564 ON THE FORCE OF THE RUDDER, &C, 



SECTION IX. 



On the force of the Rudder, and the manner in which it act&'. 



THE force by which the rudder of a ship makes her move in 

 any direction, at pleasure, excites no small degree of astonishment ; 

 especially when we consider the weak action of the enormous rud- 

 ders with which some of the barges lhat navigate our rivers and 

 canals are furnished. The cause of this phenomenon we shall here 

 endeavour to explain and illustrate. 



The rudder of a barge or vessel has no action unless impelled by 

 the water. It is the force resulting from this impulse, which being 

 applied in a direction transversal to the poop, tends to make the 

 vessel turn around a point of its mass, called the spontaneous centre 

 of rotation. The prow of the vessel describes around this point an 

 arc of a circle, in a direction opposite to that described by the 

 poop ; hence it follows that the prow of the vessel turns towards 

 that side to which the rudder is turned, consequently opposite to 

 that side towards which the tiller or lever of the rudder is moved. 

 Hence, when the tiller is moved to the starboard side, the vessel 

 turns towards the larboard, and vice versa. 



A force, and even a certain degree of intensity, must therefore 

 be applied to the rudder to make the vessel turn ; and on this 

 account the construction of the vessel is so contrived, as to incrfase 

 this force as much as possible ; for while the barges which navigate 

 our rivers are in general very broad behind, and screen as we may 

 say the rudder, so that the water flowing along their sides can 

 scarcely touch it, the stern of vessels intended for sea are made 

 narrow and slender, so that the water flowing along their sides must 

 necessarily strike against the rudder, if in the least moved from the 

 direction of the keel. Let us therefore endeavour to estimate nearly 

 the force which results from this impulse. 



A vessel of 900 tons, when fully laden, draws 13 or 14 feet of 

 water, and its rudder is about two feet in breadth. Let us now 

 suppose that the vessel moves with the velocity of two leagues per 

 hour, which makes 17^ yards per minute, or about nine feet per 

 second ; if the rudder be turned in such a manner as to make with 

 the keel a continued angle of 30 degrees, the water flowing along 



