124 



MVDRODVNAMICS I.\ SHIP DESIGN 



Sec.4-y.20 



the battlciihip Tennessee. Curve C was dniwri 

 midway between B and the baseline as a rougli 

 fi:uess, with some scattcretl supporting data. 

 Lewis' graphs A and C indicate a progressive 

 increase in SrCr per day for the early months 

 of the perioil between tioikings while graph li, 

 for the heaviest fouling, indicat<'s a linear increase 

 with time. The graphs for the three combatant 

 vessels show a slight increase in rate with eliip.sed 

 time for the entire interval, and a rate that 

 increases as the next docking time approaches. 



Fig. 45. L is a duplicate of the three Lewis 

 graphs with the fouling-efTcct predictions for the 

 ABC ship of Part 4, under the conditions set 

 fourth in items (18) and (19) of Table 64. c and 

 item (20) of Table 64. d. This ship is expected to 

 have what Lewis terms "average voj'ages" 

 except that at one end of the route the ship 

 spends an appreciable time in a fresh-water river. 

 The fouling rate may be expected to lie somewhat 

 below that of line A. 



The broken line with long dashes of Fig. 45. L, 

 for the ABC ship, involving a fouling rate that 

 increases slowly with time, applies to the hull 

 with a final bottom coating of anti-fouling sclf- 

 leveUng paint {7wt a plastic type). 



The dot-dash line of the figure, representing 



what may be expectetl of hot jjlastic paint in the 

 way of fouling roughness only, involves a fouling 

 rate that limits the roughness to a value which, 

 in 4 years, would be e(|ual to that expected of 

 "commercial" anti-fouling paint in only 'J months. 

 However, the inilial roughness of the hot plastic 

 paint is extremely high, so that this paint is at 

 a di.^^ad vantage compared to the "commercial" 

 anti-fouling paint until sucii time as tiie latter 

 has acquired considerable roughness due to its 

 increa.sed rate of fouling. It is estimated that, for 

 the average application, and as the ship comes 

 out of dock with a new coat of paint, the ScCf 

 of the hot-plastic paint exceeds the AcCy of the 

 commercial anti-fouling paint by the order of 

 ■0.5(10"^). For the purpo.se of this discussion, the 

 augmented AcCy for the hot-plastic paint is 

 considered in the nature of a fouling roughness. 

 To make up for this increiused resistance of the 

 hot-plastic coating in the first few months out 

 of dock, it must have a much smaller fouling 

 allowance ApCy for the remainder of the interval 

 between dockings. Fig. 45. L indicates that the 

 two predicted ABC ship curves cross each other 

 at 5.4 months out of dock. If the hot-plastic 

 paint is to pay its way, so to speak, the interval 

 lictween dockings must be long enough so that 



10 



bne B is for ships which 



enter a number of ports 



-where severe fouhrx] 



is encountered 



Line A is for ships on overoqt 

 vofoqes between temperote 



ports with overoqe 



rates of fou 11 no 



Line C is for ships hovinq 

 unusuoll^ short stoys In port 

 . ortrodincj requlorlij between 

 ports free from fouling or ports, 

 havino a scourinq action 



_a5 



6 7 5 9^ 10 M 



Months out of Drydock 



Fm. 4.1.1, I'liKKirTKn Si-Kcmc T'oiiUNn Ukmistantk Ai.i/iwanck.s koii .MK" Siiir, with Two Kinhs ok I'mnt 



