EXPERIMENTS ON THE FROUDE. 101 
efficiency of 0.9. The effective power on line eleven is therefore found by 
multiplying the propeller power by this factor. 
In order to compare the results from these trials with the tests of the 
23.5-foot model it is convenient to separate the effective power into surface 
friction power and residual power. Using Tideman’s factors for copper 
in good condition the former may be calculated by the expression 
0.00307fS V"*" 
where the friction factor f=0.00943 and the exponent n=1.827; while 
S represents the wetted surface in square feet and V is the speed in knots 
per hour. The results are given on line twelve of the analysis, and the 
residual power is found on line thirteen. To complete the analysis the 
factor 6 is computed by aid of the formula 
’ 
0.0307 oe Wwe 
where D is the displacement in tons, and Z is the length in feet, while V 
is, as before, the speed in knots. This last step, though useful when an 
analysis is to be used as a basis for new design, is not very interesting for 
our present purpose. 
In order to compare the results of towing the model at the Model 
Basin with the trials of the Manning the residual resistance of the model 
was computed by allowing for the surface friction resistance, taking 
f=0.0092 and n=1.85. 
The residual resistance was then increased by the ratio of 
35 336 
to allow for the change from fresh water in the Basin to sea-water in which 
the Manning was tried. In this way the residual powers of line sixteen 
were developed. ‘The discrepancy between the quantities on line sixteen 
and line thirteen show the difference between work with a model and the 
analysis of a progressive speed trial. 
It may be mentioned in passing that had a hull-efficiency of unity been 
assigned to the analysis of the trials of the Manning there would have been 
a close concordance between the residual resistances so computed and those 
derived from the tests of the 23.5-foot made at the Model Basin; but such 
a concordance would be gale misleading. 
