Case Study: CODAG Power Plant for a Pleasure Boat: Theory - Project - Results

The gas turbine engine first went to sea in 1953 when the British Shipbuilding Company VOSPERS was selected to experiment with the gas turbine as a form of marine propulsion. The former Steam Gun Boat (SGB) HMS Grey Goose was rebuilt by the shipyard with the surprisingly sophisticated Rolls-Royce RM60 engine. Today, for any vessel project whose design function makes its speed or volume critical, the gas turbine has become a strong candidate for selection as a prime mover, due to its inherent power density and low weight. For marine use, the gas turbine is equipped with a power turbine in order to convert the energy in the exhaust gases into rotational energy. This energy is applied, via a shaft line, to a suitable propulsion system either by a direct mechanical drive system or by the intermediary of an electrical system: this is the case of yachts and mega yachts characterized by high speed performances. The case study concerns the motor yacht “OCI CIORNIE”, built by the shipyard PALMER JOHNSON in 1999 and powered with a CODAG (Combined Diesel and Gas) type propulsion plant made up of two diesel engines coupled to waterjets, via a reduction gear and by a gas turbine TF40, driving a surface piercing propeller (SPP) with Arneson transmission, via a reduction gearbox. According to expectations of the designer the top speed with the CODAG configuration was to be more than 55 knots while the speed with the only diesel engines running would be around 25 knots, but enough to reach the take off speed anyway. Instead, because of weight increase during the construction, the current maximum speed with the diesel propulsion is only 14÷16 knots, depending of the displacement and the sea condition, while the CODAG propulsion top speed is around 50÷52 knots. The paper will analyze the main steps of the development of the OCI CIORNIE project and will compare the performances of the planning stage with the final operating conditions of the yacht. Besides, the paper will consider, for the current displacement reached, the optimum distribution between the power of the diesel engines and the power of the gas turbine in order to obtain, for different speeds, the maximum range, the minimum wear and tear of the machines and, as a consequence, the minimum operating expenses, respecting the restrictions of the maximum torque on the gas turbine and on the diesel engines.