The powering performances of large waterjet and propeller driven catamarans at medium-speed

Wave-piercing catamaran builder INCAT has been developing new energy-efficient hull forms for medium speeds operation (Froude number between 0.25 to 0.50), to reduce the environmental impact of fast sea transportation. This research focused on the investigation into the performance of the potential propulsors for these new medium-speed catamarans which could either be waterjets or propellers. The challenge is in defining the appropriate speed changeover point between using waterjets and propellers for large catamarans operating in the medium speed regime. The main approach in determining which propulsor is more efficient was to perform experimental model testing of a self-propelled catamaran using a towing tank. These tests were done for both propeller and waterjet propulsion. A single demihull test using the tank wall as the plane of symmetry was employed rather than using the whole catamaran model in order to maximise the size of the model. The comparison of these two types of propulsion required new studies in several areas. A study was made as to investigate the magnitude of the scale effect on the small propeller used in the self-propulsion test. A method of scale effect correction was proposed as the scale effect corrections on the propeller proposed by ITTC 1978 method are inappropriate for corrections over large scales for variations in propulsors. The self-propulsion test (SPT) only method was chosen over the traditional ITTC 1978 method as some of the empirical approximation in the ITTC 1978 method were not applicable; uncertainty analysis revealed that the SPT method has lower uncertainties than the traditional ITTC 1978 method. The two catamaran models had a different hullform designed to the specific propulsion method. So both of these catamarans could not be directly compared as the length to displacement ratios and the wetted surface area ratio were not identical. Therefore comparative methods that can adequately compare these two catamarans were sought. Three approaches were made in order to assess the merit of the waterjet and the propeller as the propulsor for the medium-speed catamaran: an approach using the propulsive efficiency, an approach using transport efficiency or transport factor and an approach using a scaling to a similar size method. The study on the propulsive efficiency showed that the propeller catamaran is more efficient than the waterjet catamaran up to a Froude number of 0.33. In the transport efficiency approach, the propeller catamaran was found to be more efficient than the waterjet catamaran up to a Froude number of 0.44, at higher speeds waterjets become superior. Similar results were found using the scaling to a similar size method. These findings suggest that the propeller propulsion is considered to be the best option for a large energy-efficient catamaran operating in the medium-speed regime. Finally, methods of improving the ship powering prediction method for catamarans are proposed, based on the reasons for the variation in Froude number cross-overs from the different comparison techniques.