Roughness allowance (CA)

The roughness or correlation allowance C_A is the increment added to the model-predicted resistance coefficient to account for ship-scale hull roughness and to reconcile the prediction with measured trial power. In the original ITTC-1978 method it appears as a roughness term delta C_F = 0.044 (k_S / L)^(1/3) - 0.000125, with standard hull roughness k_S = 150 microns. The 19th ITTC recommended a correlation allowance form C_A = (-5.68 - 0.6 log10 Re) x 10^-3 that excludes the explicit roughness term but reproduces similar values of delta C_F + C_A. It typically adds a few tenths of a millicoefficient and grows as the hull fouls in service.