330 HYDRAULICS AND ITS APPLICATIONS 



stern of the latter, and this is increased by the influx necessary to pro- 

 vide the water thrown astern by the propeller. Further astern the impact 

 of the streams converging from the two sides of the hull produces a region 

 of outflowing currents. 



The bows of the overtaking boat first come within the influence of this 

 region which tends to produce a slight outward sheer. As it creeps 

 further ahead the bows come within the influence of the inflowing 

 currents while the stern is still being repelled, with a resultant tendency 

 to inward sheer which has often led to serious collisions. This effect is 

 a maximum where the bows of the follower are about one-third of its 

 length aft the bows of the leading ship. As the follower draws further 

 ahead the tendency to sheer diminishes and is replaced by a tendency to 

 bodily inward drift, while when almost abreast, the bows of the follower 

 become exposed to the outflowing currents from the leader's bows while 

 its stern is still being attracted, with a consequent tendency to an out- 

 ward sheer. 1 



The effect depends largely on the sizes, speeds, and relative speeds of 

 the vessels, increasing with the size of the leader, and with the common 

 speed, and diminishing as the relative speed increases. 



ART. 94. FLOW ROUND RIVER BENDS. 



A river flowing through an alluvial plain always tends to gradually 

 increase any winding which may occur in its course, until finally a new 

 channel is cut through the narrow neck of land thus formed. The 

 following explanation of this scouring of the outer bank of a bend and 

 the deposition of detritus on the inner bank has been given by 

 Professor James Thompson. 2 



In consequence of the centrifugal force, the pressure at any level in a 

 transverse section of the stream increases outwards, so that the level of 

 the free surface is highest near the outer bank. Near the bottom, how- 

 ever, the resistance ot the bed reduces the velocity and consequently the 

 centrifugal force of the water, which now becomes insufficient to overcome 

 the tendency to inward flow produced by the higher level of the free 

 surface at the outside of the curve. The water near the bottom, then 

 has a tendency to flow inwards and to carry with it gravel and other 

 detritus which is left at the inner bank. Experiments in a model river 



1 For a full discussion of the phenomena of interaction an article in " Bedrock," Vol. I., 

 No. 1, pp. 6687, may be consulted, 

 a Proceedings Royal Society," 1877, p. 356. 



