Model and Full Scale Ducted Propeller Cavitation Observations on a 280, 000 dwt. Tanker

Abstract

The application of ducted propellers to large full ships has recently been intensified. However, these highly loaded propellers using an accelerating duct are susceptible to the occurrence of propeller cavitation. Therefore, in adopting the ducted propeller for a 280,000 dwt. tanker, the M.S. 'THORSAGA' the authors conducted model tests in the cavitation tunnel and in the NSMB depressurized towing tank (vacuum tank) to study the ducted propeller cavitation. Also full scale cavitation observations were carried out succesfully during the sea trials of the M.S. 'THORSAGA', using a T.V. camera scanning the propeller. The observed results were recorded on video tape. Results of the cavitation tunnel and vacuum tank experiments are presented together with the full scale cavitation observations, and the correlation between the model and full scale observations is discussed. The model test procedures are also described. The correlation of the vacuum tank cavitation pattern with the full scale observations can be seen to be satisfactory, although the extent of the full scale cavitation is slightly larger than observed on the model propeller. Moreover, the vacuum tank/full scale observations appear to have a better correlation than cavitation tunnel/full scale observations. This difference is regarded to be mainly due to lack of simulation of transverse velocity components and the propeller-hull interaction in the cavitation tunnel. Thus for large full ships, the presence of the ship model in front of the propeller during the cavitation tests seems to be very important.