OF SHIPS WITH CONDITION OF WETTED SURFACE. 39 



The planes were sandpapered smooth and coated with shellac, and in this 

 condition were each towed in the water at speeds ranging from i to 1 1 knots. 



One plane was next coated with black lead or graphite, which was mixed in 

 sour milk and applied with a brush to the shellacked surface of the plane. When 

 dry, the lead was rubbed down to a fine, smooth polish. Ten runs were made with 

 this plane at speeds from 5.1 to 6.5 knots. The graphite seemed to cling well to the 

 surface and showed no abrasion or wear. When wet, however, it was easily rubbed 

 off. 



The next material tried was ordinary Ivory soap, which was mixed as a thick 

 emulsion in water and applied with a brush. The plane was run at speeds from 6 

 to 9 knots. The soap left a whitish streak in the water the first run, but not 

 thereafter. The resistances obtained on all of the runs were consistent, so that the 

 quality of the surface was probably unchanged during the course of the experi- 

 ments. 



Two other lubricating materials experimented with were heavy cylinder oil 

 and light cylinder oil. The oil was applied to the shellacked surface of the plane 

 when dry with a brush, the plane being suspended over the position in which it was 

 attached to the towing carriage, so that it could be run in the water as quickly as 

 possible after the application of the oil. The plane with the heavy engine oil was 

 run at speeds from 6 to 8}^ knots. The observations obtained were consistent, but 

 the slope of the resistance line when plotted on log paper indicated that at speeds of 

 about 3 knots the frictional resistance would fall below that of the smooth, shel- 

 lacked surface. This peculiarity is possibly due to the fact that parts of the oil 

 formed in drops or globules and wavelets, increasing as the speed increased, thus 

 changing the condition of the surface which might, if tested at lower speeds, not 

 have given results lower than shellac. 



The experiments with the light engine oil indicated that for the first two runs 

 the resistance was a small percentage greater than for the shellacked surface. Ap- 

 parently as the oil was washed off the resistance decreased until the resistance 

 agreed with that previously obtained with shellac. 



The results of the experiments indicate that no advantage over a smooth var- 

 nish or shellacked surface is obtainable with the materials used. For each lubri- 

 cated surface the resistance is greater than for the shellacked surface. The re- 

 sistance increases in the following order: — Black lead, light engine oil. Ivory soap, 

 heavy cylinder oil. 



The gross resistances for the different surfaces, with actual observed spots as 

 plotted on logarithmic paper, are shown on Plate 28. 



The lubricated surfaces were tried only over a limited range of speeds, from 

 6 to 9 knots, as it was thought that the exponent in the formula R = SfV" could 

 not be used, owing to the changing condition of the surface due to the washing off 

 of the lubricating material. Actually it was found, however, that the light engine 

 oil was the only material for which the resistance changed rapidly, and that washed 

 off quickly so that the resistance came down to that for shellac. 



