Making the Dixie
Exact, Careful Work, Not Luck, the Secret
One point has not been sufficiently emphasized in the news accounts of the race--Dixie II has a 200-hp. motor. Wolseley- Siddeley has 400-hp. and Daimler II is credited with over 500-hp. The hulls differ in length and beam not more than a few inches. The English motors are run at about 1,000 r.p.m., while the motor in the Dixie is said to have turned about 750 during the race. This difference in power is more than a discrepancy, and is so large that few men would care to come up against it when racing, boat against boat, without time allowance. How is it possible for a 200-hp. motor to drive this hull so much more swiftly and easily than the English challengers are driven with fully twice the power? There must be some explanation and it may perhaps best be summed up by the one word, "brains."
There are two ways of designing a speed boat. The English have chosen that of piling up horsepower upon horsepower. The finer details which make for speed have been, to some extent, neglected. The predominant feature in the Crane design is that of refinement in every detail, no matter how small and seemingly insignificant.
It is not usually thoroughly understood that the science of naval architecture, especially that part which relates to the proper lines of a vessel, the speed to be expected with the given horsepower, and similar questions, is not an exact science, but one which grows step by step, built up by results obtained from actual trials of similar forms. No one in the world can say that he has the best form of vessel for any given speed and power. It is a matter of research and investigation. Of late years the designers themselves have recognized this, and it resulted in the establishment of the Government Experimental Model Basin at Washington D.C. Fortunately, there is a law by which we may apply the results obtained from models to full size vessels. In this basin exact models of the proposed vessels are tried, the resistance they offer at various speeds measured by the most accurate of scientific instruments, and the horsepower required for the full-sized vessel calculated from these model trials. This has proved of the utmost value in recent battleship designs where the performance of a 20-foot model was taken as a criterion of the performance of a 400-foot battleship.
In designing a speed boat of extreme type, such as Dixie II, the designer has no data for an accurate estimate. In order to be sure of his results, Mr. Crane decided to carry out trials on several models in this Government tank. The models were approximately ten feet in length, being about one-fourth the full size. In order to compare the resistance of the 10-foot model in the tank with that of the 40-foot boat, it was necessary to run this model at one-half the expected speed of the full-sized boat. The tests were started at low speed, and the speed increased gradually until the model was run at a speed of 16 knots.
The model basin is a long, rather low building, the basin itself being about 600 feet long, 30 feet wide and 10 feet deep. Spanning this basin and running on a track at each side is a carriage driven by electric motors which are capable of close regulation as regards speed. Automatic instruments record the speed of the carriage throughout its entire run. The model is towed by arms projecting below the carriage, the arms being carried on frictionless joints and coupled to the pressure recording apparatus. By reading the records of a run, it is thus possible to know exactly the speed at which the run was made and the exact resistance of the model being towed. A special device is also installed by which a photograph of the model is obtained when under way. The photograph gives an idea--not an approximate but almost an exact one--of what wave formation may be expected with the hull under consideration.
The first model tried by Mr. Crane did not seem suitable at extremely high speeds. A second model, that of Dixie II, was tried with what seemed to him extremely good results, and upon this basis the actual design was worked out. it will be seen, then, that this clean, easy-driving hull is not a matter of luck, but one which is the result of careful and patient investigation. In other points as well as hull form this same careful work is apparent. The shaft angle of the Dixie, for instance, is small, which is an absolute necessity for proper propeller conditions. When it was announced that the Wolseley- Siddeley had a shaft angle of eleven degrees, it was seen that in this point the American boat would have a decided superiority.
Fully as remarkable as the hull is the motor. This is the first large motor of a comparatively new and growing concern. That it should have been so uniformly successful and reliable is unprecedented. This motor is of the most modern type, having eight cylinders arranged in a 90-degree V, the advantages of this type being that the weight of the crank case is scarcely more than half that when the eight cylinders are arranged in a direct vertical line.
Another striking feature is the auxiliary exhaust port. it is said that this auxiliary port takes care of 70-80 per cent of the exhaust gases, and this relieves the main valves of all tendency to pit and burn out. The whole motor installation, including the clutch, tail shaft, etc., with a pulley in place of the propeller, was carefully run on the brake until everything was working perfectly before being placed in the boat. The brake was placed on the pulley and in this way the same strains were brought upon the shafting as would be caused by the propeller. Without this careful testing it would have been impossible for Dixie II to have tuned up to racing pitch inside of a week as she did. Crane & Whitman seems to have brought forward a marine motor which is better than the best of the foreign manufacturers.
(Transcribed from MotorBoat, Aug. 10, 1908, pp. 10-11.)
[Thanks to Greg Calkins for help in preparing this page LF]
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