Question: A sailing vessel is 64 m long, and its hull has a wetted surface area of 560 m2. Its top speed is about 9 m/s. Assume that the equivalent sand-grain roughness ks of the hull is normally about 0.2 mm.
The total resistance of the hull is composed of wave drag plus skin-friction drag. Assume that the latter can be estimated by taking it to be the same as the equivalent flat plate. The skin-friction drag is exactly half the total drag when sailing at top speed under normal conditions. If the water density and kinematic viscosity are 1000 kg/m3 and 1.2 × 10-6 m2/s, respectively, estimate
(a) The admissible roughness for the vessel.
(b) The power required to maintain the vessel at its top speed when the hull is unfouled (having its original sand-grain roughness).
(c) The amount by which the vessel's top speed is reduced if barnacles and seaweed adhere to the hull, thereby raising the equivalent sand-grain roughness ks to about 5 mm.