Roll damping coefficient

The roll damping coefficient measures the dissipation in roll, identified from inclining or free-decay tests as the rate at which successive roll amplitudes decay. Because roll is lightly and nonlinearly damped, it is modeled either as an equivalent linear b_44, as a percentage of critical damping (often 5 to 10 percent), or as combined linear-plus-quadratic terms. The Ikeda method predicts it as the sum of wave-radiation, eddy-making, skin-friction, lift, and bilge-keel contributions, with bilge keels usually the largest. The coefficient sets peak roll near resonance and is a primary input to intact-stability weather and parametric-roll assessments, since radiation damping alone underpredicts the real decay.