304 THE INFLUENCE OF SHAPE OF 



gave the best results from a resistance standpoint, for the range of speeds suitable for the 

 present series of models. Each model was tested at four different draughts and consequently 

 represents four types of vessels with varying ratios of beam to draught, and varying pris- 

 matic coefficients. 



The particulars of the parent form are given in the following table, the prismatic co- 

 efficients being for the whole vessel, and a parallel middle body of 32.5 per cent. 



Boui sections Stern sections 



varying varying Bjd 



IS-foot draught U, .734 .7371 



M, .730 .728}- 3.73 



V, .727 .72 ij 



20-foot draught U, .756 .7571 



M, .754 .753 V 2.8 



V, .753 .750j 



24-foot 9 inch draught U, .772 .7721 



M, .772 ~ .772!- 2.26 



V, .772 .772J 



27-foot 9 inch draught. U, .785 .7831 



M, .786 .784}. 2.02 



V, .787 .785j 



For other variations of percentage of middle body and ends the change in prismatic 

 coefficient is shown in Fig. 2, Plate 54. 



METHODS OF PLOTTING RESULTS. 



There has been some discussion in this and other countries as to the various methods 

 used for recording the results of tank experiments. Here it has been customary to give 

 the residuary resistance in pounds per ton of displacement on a speed-length ratio base. 

 Abroad, the Froude constant system or [c] values, or a modification by Baker, or [p] values 

 have been used. Arguments pro and con for each system can be made, but the deciding fac- 

 tor rests in the use to which the results are to be put. If for general comparative purposes 

 in tank work, then the [c] values on a speed-length ratio base, or even [p] values possess 

 certain advantages. In other words, the "ideally best" vessel may be readily deter- 

 mined. If, however, the results are to be used by the practicing naval architects, it would 

 seem that a somewhat more simple form would be preferable, especially as limiting condi- 

 tions of design and the necessity of arriving at the results of minor modifications quickly 

 are the governing factors. 



The following results have therefore been plotted in the form of effective horse-power 



FHP V 



per ton of displacement or — upon a speed-length ratio or — = base, and apply directly 



^ Tons yJL 



to a 425 -foot ship. 



They can be readily modified for any other length by multiplying by the square root 

 of the length ratio up to a difference of 10 per cent either way, and within the limits of, say, 

 2 or 3 per cent either way may be taken as practically constant for a given speed-length 

 ratio. 



