TRANSVERSE SECTIONS UPON RESISTANCE. 



307 



APPENDIX. , 



The effective horse-power of any ship taken as a model will be 

 E.H.P., = (AS, K'''+ ^wF.) X .00307. 



/, = Coefficient of friction for model ship. 



S, = Wetted surface of model ship. 



K, = Speed of model ship in knots. 



./?w = Wave resistance of model ship in pounds. 



At the same speed-length ratio the effective horse-power of a similar ship whose dimen- 

 sions have the linear ratio /I to the model will be 



B.H.P., = (/. S,:^' {V,^-T'' + R^2.^ V^n X .00307 



= (AS, V-''' V, '■'' -H i?w F. ;i '•= ) X . 00307 



.-. E.H.P., = (E.H.P.,;i3.5 _ (y;;^3.5 _ /^x^.4i5) s, V,'-^' X .00307. 

 In terms of effective horse-power per ton of displacement, since displacement varies as T}; 



E.H.P., E.H.P., 



Dispt. 



Dispt 



^A-.-(/;;..>-/.x-)5|j-j^-«x. 00307 



Where the similar ship is only 10 to 12 per cent longer or shorter than the model ship, 

 the second term can be neglected without introducing any serious error, and the effective 

 horse-power per ton obtained by multiplying the similar quantity for the model ship by ^■^. 

 The power so obtained is for the same speed-length ratio as in the model ship. 



As an illustration, the following table has been prepared for a model ship of 425 feet: 



At the deepest draught the ratio of wetted surface in square feet to displacement in 

 tons does not exceed 2.9 for the model ship, and for the lightest draught rarely exceeds 4.0. 



At the high speed-length ratio for this type of .68, the value of V^-^^ is about 2,000. 

 Using these values for a similar ship where ^ = .882 the value of the corrections becomes : 



-f .00014 (4 X 2000 X .00307) = .0034. 



At the lowest draught the effective horse-power per ton of displacement of the model 

 ship is about .30. If the simplified expression is used, the effective horse-power per ton for 

 a similar ship will be .3 X .94 = .282 ; and if the complete expression be used it will be 

 .3 X .94 H- .0034 = .2854 at the same speed-length ratio. 



In this extreme case the correction amounts to a trifle more than one per cent. 



