LOGARITHMIC SPEED-POWER DIAGRAM. 337 



The diagonal lines, which may be called reference lines, are laid out to 

 such slopes as to give numbers proportional to the following powers of 

 speed: — 2, 2.83, 3, 4, 6, 6.735, 6.866, and 7. Of these, the first is propor- 

 tional to length or any linear dimension. The fourth power of speed is 

 proportional to wetted surface or any area measured on the ship. Displace- 

 ment or volume is proportional to the 6th power. 



For any type of ship for which full-power data is available a diagram 

 such as Fig. 2, Plate 130, may be constructed, which are to be superposed on 

 Fig. I, Plate 129. Ordinarily tracing cloth would be used for the diagram 

 corresponding to Fig. 2, Plate 130. On this is seen a diagonal marked 

 "Shifting Line." This is to be placed upon the "Shifting Line" on Fig. i, 

 Plate 129. It will now be observed that a vertical line on Fig. 2, Plate 130, 

 marked "Displacement" intersects, at some point, the line marked similarly 

 on Fig. I, Plate 129. To the left of this point on the margin of Fig. i, Plate 

 129, will be read the displacement of the vessel. Similarily length may be 

 read to the left of the intersection of the two length lines; likewise for wetted 

 surface. Other desirable dimensions may be shown, such as beam and 

 draught. Next, the curve marked effective horse-power on Fig. 2, Plate 130, 

 reads directly for any speed, the corresponding effective horse-power by 

 reference to the speed scale at the bottom and the power scale to the left of 

 Fig. I, Plate 129. The assumption is that the effective horse-power varies 

 as the 6.866 power of the speed, for two similar vessels at corresponding 

 speeds. 



If now a similar boat of another length or displacement is desired, it is 

 only necessary to shift Fig. 2, Plate 130, on Fig. i, Plate 129, being careful 

 that the shifting lines still coincide, until the desired displacement or length 

 is obtained. From the effective horse-power curve, the power required for 

 the new vessel may be read directly. 



Assuming that the propulsive efficiency is the same for similar ships it 

 will be equally correct to plot the indicated horse-power, as has been done 

 on Fig. 2, Plate 130. 



The convenience of this method of coaversion from one size to another 

 is quiet evident. For each boat or series of boats a separate sheet should 

 be used. 



To further illustrate and to show some results of comparison available 

 by this method Figs. 3 to 7, Plates 131 to 135, are given, showing a number of 

 types taken from actual designs or from hypothetical designs which have 

 been worked out from Mr. Taylor's standard series experiments. 



If it is desired to compare two models at the same displacement it is only 

 necessary to superpose the two and bring shifting lines and displacement 



