RUDDEK ACTION 



391 



wheels, and the gliders of aeroplanes, since, area for area, a short (in the 

 direction of motion) wide surface is considerably more efficient than a long 



* W a v 2 

 narrow one, the total pressure approximating more nearly to ^ as the 



surface is made shorter and wider. 1 



As applied to the design of rudders it explains why a deep narrow rudder 

 gives better results than one which, having the same area, is shallower 

 and wider. 



The following table shows the ratios of P' to P experimentally obtained 

 by Stanton on rectangular planes. 



The maximum ratio, 1*15, for the former plane was attained when 

 was 44, while with the latter plane the pressure curve had a 

 characteristic hump (ratio = *74) when was 25. 



AKT. 112. RUDDER ACTION. 



The normal pressure which is produced on an oblique plane by its 

 motion through water is taken advantage of in the ordinary rudder. 

 Since the flow of water to the rudder is guided by the form of the stern of 

 the vessel, the distribution of pressure is entirely different to that occur- 

 ring in the cases previously considered. In any case, however, the effect 

 of the motion is to produce a distribution of normal pressure over the 

 rudder which has a single resultant tending to turn it about its point of 

 attachment to the stern post. This action being resisted by the rudder 

 chains, the nett effect is that of a single force P acting on the vessel at B 

 in the direction A B (Fig. 177 a). 



This is equivalent to an equal and parallel force applied at the centre of 

 gravity G of the vessel, altogether with a couple of moment P X A G 

 tending to rotate the vessel about G. It is this moment which tends to 

 turn the vessel. The single force simply tends to produce a bodily drift of 



1 For curves showing the effect of varying the manner of presentation of glider surfaces for 

 aeroplanes, see Lord Rayleigh's paper, ante loq. 



