The presence of marine growth increases submarine structures diameter and roughness and thus the hydrodynamic loading due to wave and current excitation. In this paper, two kinds of hard roughness (mussels and corals) are studied experimentally. In order to cover a wide range of the dimensionless numbers (Reynolds number, Keulegan–Carpenter number and reduced speed), three kind of trials are carried out: current only, forced motions and a combination of forced motions and current. Two equivalent diameters are used for the calculation of the hydrodynamic coefficients: an equivalent diameter corresponding to the closed volume of the system, where no fluid is permitted, and an equivalent diameter based on the projected area of the colonized structure for the drag coefficient, and on the total volume for inertia coefficients.Results show that these coefficients are up to 4 times higher than those obtained for a reference cylinder. The use of a different specific diameter for inertia and drag coefficient leads to fit all results with the reference cylinder results. This standardization prove that the knowledge of the volume and the projected surface of the cylindrical structure could simplify the forces estimation applied on the system just by knowing the corresponding smooth one.