62 



d. For moderate P the resistance properties of the forebodies (judged 

 by the resistance qualities of symmetrical models) yield an estimate of the 

 success obtained by moving the CB. 



Conditions for forms with higher prismatics are especially compli- 

 cated, since here the viscous-pressure drag may become as sensitive to form 

 variations as the wave resistance. These two resistance components influence 

 the total resistance in opposite ways. For low wave making we need a fine 

 forebody; for low viscous-pressure drag a fine run. The LCB of full slow 

 ships lies normally forward of the midsection, although the wave resistance 

 is increased by this location, since in this case, the viscous-pressure drag 

 is more important than the wave resistance. 



An additional complication is caused by a possible instability of 

 flow, to which this kind of model is especially subject. Thus, the analysis 

 is peculiarly unsatisfactory when dealing with the important class of slow and 

 moderately fast cargo ships, because: 



a. The theory of wave resistance can be applied only with great re- 

 strictions on account of their full forms and low Froude numbers. 



b. Our knowledge of viscous-pressure resistance and frictional-form 

 drag is completely inadequate. 



c. Numerous earlier experiments are questionable as to their accuracy. 



Various attempts have been made by Havelock to estimate the influ- 

 ence of viscosity on wave phenomena. In the first place, Havelock pictured 

 the process phenomenologically in terms of a friction belt whose effect may 

 be equivalent to reducing the slope of the ship towards the stern; the equiv- 

 alent shape of the hypothetical body was rather arbitrary.®^ The correspond- 

 ing resistance curve showed much less waviness; the greatest part of this ef- 

 fect can be explained by the heavy reduction of the prismatic due to the vir- 

 tual lengthening of the form. 



The next step consisted in introducing a correction factor p * to 

 allow for a decrease in efficiency of the elements of the ship's surface in 

 going from bow to stern; ^^ the frictional effect is treated as a diminution 

 in the effective relative velocity of the model and the surrounding water. 

 This method was developed by Wigley, who applied the same factor p * for the 

 decay of the bow waves.** He deduced the reduction factor yS * from resistance 

 curves by finding the values necessary to give reasonable agreement between 

 theory and experiment. Important practical results thus obtained are dis- 

 cussed later. 



