VARIATION OF FRICTIONAL RESISTANCE OF SHIPS WITH CONDI- 

 TION OF WETTED SURFACE. 



By Naval Constructor William McEntee, U. S. Navy, Member. 



[Read at the twenty-third general meeting of the Society of Naval Architects and Marine Engineers, held in 



New York, November 18 and 19, 1915.] 



Considering that frictional resistance is the most important element in the 

 resistance of practically all ships, it is notew^orthy that investigations of condi- 

 tions affecting it have been relatively few. Although there have been many at- 

 tempts made to reduce this resistance by different methods and there are many 

 references made to its increase from fouling, there appear to be relatively little 

 actual data as to the quantitative variations obtained. 



The results here given are based on a series of tests made at the United States 

 Experimental Model Basin during a period extending somewhat over a year. The 

 investigation was begun by Chief Constructor D. W. Taylor, U. S. Navy, before he 

 gave up the immediate direction of the Model Basin work as a result of his appoint- 

 ment as Chief of the Bureau of Construction and Repair in the Navy Department. 

 The writer therefore thanks him for permission to use the data given in the pres- 

 ent paper. 



The coefficients of friction for different kinds of ordinary surfaces, and for 

 variation in length of surface, were determined, as is well known, in experiments 

 made by William Froude years ago, and subsequently confirmed by other experi- 

 menters. These are the coefficients used in estimating the frictional resistance of 

 a new ship with clean, well-painted bottom. When, however, the condition of the 

 bottom changes, the resistance changes, and the extent of this change is a matter 

 of prime interest. 



In considering this general subject, two queries naturally arise: — (i) On actual 

 ships, what is the increase in resistance due to fouling? (2) Is it possible to reduce 

 the frictional resistance by modifications in the condition of the frictional surface, 

 or by the application of materials which would tend to make the surface smoother? 



EFFECT OF FOULING. 



The method adopted to investigate this subject was to expose to fouling in sea 

 water twelve lo-pound steel plates 2 feet wide by 10 feet long, painted with two 

 coats of anti-corrosive paint. The plates were placed in the water July 10, 1914, at 

 a point in the Chesapeake Bay near the navy yard, Norfolk, Va., where the water 

 ordinarily has a density of 1.02; that is, contains about 75 per cent as much salt as 

 is ordinarily found in sea water. 



