38 VARIATION OF FRICTIONAL RESISTANCE 



The plates were removed from the water at intervals of one month, carefully 

 packed so as not to remove any of the fouling, which consisted mostly of small 

 barnacles, and shipped to the Model Basin, where they were tested to obtain the 

 frictional resistance. The appearance of three of the plates removed from the 

 water at the end of three, seven and twelve months, respectively, is shown on 



Plate 29. 



Each plate as received at the Model Basin was tested at speeds ranging from 

 i>4 knots to 8 or 9 knots, and gave results which are shown typically for one 

 plate on Plate 26. After this the plate was carefully cleaned, scraped and repainted 

 with a coat of Norfolk anti-fouling paint and retested at the same range of speeds 

 as when fouled, and the coefficient and exponent for the clean condition determined. 

 The fouling matter removed from the plates was weighed so as to give a measure 

 of the amount of fouling. The weights given by the dotted line on Plate 27 are for 

 this fouling matter when dry. 



On Plate 27 is shown the resistance of the fouled plate at 10 knots as a percent- 

 age of the resistance of the same plate when cleaned and painted. These resist- 

 ances are plotted on the time during which the plate has been exposed to fouling. 



In Table I the data obtained at different speeds are given in tabular form. 



It will be seen that the maximum increase is such that the resistance when 

 foul is four times as great as when the plate is clean and freshly painted. 



Referring to Table I it will be seen that not only does the coefficient of friction 

 increase greatly but that the exponent in the formula, R = SfV", also increases and 

 has a value averaging about 2.00 as compared with the value of about 1.88 for 

 the clean plates. 



These results indicate the importance of keeping a vessel's bottom clean and 

 well painted, though it is not considered that the relative efifect on a ship's resistance 

 due to an equal amount of fouling would actually be as great as that obtained in these 

 experiments, as it seems probable that the decrease in frictional coefficient due to 

 increase of length would be relatively greater for the foul condition than is found 

 to be the case for clean, smooth surfaces. 



EFFECT OF LUBRICATION. 



At various times attempts have been made to reduce the frictional resistance 

 of ships by lubricating the under-water surface or by polishing it to a high degree 

 of polish with various materials. One favored method used for racing yachts is 

 to polish the bottom with black lead or graphite. An investigation of the benefits 

 to be expected from such polishing or lubrication has recently been made at the 

 Model Basin with friction planes of the dimensions shown on Plate 28. The two 

 planes used were those with which the frictional coefficients of several ship bottom 

 paints were determined a few years ago. These planes are roughly 20 feet by 2 feet 

 by % inch, with ends sloped forward at an angle of about 30 degrees. Total wetted 

 surface for each is about 82 square feet. 



