EXPERIMENTS ON THE EUETON AND THE EROUDE- 9 



As was pointed out in last year's paper the propeller with maximum 

 pitch-ratio (1.5) as used for towing on the Froude had a small clearance from 

 the fair-water on the stem-post and consequently it demanded a dispropor- 

 tionate power. This does not affect our discussion of the effect of area-ratio, 

 but throws it out of line the discussion of the effect of pitch-ratio. Com- 

 paring, now, the other two propellers having pitch-ratios of i.i and 0.8, it 

 appears that at 7 knots speed they required respectively 8.3 and 8.5 brake 

 horse-power; this result may be attributed directly to the poorer efficiency 

 of the low-pitch wheel, with only the reservation that experiments on pro- 

 pellers would lead us to expect a somewhat larger disadvantage. 



The running trials on the Fulton, shown on Plate 10, indicate quite 

 clearly the disadvantage of a small pitch-ratio which is associated with a 

 poorer propeller efficiency; thus the propellers with pitch-ratios of 1.3, i.o, 

 and 0.8, at 6.5 knots, require 10.7, 10.9, and 11.5 brake horse-power respec- 

 tively, or, inverting the comparison, with 1 1 horse-power propellers with those 

 pitch-ratios would give the Fulton speeds of 6.55, 6.51, or 6.37 knots per 

 hour. The difference, though not large, might be important in getting a 

 tow away from a competitor. 



On the same plate there are shown curves of propeller thrust for the 

 two propellers with lower pitches; for the pitch-ratio of 1.3 only the upper 

 end of the curve appears as the lower part was not properly recorded by our 

 thrust mechanism. These curves are quoted and also the resistance (as 

 determined by the theory of mechanical similitude) from the towing of the 

 Sotoyomo's model at Washington. 



TOWING. 



The performances of ocean-going and harbor tow-boats towing at full 

 power and at about 0.6 full speed are shown by Plates 4 and 11. Plate 11, 

 which represents results of experiments on the Fulton, shows a small advan- 

 tage for the low-pitch propeller. Plate 4 shows a corresponding advantage 

 for a propeller of the Froude with a pitch-ratio of 0.8 as compared with 

 one having the pitch-ratio i.i ; the coarse pitch propeller with a ratio of 1.5 

 appears to be at a great disadvantage, but I mistrust that this is to be 

 attributed in part to a defective clearance from the stern-post as aheady 

 indicated; such a pitch-ratio is not fitted for an ocean-going tug and the 

 importance of the comparison is therefore diminished in consequence. 



Plate II shows also the propeller thrust and the pull on the tow-rope 

 measured by our weigh-beam, which are given as a matter of interest. The 

 curious inversion of order of the thrust curves is yet to be explained. 



