CATAMARAN RESISTANCE A COMPARISON OF MATHEMATICAL THEORY AND EXPERIMENTAL RESULTS

This paper presents the results of wave-resistance calculations and model experiments on a seagoing catamaran which had a conventional form with prismatic coefficient 0.645.The waveresistauce of a catamaran ship is composed of two parts: the wave-resistance due to each hull itself and that arising from the mutual action between the two hulls.It is then derived that the resistance of catamarans should be the functions of ship form, Froude number and separation-length ratio. A series of computations based on the Michell-Havelocks thin-ship theory were made on the digital computer to investigate the effects of hull separation and speed on the resistance.The calculations show that in the speed range from Froude number 0.30 to 0.36, the interaction between the two hulls may result in favorable interferences if a suitable hull separation is chosen.The paper shows that the optimum hull separation decreases with increasing speed.In the speed range from Froude number 0.36 to 0.50, the interference resistance is always positive and thus the resistance of the twin-hull is greater than that of the corresponding one which has a hull separation equal to in finity. The above conclusions are confirmed by model experiments.But quantitatively, the measured resistance and optimum separation are somewhat different from the theoretical predictions.The optimum separation as determined from the model experiments is greater than that obtaind from mathematical computations.The model experiments also show that when the Froude number is less than 0.30, the effect of hull separation on the catamaran resistance is not significant provided that the distance between the two hulls (from centre to centre) is greater than one quarter the ship length. A comparison is also made between the resistances of a catamaran and that of a single-hull ship which has the same length, draught and displacement as that of the twin-hull ship.Althougth the catamaran ship has a larger wetted surface which introduces higher frietional resistance, but its resistance quality is superior to the corresponding conventional ship when the Froude number is beyond 0.34.