COEFFICIENT CONSTANT AND OTHER COEFFICENTS VARIED. 3 



It will be observed that except for very low speeds, where the smaller wetted 

 surface associated with the fine midship sections has predominating influence upon 

 resistance, the least effective horse-powers are found associated with the large mid- 

 ship section coefficients. In fact, for the vessels of large block coefficient less power 

 would be required with midship section coefficients greater than unity. I think, how- 

 ever, we may safely say that the small powers found associated with the large mid- 

 ship section coefficients are not due entirely to the large coefficients. The larger 

 midship section areas resulting from the larger coefficients enable us to obtain finer 

 ended vessels, or vessels of finer longitudinal coefficient, and this more advantage- 

 ous longitudinal distribution of displacement for the speeds at which tested helps 

 to reduce the power. 



I come now to shallow water results. The depth of water over the false bot- 

 tom was 20 inches while the draughts of the models, as shown in Table I, varied 

 from a little over 13 inches to about 15 inches, leaving comparatively small clear- 

 ance under their keels. 



It was found impossible to obtain consistent results for these models if they 

 were run at speeds appreciably above three knots. At such speeds unstable eddies 

 caused resistance for a given speed to vary radically. Accordingly these models 

 were tested up to three knots only, and even then the results were materially more 

 erratic than in deep water and required some cross-fairing of the experimental spots 

 to make them consistent. 



In Plates 8, 9, 10 and 11 will be found shallow water results expressed as 

 curves of effective horse-power for vessels 300 feet long, of 3,857 tons displace- 

 ment, and in water 25 feet deep. Examination of these curves discloses the fact 

 that in this shallow water the large midship section coefficients are at a disadvan- 

 tage. Except for the highest block coefficient the curves of effective horse-power 

 arrange themselves in order with the smallest midship section coefficient at the 

 bottom. 



There is another material departure from the deep water results. In deep 

 water, broadly speaking, the finer the block coefficient the less the effective horse- 

 power, but in the shallow water the effective horse-power at ten knots, for instance, 

 is very materially greater for the vessels having a block coefficient of .56 than for 

 those having a block coefficient of .64. 



The differences between deep and shallow water are perhaps more clearly 

 brought out by Plate 12, which shows curves of residuary resistance in pounds per 

 ton plotted on 



V 



y/T 

 Needless to say that the higher curves for each block coefficient are for shallow 

 water. The difference is quite remarkable. 



I should like to explain the shallow water results, but this is almost the first sys- 

 tematic investigation which we have been able to make of shallow water resistance 

 and I do not feel that the data so far obtained are sufficient to enable any final con- 

 clusions to be drawn. 



