Roll Decay Test

A roll decay test measures a ship’s natural roll period and its roll damping by inducing a heel, releasing the ship, and recording the decaying oscillation that follows. It is the standard experimental route to the damping coefficients that seakeeping and dynamic stability analysis need, and it is performed both at model scale in a basin and at full scale on the ship.

The test yields two things. The period between successive peaks gives the natural roll period, which combines with the metacentric height to give the roll radius of gyration. The rate at which successive peaks decay gives the damping, which is amplitude-dependent, so a decay curve is fitted with linear and quadratic or cubic terms rather than a single coefficient.

The full article will cover the conduct of the test at model and full scale, the excitation methods used, the extraction of linear and nonlinear damping coefficients from the decay curve, the scale effects that separate model damping from ship damping, the relationship to the Ikeda component method, and the use of the result in the second generation intact stability criteria.