Hull roughness and added resistance
How coating condition and biofouling raise required shaft power, the Schultz condition table, why barnacle height beats coverage, and the sea margin that covers it.
Hull roughness is the departure of the wetted surface from a smooth reference, from coating condition, mechanical damage and biofouling, and it raises frictional resistance and therefore the power needed to hold a given speed.
Schultz (2007) gives the increase in required shaft power by condition, which is more useful than a single range because it identifies the state the hull is in: 10 to 11 percent for deteriorated coating or light slime, 16 to 21 percent for heavy slime, 34 percent for small calcareous fouling and 55 to 86 percent for heavy calcareous fouling. Demirel and others (2017) showed that roughness height dominates coverage, with 10 percent coverage of 5 mm barnacles costing about the same added power as 50 percent coverage of 1.25 mm barnacles. Full-scale before-and-after cleaning trials measured biofilm alone raising the powering requirement by up to 18 percent.
The full article will cover Granville scaling from flat-plate roughness functions to full scale, the roughness measurement methods used at drydock, the interaction with coating type, and the sea margin convention of about 15 percent of propeller design power that the industry uses to absorb it. See slow steaming and sea margin and engine margin .