Research On Seakeeping Performance Of Longitudinal Motion Of SWATH Ship

In recent years,with the rapid development of the marine economy and the gradually deepening exploitation of marine resources,the requirements for ship performances have been improved,and the seakeeping is a very important performance index.As high-performance ship,the SWATH ship is favored by ship owners for its excellent seakeeping performance and small added-resistance in wave,wide deck area and modular construction Due to the special structure,it will produce violent longitudinal movement during navigation,so it is important to accurately predict the longitudinal motion of the SWATH.At present,optimizing the shape of the submerged body of the SWATH ship to improve its resistance and seakeeping performance is a hot spot in the research field.In this paper,a numerical tank was established based on the CFD(Computational fluid dynamics)method to calculate the resistance performance of SWATH-6A ship in calm water.The result is in good agreement with the experimental data,and the maximum error is 2.09%,which proves reliability of the CFD method.Based on the 3D potential theory,this paper only considers the hydrodynamic lift and inertial force of the stabilizing fin and the effect of ship damping.This paper calculates the heave and pitch RAO curves at zero speed and fixed speed in regular head waves,and comparing with results of STF method and experiments.The results of 3D potential theory are in good agreement with the experimental data,which verifies the accuracy and validation of the 3D potential theory and provides the basis for the following calculations.This paper uses SWATH-6A as the parent ship,the influence of the camber of the lower hull of SWATH ships on the resistance and longitudinal motion performance is studied under the condition that the displacement and the shape of waterplane remain unchanged.With airfoil of NACA0008,1408 and 2408 as the middle longitudinal profile shapes of lower hull,three different models,named Model #1～3,are established.The calm water resistance of four SWATH ship models are calculated and the wave resistance of the model which has best calm resistance performance among Model #1～3 by using CFD method.The performance of added resistance in waves at designed inflection point was calculated and compared with that of SWATH-6A.It is found that the total resistance curve and residual resistance curve of SWATH-6A decrease obviously in the stage of Fr=0.340～0.397,showing favorable wave interference.Among the three models,Model #2 has the best calm water performance.Compared with SWATH-6A,Model #2 has smaller wave added-resistance response coefficient and smaller drag vibration amplitude,so its resistance performance in waves is better than SWATH-6A.In this paper,the AQWA-Diffraction module is used to calculate hydrostatic coefficient,heave RAO and pitch RAO of the four models at 0kn and 17.5kn in regular head waves.It is found that the longitudinal movement performances of models #1～3 are better than those of SWATH-6A and the peak value of longitudinal motion response with speed is obviously smaller than that without speed.The AQWA-Response module is used to simulate random waves distribution of the sea state of level 3,4 and 5 in China’s offshore waters by using the JONSWAP wave spectrum.Through the method of spectrum analysis,the significant value of the longitudinal motion response amplitude and the significant value of longitudinal acceleration response amplitude of the four models in head wave at 0kn and 17.5kn are calculated and compared,showing that longitudinal motion response of the SWATH ship still meets the seakeeping requirements under high sea conditions and has a high comfort The heave and pitch RAO and longitudinal motion response in irregular wave of Model #2 are better than those of SWATH-6A,which indicates that the curvature of the lower hull has an influence on the seakeeping performance.It is found that the longitudinal motion response performance and wave added resistance performance of Model #2 are better than those of SWATH-6A,which lays a foundation for subsequent ship shape optimization and also provides a new design idea and scheme for SWATH ship profile.