CHAPTEE XL 



Impact of Jtts on Stationary and Moving Vanes Distribution of Pressure over Plate- 

 Graphical Construction to determine Pressure on a Vane Centre of Pressure on Vane 

 Compounding of Jets Resistance of Submerged Plane Surfaces Rudder Action 

 Resistance of Submerged Bodies Resistance and Propulsion of Ships Jet Propulsion. 



ART. 104. IMPACT OF JETS. 



WHEN a steady jet of water impinges on any solid surface there is none 

 of the rebound which follows the impact of two solid bodies. Instead 

 of this, a thin stream is formed w r hich glides along the surface until it 

 reaches the boundaries, when it leaves approximately tangential to the 

 surface. 



In theoretical discussions it is usual to assume that the sheet of water 

 leaves the surface tangentially. Actually the action is similar to that 

 indicated in Fig. 163, where the dotted lines indicate the true path of 

 the sheet. 



This digression from a straight path is due to the force exerted by the 

 surface tension of the film of water clinging to the outer periphery of the 

 plate. It may be reduced by making the edges of, the plate extremely 

 thin, and is of less consequence as the velocity of the issuing stream 

 is increased, and as the inertia of the water becomes, in consequence, of 

 greater relative importance. 



When the initial and final directions and velocities of an impinging jet 

 are known, the pressure which it exerts on the surface in any direction 

 may be calculated by equating this pressure to the total change of 

 momentum of the jet per second in this direction. 



It is important at this stage to differentiate between the " absolute " 

 and "relative " velocities of a jet. Thus, if the jet be projected from a 

 fixed nozzle and strike a moving surface, its velocity may be considered 

 from the point of view of a person standing by the nozzle and who notes 

 its velocity relatively to the earth, or from that of a person moving with 

 the surface struck, and who notes the motion relative to this moving 

 surface. The first person then notes the absolute, and the second the 

 relative velocity of the jet. 



In the case of impact on a fixed surface the change of relative velocity 

 at impact is identical with the change of absolute velocity, and in 



