Incremental hull resistance

Incremental hull resistance is the added drag a ship accumulates in service beyond its clean trial condition, driven by hull roughness, coating breakdown, fouling, and corrosion. In the ITTC framework the roughness penalty enters as a friction increment, delta C_F = 0.044 (k_S / L)^(1/3) - 0.000125, with hull roughness k_S taking a standard value of 150 microns when unmeasured. Marine growth adds far more: heavy slime or weed and shell fouling can raise total resistance by 20 to 40 percent and is the case ISO 19030 hull-performance monitoring tracks. This increment is the engineering basis for the service margin added to trial power.