84 THE RESISTANCE OF SOME MERCHANT 
Cross curves representing the variation of resistance in terms of depth 
of water are given on each diagram; also curves showing the approximate 
speed-length ratio at which the residuary resistance begins to increase as 
compared with deep water. Owing to the nature of the curves and the lack 
of full data, no claim is made for the absolute accuracy of these last curves, 
but they may be taken as representing the region of speed above which one 
may expect the resistance to show a marked increase. As a matter of 
interest a second set of curves showing the speed-length ratio in terms of 
depth, at which the residuary resistance is double that in deep water, have 
been added. Any other percentage increase may be readily obtained from 
the above curves. 
Plates 27, 28, 29, 30, represent the results obtained for a typical set of 
merchant ship forms varying in prismatic coefficient from about 0.5 to 0.85. 
It will be noticed that the first “hump” in the curve for a given depth occurs 
at nearly the same speed for all types. There is, however, a tendency for 
the maximum to occur at a higher speed as the form of the vessel becomes 
finer. © 
Plate 28 shows the curves for two vessels of the same dimensions and 
practically the same prismatic coefficient or curve of sectional areas, but 
differing only in area of midship section, and hence block coefficient. No. I 
is a vessel with a full section and small rise of floor, and No. II one with a 
finer section and large rise of floor. The curves of residuary resistance per 
ton of displacement in shallow water have the same characteristics as those 
of deep water; but as the humps occur at earlier speeds and are much more 
pronounced in the shallow water, the form with the finer midship section 
appears to have an advantage over the fuller type, particularly at what may 
be’termed the “practical’’ speeds for this form. 
Plate 29 shows similar sets of curves for a slightly fuller vessel than the 
previous one. ‘The same general characteristics appear to hold in both cases. 
The cross curves for models No. II in Plates 28 and 29 have been omitted 
for sake of clearness. 
Plate 30 shows a set of curves for the full type of cargo boat; but these 
were not carried up to the critical speed owing to the enormous increase of 
resistance. 
The curves, Plate 31, are for a broad vessel with practically V-shaped 
sections throughout. In this case the vessel was tried at two draughts. 
It will be noticed that the “humps” occur at the same speeds for each 
draught, also that these are not so pronounced as in the types with fuller 
midship sections. - This is probably due to the fact that the mean draught 
of the midship section is much less in this than in the previous cases. . 
