The angle dependence of the roll damping moment

Bennett, Peter (1990) The angle dependence of the roll damping moment. Masters thesis, Memorial University of Newfoundland.

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Abstract

A new approach to the analysis of the free roll decay curve, the Energy method, is examined in this paper and compared with two commonly accepted methods of analysis, the averaging technique of Krylov-Bogoliubov and the Perturbation method. The Energy method uses the equality of the rate of change of the total energy of the system to the rate of energy dissipation due to damping. This method was concluded to be the best method for analysis for three main reasons. 1) It uses the whole roll decay record in its analysis, not just the peak values, allowing for single cycle analysis and analysis of shorter roll decay records. 2) It can separate the influence of the angle and velocity dependent components of the roll damping moment. 3) It can handle non-linear restoring terms which makes it viable for large amplitude motion. -- The Energy method and the Krylov-Bogoliubov method were used to analyze stillwater roll decay test records. The results of these analyses indicate a strong angle dependence of the roll damping moment. The quadratic and cubic velocity dependent forms, which are actually functions of the roll angle, of the roll damping moment were shown to be the most viable forms for the roll damping moment. This supports the findings of a strong angle dependence. Stillwater roll decay tests with forward speed were performed and also indicate a strong angle dependence of the roll damping moment. Comparisons of forced roll tests with predictions of forced roll motion using coefficients derived from the stillwater roll decay analyses proved inconclusive.

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