Sec. 45.20 



FRICTION-RESISTANCE CALCULATIONS 



121 



world, from positive heavy fouling to slightly 

 negative scouring. 



(4) The size of the vessel, and especially the 

 operating speed for which the fouling prediction 

 is required. 



The first three items correspond to those which 

 have been recognized for many years past. The 

 fourth item is, in an analysis of fouling effects, 

 believed to be entirely new. 



One may conclude, from the discussion of Sec. 

 45.19, that the effect of fouling on ship resistance 

 should be expressed as an additive rather than as 

 a percentage term, as proposed by G. Kempf in 

 1936 and 1937. This is on the basis that the 

 fouling creates an increment of resistance depend- 

 ing upon its physical characteristics and the flow 

 around it, without regard to the amount of the 

 friction resistance of a clean, new hull to which 

 the fouling is attached, of the separation drag 

 around the hull, or of the resistance due to wave- 

 making of that hull. A convenient additive form 

 is the use of an increment ApCp , expressed in 



units of specific resistance which is to be applied 

 to the sum of the specific pressure (or residuary) 

 resistance and the turbulent-flow specific friction 

 resistance, with or without corrections for cur- 

 vature. 



Considering item (1) at the beginning of this 

 section, it is explained in Sec. 22.11 that relatively 

 little is known about the quantitative effect of 

 surface slime. At the other end of the roughness 

 scale, a very heavy, thick, and rough deposit is 

 neceSsary to eliminate entirely the effect of 

 viscosity. Such a deposit has the effect of making 

 the sum of the smooth-plate friction resistance 

 and the fouling resistance constant, independent 

 of the Reynolds number. For the normal ship 

 situation, the viscosity effects are retained, so 

 that AfCf is roughly constant in the relatively 

 narrow range of speed for any one ship for which 

 fouling effects are to be predicted. 



The ultimate solution to this problem, despite 

 the demand for simplicity, seems to call for a 

 subdivision of items (2) and (3) of the summary, 

 to take care of conditions which vary widely in 



TABLE 45.g — Proposed Form of Tabulation for Determining the Value of ApCp per Day Due to 

 Fouling in an Average Port 



