6 EXPERIMENTS ON THE EULTON AND THE FROUDE. 



pellers given in the table on page 2, for each of the three several duties of a 

 tow-boat, namely, running free, towing, and pulling (or pushing). 



For running free we may refer to Plates 47, 48, and 49 of volume 19 of 

 the Transactions, which give the results of tests as reported last year with 

 the propellers given on page 2 of this paper. A reference to those plates will 

 show that the curves of power on speed for a given pitch-ratio and for all 

 projected area-ratios from 0.44 to 0.60 are practically identical; the several 

 curves for the various areas can be distinguished but the separation is no 

 more that must be attributed to the unavoidable error of experiment. This 

 result is unexpected because the increase of area-ratio for the experimental 

 propellers of Mr. Taylor is accompanied by a loss of efficiency amounting to 

 three or four per cent out of sixty to sixty-six per cent. 



The effect of area on towing is shown by Plate 4, which gives all the 

 points representing the separate runs and also mean curves, with brake horse- 

 power plotted on speed. Points for tests with the original width of blade 

 are represented by triangles, the intermediate widths have crosses, and the 

 least widths have circles. For the pitch-ratios 0.8 and i.i all points lie on 

 or closely adjacent to the curves; for the pitch-ratio 1.5 the points show a 

 little dispersion. It would be possible to draw three separate curves for this 

 pitch-ratio of 1.5, and in that case the curve for the least area-ratio (0.44) 

 would lie the lowest and indicate a slight advantage for the narrower blade. 

 In reading these diagrams, which for convenience are put on one plate, it 

 will be noted that there are three base-lines selected so as to separate the 

 curves for the several pitch-ratios. 



On Plate 5, the results of our measurements of thrust of the propeller- 

 shaft are plotted in the same manner as the power. These results are quoted 

 because they represent a direct measurement in which we have great confi- 

 dence, and as the points and curves have the same characteristics as those 

 for power on Plate 4 they give direct evidence of the reliability of our power 

 curves. 



For our pulling experiments the Froude was secured by a line from our 

 weighing-beam to a pile of the draw of the Harvard Bridge. The motor was 

 allowed to run at various powers and the tow-line puU was read directly on 

 the spring-balance, allowing, of course, for the inequality of the arms. 

 The results are shown by Plate 6, for the original area-ratio of about 0.6 

 only; similar experiments with narrower blades were not made, partly 

 because of lack of time and partly because their importance was not realized. 

 The experiments gave no indication of breaking of the propeller race except 

 at high powers for the largest pitch-ratio. 



From these experiments the conclusion to my mind is direct and positive, 



