THE PROBLEM OF THE HULL AND ITS SCREW PROPELLER. 205 
wake which may occur due to the vessel altering her lines of flotation. This 
variation of wake may be due to a change in trim, causing an increase in wake 
during a portion of her career, but the wake usually returning to normal after an 
upper limiting speed is attained, or it may be caused by the vessel lifting bodily 
in the water, decreasing its submerged fullness and tending to plane. This latter 
action is accompanied by a heavy decrease in wake and a corresponding falling 
off in efficiency in propulsion. 
Vessels which slightly alter their trim at a speed depending upon the ratio 
of v to VL. L. W. L. may accomplish this change in trim without an increase in 
the wake. Such a vessel is vessel H, while another ship of similar type may show 
a decided gain in efficiency during this period of increased wake due to the resulting 
increased thrust of the propeller produced by the increase in wake. Such a gain 
is spoken of as ‘““wake gain.” Such gain is shown by vessel L, following. Vessels 
H and L have effective horse-power curves practically identical in form. Both 
show a slight hump between 15 and 17 knots. The performances on trial of the 
two ships were radically different. The writer was aboard both of these vessels 
and noted that vessel H, which was the lighter and smaller of the two ships, after 
passing 15 knots developed a very peculiar vertical vibration which gave one the 
impression that she was trying to break her back. This vibration disappeared 
after passing 17 knots. Vessel L, on the contrary, passed through the range of the 
hump smoothly and with a total lack of this peculiarity of vibration. Analysis 
of the performance of vessel L shows that when the estimate of power and revolu- 
tions are made for the speeds and effective horse-powers obtained from the model 
tank, the revolutions obtained agree with the revolutions corresponding to the 
actual speeds corresponding to these estimated powers. 
When a vessel lifts bodily in the water and begins planing as she lifts, the model 
tank curves indicate a rapid decrease in the increment of increase of the resistance. 
Should the estimate of power required be made for the e. h. p. and speed given on 
this curve of deflection, a bitter disappointment will be encountered. Rather, 
the curve before inflection should be extended by means of a batten and the v’s 
corresponding to this extended curve of e. h. p. will be the actual speeds obtained by 
using the model tank curve of e. h. p. and v. After planing is accomplished, the 
wake gradually increases to normal as the speed is increased. 
Gain by possible wake increase should never be counted on as a certainty, 
but loss by wake decrease should always be allowed for. 
VESSEL L. 
In the case of this vessel, the estimated powers and revolutions are those 
obtained from the model tank curve while there are also given columns of the 
actual S. H. P.,, speeds with corresponding revolutions, obtained by fairing the 
hump out of the model tank curve and using the new speeds as those to be expected 
rather than those used in the estimate. 
