Sec. 45.19 



FRICTION-RESISTANCE CALCULATIONS 



117 



and Eq. (45. ii). Although adequate means do not 

 yet exist for selecting these separate roughness 

 allowances independently and exactly, it is 

 possible to assess reasonable values, within not- 

 too-close limits, to: 



(a) A combination of plating and structural 

 roughness 



(b) Item (a) plus coating roughness 



(c) Item (b) plus fouUng roughness. 



These serve to subdivide: 



(i) The roughnesses built into the ship 



(ii) Those inherent in the coating(s) applied after 



construction or during docking 



TABLE 45.f — Tentative Individual Allowances 

 FOR Roughnesses of Three Types 



These correspond to three of the types listed at the end 

 of Sec. 45.7. FouHng-roughness predictions are listed in 

 Table 45. g. The present values are intended to apply only 

 to ships of medium and large size, having ship Reynolds 

 numbers at designed speed in e.xcess of about 100 million. 



(a) Metal plating, ApCj?(10=) 



(1) Pickled, sandblasted, or galvanized 0.01 to 0.05 



(2) Rusty, pitted, mill scale 0.05 to 0.1 



(b) Wooden planking, ApCfClO^) 



(1) Plastic, or molded pl3Tvood, with 



few seams or the equivalent, very 

 smooth 0.00 



(2) Flush seams, planks in the direction 



of flow, no open seams, and no 



calking 0.00 to 0.01 



(3) Flush seams, calked, planking not 



in direction of flow 0.01 to 0.05 



(4) Soft planks, slash-grained, rough 



finish 0.05 to 0.1 



(c) Structural roughness, AsCf(IO^) 



(1) Metal plating, lapped riveted or 



welded seams, smooth welded butts 0.06 to 0.1 



(2) Metal plating, lapped riveted 



seams and butts 0.08 to 0.15 



(3) Wood planking, lapped or clinker 



style, discontinuities in line of flow 0.12 to 0.18 



(d) Coating roughness, AcCp(lO') 



(1) Varnish, yacht-racing enamel, pol- 



ished metal 0.00 



(2) Red lead, metallic oxide, zinc 



chromate 0.05 to 0.18 



(3) Self-leveling varnish-type bottom 



paints 0.02 to 0.12 



(4) Vinyl resin 0.05 to 0.3 



or 0.4 



(5) Cold plastic 0.1 to 0.3 



(6) Hot plastic 0.3 to 0.8 



(iii) Those accumulated in service, after undock- 

 ing. 



The matter of fouling roughness is discussed in 

 Sec. 45.20 following. 



Using the best information available in 1955, 

 the plating, structural, and coating allowances 

 may be assigned tentative individual values as 

 listed in Table 45.f, for values oj R„ greater than 

 about 100 million. For a modern vessel, with 

 clean plates, riveted lapped seams, and flufsh- 

 welded butts, coated with a self-leveling bottom 

 paint, this gives a SACf (10^) = (Ap + Ag + Ac) 

 (10^) Cf. (excluding fouling) having a minimum 

 value of 0.09 and a maximum value of 0.32. For 

 a clean, new steel ship with both flush-welded 

 butts and seams, also coated with a self-leveling 

 bottom paint, the value of SACp(lO^) may 

 approach but usually does not exceed 0.2 [Vincent, 

 S. A., unpub. Itr. to HES, 26 Jun 1953]. These 

 values do not apply to ships coated with plastic 

 bottom paint of the kinds in use at the time of 

 writing (1955). 



Values of 2ACp for a number of merchant 

 vessels of varied size and type, in the range of 

 R„ from 150 to 1,800 million, are plotted by 

 R. B. Couch on a plate accompanying Appendix 

 XXVIII of the published Minutes of the 1953 

 ATTC meeting. The ranges of speed for some 

 individual ships are sufficient to show changes 

 in ACp with R„ . 



Manifestly, it is seldom logical to predict a 

 roughness allowance by adding all the Ap , As , 

 and Ac factors. The preservative coating on the 

 bottom may cover up (or even exaggerate) some 

 of the plating (or planking) roughness, just as the 

 fouling may later cover up the plating and coating 

 roughness and perhaps even the structural 

 roughness. A designer may well predict two or 

 more roughness allowances, one for trial and the 

 other (s) for service conditions. The latter should 

 correspond to (1) the ship just out of dock, and 

 (2) the ship with x months in service, following 

 the last docking. In any case, a good prediction of 

 bottom roughness, excluding fouling, requires 

 rather unusual judgment and a better background 

 of reliable roughness and resistance data than are 

 available at present (1955). 



45.19 Factors Affecting Fouling Resistance on 

 Ship Surfaces. W. J. M. Rankine, in his 1866 

 book on "Shipbuilding: Theoretical and Prac- 

 tical," page 5, says of fouling that "It is very 

 common to find the resistance increased by about 



