Design of Baltic Ferries for the Eighties, Propellers and Aft End Vibrations

The Wartsila Shipyards are building seven Baltic ferries between, 1979 and 1981, four of them are prototypes. The Turku Shipyards are building six of these ferries. Some of the main design criteria are high propulsive efficiency at various sailing conditions, low vibration and noise level, enough power for ice-navigation without problems, good maneuverability. To meet these criteria a lot of attention has to be paid for the design of hull form, propellers and aft ship construction. Results of theoretical propeller calculations are presented. Model test results for three prototype ferries are presented, self-propulsion, cavitation and pressure pulse measurements. Comparative measurements were performed with a convential and a so called highly skewed c.p. propeller model. Calculations of natural frequencies and modes and forced vibrations caused by propeller pressure pulses are presented. Estimates made for the vibration level are presented. Forced vibration tests made with a vibration generator are presented. During the sea trials the cavitation phenomenon has been photographed, pressure pulses and vibrations have been measured. Comparisons between theoretical propeller calculations, model tests and full scale measurements are presented. Also vibration calculations, forced vibration tests and sea trial measurements are compared. Conclusions are drawn of the applicability of special c.p. propellers to reduce the vibration level in ice strengthened high-powered ferries.