CHANGE OF LEVEL IN CANAL 



329 



ward bottom current must now be sufficiently great to supply an additional 



mass of water equal to this, so that its velocity at the minimum section 



/ a' \ v, its velocity at the bows being K v. Here 



a' = a + i where i is the area of the channel occupied by the forward 

 current at the amidships section. A a' is then the effective area of the 

 backward current. 



But to produce a backward velocity of flow at the bows of the boat the 

 surface level at the bows must be less than at some distance ahead, and 

 will thus be below the normal. The result is that the water level in the 

 canal falls as the boat approaches, has its minimum value near the 

 amidships section, and then rises to attain its normal value. The 

 effect of the bow waves in modifying the level at the bows is here 

 neglected. 



By applying equation (3) (p. 325), the difference of level at any two 



FIG. 144. 



points in advance of the boat may be deduced in terms of K v, since Q = 

 K v b hi, where hi is the depth of water at the bows. 



Fig. 144 shows the surface curves for a perfect fluid (dotted lines) and 

 for water. 1 



If the boat is nearer to one side of the channel the velocities of flow 

 are greater on this side of the boat, the pressures, particularly abaft its 

 beam are consequently less, with the result that it tends to sheer off 

 towards the further side. 



ART. 93A. SUCTION EFFECT BETWEEN PASSING SHIPS. 



Even in open water, where one boat is overtaking another in moderately 

 close proximity on a parallel path their mutual action, due to interference 

 of currents between their hulls may have serious effects. The mass of 

 water displaced by the forepart of the leading boat, returning to fill the 

 space vacated by its stern causes a continual influx of water towards the 



1 For the further investigation of the change of level round a moving 

 Tlie Engineer, vol. 63, p. 252, may be consulted. 



a paper in 



