. 
After a thorough study of the phenomenon, Mr. S$. W. Barnaby, of Thornycrofts, 
advanced the theory that the failure was due to a lack of flow of water to the pro- 
peller caused by insufficient supply head, this failure resulting in the formation of 
cavities in the water in which the propeller was working. He gave the name of 
“Cavitation’’ to this action and fixed a limit of effective thrust, 1144 pounds per 
square inch of projected area, as that at which cavitation could be expected to 
occur. Later experience has demonstrated that this limit is not correct but that 
it depends upon the projected area ratio; the higher this ratio the higher the effec- 
tive thrust before cavitation happens. Taylor supposes that both tip speed of the 
propeller and effective thrust enter into the solution of the problem. By inspection 
of Fig. 4, curves of design, it will be noted that tip speed increases with indicated 
thrust and with effective thrust as the projected area ratio increases. It is also a 
fact that, in changing from the basic conditions of resistance, for equal powers the 
indicated thrusts vary inversely as the tip speeds. Therefore, with a vessel loaded 
down very heavily beyond the designed condition the tip speeds will, as a rule, be 
less than the designed tip speeds and yet, as experience has taught, ‘“‘cavitation”’ 
may occur. 
It is not the intention of this paper to advance any theories as to cavitation, 
but rather to have the student form his own theories after having had pointed out 
to him the various phenomena that actual performance data have shown to occur. 
For convenience in discussion, the phenomena encountered where the load 
