The Research On The Seakeeping Performance And Longitudinal Motion Stability Of Small Water-plane Area Twin Hull(SWATH)

For a long time, how to improve the seakeeping performance and extend the ship operating time has been an important issue for ship designers. To ensure the safety of cargo and persons onboard during ship navigation, the research and analysis of seakeeping performance is of great importance. SWATH vessel is made up of three important parts: two submerged bodies away from the waterline, the upper structure and the connecting struts. them. Comparing with the considerable displacement of conventional monohull, there are several advantages for SWATH, such as low wave- making resistance, rich deck area, excellent seakeeping performance and so on. Since the geometric characteristics of SWATH structure, different layout forms of SWATH under the same displacement can be easily got.Based on three-dimensional potential flow theory, SESAM software was used to study the cross-section of different submerged bodies with transverse section of circular and ellipse; different struts with inclined angles(0°,15°,30°,45°) and different struts location’s influence on seakeeping performance at the design speed. By comparing the frequency response curves, these factors affect are considered on the influence of SWATH’s seakeeping performance, it is concluded that the seakeeping performance of SWATH will be improved as the struts inclination angle increases.as, it can effectively improve the seakeeping performance of SWATH;comparing the submersible body cross-section with ellipse and circle,the seakeeping performance with ellipse cross-section is slightly improved in some wave direction; move the SWATH’s strut from the centerline position to the outside of the center line,the roll degree has a little improvement, for heave, pitch degree, it has a smaller impact. In addition, combined with SWATH navigation area’s sea condition, we completed the seakeeping short-term forecasting at the design speed and get the significant motion magnitude and the most probable maximum amplitude etc.Combined with the global wave statistics scatter diagram, using relevant mathematical statistics theory, we accomplish the long-term forecast of ships under specific sea area. This article gets ship’s roll, pitch, heave response amplitude at a certain probability of exceedance.Compared with the considerable displacement monohull, SWATH vessel has smaller Water plane area, which causes its ability to resistant heave and pitch motion is weak. Therefore, it is necessary to study its longitudinal motion stability.Calculation and experimental studies have shown that when ship Froude number exceeds 0.38 to 0.44, the longitudinal motion of SWATH is generally unstable. The inclined strut angle of 45° with elliptical cross-section SWATH is selected for the study.Hydrodynamic coefficients of motion equations is substituted into the Routh stability criterion to verify its longitudinal motion stability at the design speed. And heave and pitch motion simulation in different sea states at design speed in heading waves are carried out. And its motion attitude in waves is obtained.