An Experimental Study of Roll Damping Using a Pendulum-Based System

Abstract A hull-pendulum system is introduced to investigate the roll damping of a hull through roll decay. Compared to conventional roll decay tests, this method can reduce the decay of the KC number (roll amplitude) and more conveniently allows for a range of oscillatory frequencies to be investigated. Further, the setup allows for roll damping experiments with low mechanical friction and at low cost. The study primarily focuses on flow separation and vortex shedding. Two types of sharp bilges were investigated: a square bilge and a round bilge equipped with a bilge keel. The results demonstrate that, in most of the scenarios studied, the roll damping coefficient is proportional to the roll amplitude, which aligns with the assumption and prediction of the Ikeda method. However, for bilge keel configurations, the roll damping coefficient decreases rapidly at ϕ ≤ 2°. These findings are consistent with the prediction of isolated edge theory that the roll damping coefficient is proportional to KC2/3 in the case of the bilge keel with ϕ ≤ 2°. For ϕ 2°, the isolated edge theory suggests that the damping coefficient, effectively reverting to that of a square edge, should be proportional to KC, as corroborated by experimental observations.