Seakeeping performance of semi-swath in following sea using controlled fins stabilizer

Abstract

Semi-SWATH ship design is a result of combining the good features of SWATH and Catamaran designs. However, the disadvantage of semi-SWATH is that she has low restoring force at bow that causes a tendency to bow-dive when running in following seas. In some critical conditions, the foredeck was found to be immersed underwater. One of the efforts to improve the ship’s performance is to install fin stabilizers at bow and stern. The fin stabilizers are used to compensate for the low restoring force at the bow by increasing the lift force and damping force. A fuzzy logic controller developed for the system gives the ability to transform human knowledge and experience into the controller system and also to regulate the fin angle. A numerical simulation program developed in time domain for surge, heave, and pitch motions are then validated by seakeeping tests in towing tank of Marine Technology Centre, Universiti Teknologi Malaysia. Using the simulation program, parametric study was conducted to relate the ship and wave parameters with the ship’s performance characteristics. The ship with fixed fin stabilizers has the bowdive and immersed foredeck conditions at the following situations: for wave height to length ratio, Hw/Lw = 0.07, the wave to ship length ratio is 1.1 = Lw/Ls < 1.4, and for Hw/Lw = 0.08, the wave to ship length ratio is 1.0 = Lw/Ls < 1.7. For ship with active fin stabilizers, at Hw/Lw = 0.08 and Lw/Ls > 1.6, it was found that the foredeck was immersed with low surfing speed. Meanwhile, for the ship with fixed fin at bow and active fin at the stern, it was found that bow-diving and immersed foredeck did not occur.