Photographs were taken with a camera made for the purpose, with a focal plane shutter giving an exposure of about one thousandth of a second, the illumination being by sunlight concentrated on the propeller from a twenty-four-inch concave mirror.
Photographs were also taken by intermittent illumination of the propeller from an arc lamp, the arrangement consisting of an ordinary lantern condenser, which projected the beam on to a small concave mirror, mounted on a prolongation of the propeller shaft, the reflected beam being caught by a small stationary concave mirror at a definite position in each revolution and reflected on to the propeller. By this means the propeller was illuminated in a definite position at each revolution, and to the eye it appeared as stationary. The cavities about the blades could also be clearly seen and traced, the photographs being taken with an ordinary camera and about ten seconds’ exposure.
A series of experiments was also made with model propellers in water at and just below the boiling point, dynamometric measurements being taken of power and thrust with various widths of propeller blade, the conclusion arrived at being that wide and thin blades are essential for fast speeds at sea, as well as a coarse pitch ratio of propeller.
The first vessel fitted with steam turbine machinery was the Turbinia. She was commenced in 1894, and, after many alterations and preliminary trials, was satisfactorily completed in the spring of 1897. Her principal features are: Length, one hundred feet; beam, nine feet; five-foot draught of water under the propellers; forty-four tons and a half displacement on trial; she is fitted with a water-tube boiler of eleven hundred feet total heating surface, and forty-two square feet of grate area, with closed stoke-holds supplied with air from a centrifugal fan mounted on a prolongation of the low-pressure turbine shaft. The engines consist of three compound steam turbines, high pressure, intermediate, and low pressure, each driving one screw shaft; on each of the shafts are three propellers, making nine in all; the condenser is of the usual type, and has four thousand square feet of surface.
When officially tested by Professor Ewing, F. R. S., assisted by Professor Dunkerley, she attained a mean speed on a measured mile of thirty-two knots and three quarters, and the consumption of steam for all purposes was computed to be fourteen pounds and a half per indicated horse power of the main engines. Subsequently, after some small alterations to the steam pipe, she was further pressed, and is estimated to have reached the speed of thirty-four knots and a half. She was, and still is, therefore, the fastest vessel afloat; she has been out in very rough weather, is an excellent sea boat, and at all speeds there is an almost complete absence of vibration.
In the Turbinia the exceptional speed results principally from two causes: 1. The engines, screws, and shafting are exceptionally light. 2. The economy of steam in the main engines is greater than usual.
At full speed the steam pressure in the boiler is two hundred and ten pounds; at the engines, one hundred and seventy-five; and the vacuum in the condenser twenty-seven inches, representing an expansion ratio in the turbines of about one hundred and ten after allowance has been made for wire-drawing in the exhaust pipe.
The first vessels of larger size than the Turbinia to be fitted with steam turbine machinery are the torpedo-boat destroyer Viper for the British Government, and a similar vessel for Messrs. Sir W. G. Armstrong, Whitworth & Company.
Fig. 4.—The Turbinia running about Forty Miles an Hour.