engleski

The Influence of Large Amplitudes on the Accuracy of Seakeeping Analisys

Experimental and theoretical investigations have shown that a linear analysis of the ship motions in waves gives acceptable predictions accuracy over a wide variety of sea conditions and vessel types if the incident wave amplitude is relatively small and if the resonance phenomenon is not present. Unfortunately both, behaving the ship in the extreme sea conditions and in the resonance range is unavoidable part of analysis in ship design. Much effort has been put in developing the suitable numerical tools to calculate the large amplitude ship response on practical and easy way. In spite of the fast recent computers and more accurate three-dimensional models, the simple preparation of input data make the traditional strip method (slender body theory) still very popular. In the article it is shown the extension of the traditional strip method for calculation of coupled heaving and pitching motion in vertical plane for large amplitudes. The non-linear effects due to large amplitude motion and variation of the wetted surface are taken into account through the variable hydrodynamic and restoring coefficients. An instantaneous draft is computed and these coefficients are interpolated for that draft. To solve the coupled non-linear time depended equations of motions, the trigonometric series and perturbation method is applied. The comparison of results with experimental values confirms the numerical results. The influence of resonant phenomena and ship speed variation on the perturbation coefficients is investigated for different wave amplitudes.

non-linear oscillatory ship motion in wave; numerical methods; perturbation method

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