Recently, the investigations on the hydrodynamic performance of floating breakwaters have great interest to
researchers because of its high advantages in protecting marinas and small craft harbors. For the purpose of improving
the hydrodynamic performance of the stepped-slope floating breakwater type, this research had been carried out as an
approach to investigate experimentally the resulting hydrodynamic performance of a composite pile supported
breakwater consists of a stepped-slope floating body moving only in heave motion overlying a fixed submerged
horizontal plate in a constant water depth, d = 30 cm. The transmission, reflection and losses coefficients had been
determined individually for each of the stepped-slope floating, fixed submerged horizontal plate and the composite
breakwaters. The results showed that the performance of the stepped-slope floating breakwater tested using different
relative draft values, Df /d, of the floating body had been significantly improved and achieved extra wave damping by
reducing the transmission coefficients by a ratio range between 23 % to 52 %, when using the submerged fixed
horizontal plate underneath the floating body, with definite improvement value depends on plate relative submergence
depth, dp /d and plate relative thickness, tp /d . For a composite breakwater using two relative submergence depths of the plate, it had been found that a significant higher wave attenuation takes place with a configuration using dp /d = 0.33
than that obtained when using dp /d = 0.50. Furthermore, plate relative thickness achieved wave attenuation
improvement for the composite breakwater especially for the configuration using dp /d = 0.33 and tp /d = 4%.