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December, 1918 THE MARINE REVIEW. $35 | i ining See 7 pe ee PARALLEL M8 2/3 re M.8. 2132 a8 05 7 a so ne 98 i i AE el3e 2135 --| ~ \ *e 2133 oa pike = EOAL Ie 17 a 135 23 \ ' é WW =H Ui | WL I WU: AP 2 a SCALE FOR PERCENTAGE OF LENGTH BETWEEN PERPENDICULARS. e 0 : FIG. 2--CURVES OF SECTIONAL AREA FOR FOUR MODELS AND THE PARENT FORM of the model, so as to steer it in a | t . straight course. The guide plates floated between the guilding points attached to the carriage, but the towing carriage did not exercise any force on the model in a fore-and-aft direction. Starting at low speeds corresponding to about 5 knots for the ship, the towing car- _fiage was adjusted to run at a uni- form speed. The rheostat control- ling the speed of the propeller dyna- mometer was then adjusted so that the thrust of the propeller would just keep the model running as fast as the towing carriage, without striking the stops, which were placed: at an in- - terval of 6 inches. .. Thus, 'starting - with the model in the mid position, it was free to gain or lose a dis- tance of three inches as compared with the towing carriage striking either stop. When the pro- _ peller was running at the proper speed to keep the model up with the tow- Ing carriage the record of thrust, torque and revolutions per minute was taken. If, in the course of the tun, the model struck either stop on E BS Carriage the run was discarded f an tained the desired data at the lowest Speed, creased another run made. Having ob- the carriage speed was _ in- I for subsequent runs and similar data taken at higher speeds. he range covered corresponded to mepecds of 5 to 121%4 knots for the Ship. About 40 different runs were Made with each model, giving a Corresponding number of points for Plotting the torque, thrust, and revo- _lutions-per-minute curves. Special Design Involved The armature of the propeller dynamometer was especially designed to reduce to a minimum the amount of magnetic thrust. This thrust in- Creased with the torque and amounted to 0.17 pounds when the armature Was displaced ve inch and the torque delivered to the shaft was 16 pounds- Inches. Neglecting this at higher Powers would have caused an error In thrust measurements of about 1.4 Per cent but would not: have caused any error in the power measurements. before' However, this magnetic thrust was separately calibrated, and corrections for it were made in working up the results of the experiments. | It is interesting to note that ex- periments of this kind: with a very NS 2423 27.2 SECONOS CAI CULATED R.P.M TORQUE PEN __, THRUST TIME Le REVOLUTIO 247'----> 12.08 LB- IN} y MANNA 2.24" > 8.85 LB. FIG. 8--DYNAMOMETER CARD TAKEN. FROM MODEL RUNNING AT A SPEED CORRESPOND- ING TO APPROXIMATELY 10% KNOTS heavy model, in this case a dis- placement of 3774 pounds, afford a very sensitive means of checking the uniformity of speed of the towing carriage. If the towing carriage it- self were not carefully adjusted, so as to eliminate ee variations in wer required to drive it, owing to ane in the level of the tracks or uneven friction of the driving wheels or guide wheels, the result- ing small accelerations and retarda- tions of the carriage were made ap- parent. As the model, driven by its own propeller, when supplied with a constant voltage, ran at uniform speeds, it was possible to see the towing carriage gaining or losing dis- tance of a few inches in 5 or 10 seconds, as shight inequalities of speed due to small variations in the resistance of the driving mechanism developed. In order to conduct self- propulsion experiments successfully with the method followed, it was necessary to have the. towing car- riage running in excellent condition. Uniform Conditions Assured Immediately after completion of the self-propulsion tests on the model the propeller was removed and. the runs to obtain the shaft friction and the thrust. without propeller were re- peated. The model was next con- - nected with the resistance dynamo- meter on the towing carriage and the usual' model resistance data taken. This insured that the conditions of test both for self-propulsion and for the resistance of the model would be uniform as regards conditions of the model, temperature of water, etc. Experiments on the four models were run on succeeding days, and the springs of the propeller dynamometer were calibrated both before and after 'the tests. In Fig. 3 is shown one'of the dynamometer cards taken for a model running at a speed corresponding to about 1014 knots for the ship. It will be noted that the deflection of the toraue spring for this 'condition was 2:47° twmches: for the thrust spring 2.24 inches, and the interval over which the revolutions were meas- ured was 1.78 inches. With a little practice it was possible to read both the thrust and the toraue deflections within 1/100 of an inch. The revo- lutions could be read to an accur- acy of less. than one-half of one per cent. Considering that the data 'taken are plotted and the whole averaged by means of faired curves, it is estimated that the results ob-