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948
[PROPULSION
SHIP BUILDING

elliptical blades varies as %+%, where r is the radius from centre of

Mr R. E. Froudein 1908, it is probable that the effect of friction would be in the direction of giving higher efficiencies for large screws than for small. The results obtained with ships' propellers are in general accordance with those deduced from model propellers, although the difficulties inherent to carrying out experiments with full-sized Screws have hitherto prevented as exact a comparison being made as was done with resistance in the trials of the “ Greyhound ” and her model. Results of model experiments have been given by Mr R. E. Froude, Mr D. W. Taylor, Sir John Thomycroft and others; of these a very complete series was made by Mr R. E. Froude, an account of which appears in Trans. Inst. Nav. Archs., 1908. Propellers of three and four blades, of (pitch ratios varying from 0-8 to 1-5, and with blades of various wi ths and forms were successively tried, the slip ratio varying from zero to about 0-45. In each case the screw advanced through undisturbed water; the diameter was uniformly 0-8 ft., the immersion to centre of shaft 0-64 ft., and the speed of advance 300 ft. per minute. Curves are given in the paper which express the results in a form convenient for application. Assuming as in Froude's theory that the normal pressure on a blade element varies with the area, the angle of incidence, and the square of the speed, the thrust T would be given by a formula such as T=a R'-bR

where R is the number of revolutions per unit time. On rationalising the dimensions, and substituting for R in terms of the slip ratio s, the “conventional” pitch ratio p, the diameter D, and the speed of advance V, this relation becomes: 2 S

TW Di” '<1-TF

From the experiments the coefficient a was determined, and the final empirical formula below was obtained- 2-l-21 I'O2S(I-~OSS)

T D'V'><(V-V'); the effective or tow rope horse-power is R XV, and the ratio of these two powers = (1 -t) (1 -1-w) is termed the hull ejiciency.

It is evident that the first factor (1 -I-w) represents the power regained from the wake, which is itself due to the resistance of the ship. As the wake velocity is usually a maximum close to the stern, the increase of w obtained through placing the screw in. a favourable position is generally accompanied by an increase in t; for this reason the hull efficiency does not differ greatly from unity with different positions of the screw. Model screw experiments with and without a ship are frequently made to determine the values of w, t, and the hull efficiency for new designs; a number of results for different ships, together with an account of some interesting experiments on the effect of varying the speed, position of screw, pitch ratio, direction of rotation, &c., are given in a paper read at the Institution of Naval Architects in 1910 by Mr W. ]. Luke. 5

The total propelling efficiency or propulsive coefficient (p) is the ratio of the effective horse-power (RV) to the indicated horse-power, or in turbine-driven ships to the shaft horse-power as determined from the torque on the shaft. In addition to the factor “ hull efficiency, " it includes the losses due to en ine friction, shaft friction, and the propeller. Its value is generally aiout 0-5, the efficiencies of the propeller and of the engine and shafting bein about 65 and 80 % respectively. The engine losses are eliminated in the propulsive coefficient as measured in a ship with steam turbines; but the higher rate of revolutions there adopted causes a reduction in the propeller efficiency usually sufficient to keep the value of the propulsive coefficient about the same as in ships with reciprocating engines.

The table on the following page gives approximate values of w, t, and p in some ships of various types. The action of a screw propeller is believed to involve the acceleration of the water in the race before reaching the screw, which is necessarily accompanied by a diminution of pressure; “Yuma and it is quite conceivable that the pressure may be ° reduced below the amount which would preserve the natural flow of water to the screw. This would occur at small depths of immersion where the original pressure is low, and with relatively small blade areas in relation to the thrust, when the acceleration is rapid; and it is in conjunction with these circumstances that so-called “ cavitation” is generally experienced. It is accompanied by excessive slip, and a reduction in thrust; experiments on the torpedo-boat destroyer “ Daring, ” made by Mr S. W. Barnaby in 1894, ' showed that cavitation occurred when the thrust per square inch of projected blade area exceeded a certain amount (11% lb). Further trials have shown that the conditions under which cavitation is produced depend upon the depth of immersion and other factors, the critical pressure causing cavitation varying to some extent with the type of ship and with the details of the propeller; the phenomenon, however, provides a lower limit to the area of the screw below which irregularity in thrust may be low values of the

disk ratio. A skewback

of the blades to an angle of 15° was found to make no material diHerence to the results., V

expected, and the data for other screws (whether model or full-size) become inapplicable.

1 Trans. I .N.A. 1897 (vol. xxxix.).