Section 6
Page 52
Appendages in 5 Strips
Figure 26.
Strip wise segmentation of appendages
The viscous resistance of each strip is then calculated from the product of the dynamic head, the local wetted surface area and an appropriate skin friction resistance coefficient. (Cf). The determination of the appropriate Cf is based on data presented in Fluid Dynamic Drag (Hoerner 1965). The calculation32 is based on 4 Reynolds Number regimes, calculated for a flat plate and t/c ratios of 10 and 20%, as shown in table 11
Reynolds No. 3.162E+03 1.00E+04 3.162E+04 1.00E+05 3.162E+05 1.00E+06 2.512E+06 6.310E+06 1.585E+07 5.012E+07 1.995E+08
Table 11.
1000*Cf Flat plate
1000*Cf t/c = 0.1
1000*Cf t/c = 0.2
Bulb
24.85 13.86 7.73 4.95 3.46 3.00 3.00 3.00 2.81 2.39 1.96
42.07 28.93 20.20 10.74 4.99 3.62 3.62 3.62 3.39 2.88 2.36
44.12 30.51 21.42 11.50 5.40 3.94 3.94 3.94 3.69 3.14 2.59
59.29 44.00 32.66 16.54 6.51 4.49 4.49 4.49 4.21 3.57 2.93
Appendage Cf. values used in the VPP
This approach works well for plain fin keels and rudders, but for keel bulbs which occupy the lowest appendage strip some further calculation must be done to ensure that appropriate characteristics are derived. The following approach is currently used: (a) Use a chord length equal to the average of the top of the bottom strip and the longest fore and aft length occurring in the bottom strip. (b) Use a maximum thickness equal to: volume / ( area Ă— 0.66) . (c) Use a reference area equal to the maximum of the strip projected area, and the wetted surface area. The total viscous drag of the appendages is determined as shown in equation [52].
32
Scheme devised by Karl Kirkman, Dave Greeley and Jim Teeters