High Speed Displacement-Type Hulls 563 
Average Results for Models 
In a first attempt to examine the effects of these design parameters, the measured model 
resistances for a varied group of about 30 round-bilge forms were plotted as in Figs. 1-11. 
The resistance coefficient © for a form of length L = 100 feet is throughout given in 
terms of the displacement/length ratio A/(0.01L)* or, what is the same thing for L = 100 
feet, the displacement A in tons. Figures 1-6 give the resistance coefficients for a series 
of speed/length ratios V/\/[, from 1.7 to 4.0, with the specific value of the beam/draft ratio 
B/d shown for each form. These diagrams show no clear evidence of any systematic varia- 
tion in resistance with 3/d at any point in.the speed range. In Figs. 8-11 the same results 
at V/V/L = 1.7 are replotted, with the specific values of block coefficient, angle of 
entrance on waterline, position of LCB and length,/beam ratio shown in turn. Again there is 
no clear evidence of any predominant form parameter. 
If resistance data were available for a larger number of hull forms, say about 150, it 
should be possible to determine the influence of each of the principal form parameters by 
carrying out a multilinear regression analysis, using a high speed computer. With the 
limited data presently available this is not possible, and as a preliminary step mean lines 
have been drawn in each of Figs. 1-6. In addition, lines representing 5 and 10 percent 
deviation from these mean lines have been drawn. These show that most of the results lie 
within 5 percent of the mean lines. These mean lines are replotted in Fig. 7 to give, for 
ships of length 100 feet, average resistance coefficients for speed/length ratios from 1.4 to 
3.5 for a range of displacements from 50 to 200 tons. It is suggested that for preliminary 
estimates these average values can be used with discretion to give the resistance generally 
within 5 percent for good standard hull forms. 
Effect of Beam/Draft Ratio and LCB Position 
The data in Figs. 1-11 indicate that it is probably unwise to assume the predominance 
of one parameter, say B/d, as has been done previously [1], and to ignore other form param- 
eters. However, an attempt has been made to assess the effect of beam/draft ratio by com- 
paring resistance coefficients for two models, each run at two displacements, which had 
different B/d values but were otherwise similar. The comparison is shown in Fig. 12 and 
Table 1, and suggests that at V/\/L = 2.5 resistance varies approximately as the cube root 
of the beam/draft ratio but at V//Z, = 3.0 the difference is much smaller. A similar com- 
parison for two forms having a chine instead of a round bilge, and a fairly high chine line 
forward, given in Fig. 13, showed a much smaller variation with beam/draft ratio. Although 
these data are inadequate, they have been used to provide preliminary guidance on the 
effect of beam/draft ratio, as shown in Fig. 14, 
Table 1 
Comparison of Resistance Coefficients for Two 100-Foot Round-Bilge Models 
with Different B/d Values 
Se i ee ©) 100’ (B-A)/A 
B-A 
(B-A)/A 
