2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



This assumption is interesting" as a means of comparison but the 

 e. h. p.'s so determined are inaccurate and exceed the true values, 

 because the frictional resistance which is an important component 

 varies with the speed to some power less than the square, approxi- 

 mately the 1.85 power. It is probable that the maximum error is less 

 than 40^ and the minimum as high as 20;^ in excess of the actual 

 e. h. p., but the discrepancies are not considered to invalidate their 

 value for comparative purposes. At the top of the plate the change of 

 level curves are plotted. 



The resistance curves were determined by towing the models at 

 " displacements corresponding to speeds," with the models at a set 

 " trim " but free to rise and fall under the influence of " suction " or 

 " planing," and the change of level curves show how much the plan- 

 ing effect changes the draft at each condition. 



All models were towed under conditions representing a full load 

 of 2,200 lbs. and the assumption that the get-away occurs at 45 m. p. h.' 

 All were of the ventilated step type. An inspection of the resistance 

 and trim curves will show the following general features : 



a. At low speeds, suction is present. 



b. This is succeeded by a condition in which the models run hard. 



c. Which is succeeded by a condition at which the model begins to 

 plane. 



d. And just before the planing is established the slope of the curve 

 lessens rapidly. 



e. And when planing is established the resistance falls off sharply 

 with one exception. 



f . Just preceding the " get-away " there is a tendency for the 

 resistance to remain at an appreciable value, which 



g. Falls to nothing sharply at the last. 



Model 1 591-3 was designed to obviate the defects of the flat scow 

 bow type, and introduces the V bottom for the purpose of parting 

 rather than pushing the water aside. The ventilated step was located 

 so as to be slightly to the rear of the center of gravity. This model 

 1 59 1 -3 was derived from 1591-1, which was a true V type. 1591-1 

 ran very well except for a remarkable- sheet of spray at a speed cor- 

 responding to 12 m. p. h. This sheet of spray is shown in plate 4, 

 fig. 2. Due to this spray it was considered necessary to modify the 

 model and this was done by making the V sections " full," the prin- 

 cipal effect of the change was to augment the sheet of spray, so the 

 opposite tack was next taken, that of making the V sections hollow 

 in wake of the position from which the sheet of spray originated, 



