Johnsson and Stfntvedt 



the tests in the towing tank. 



The results of the full scale and model experiments 

 for different ship-propeller -configurations (five -and 

 six-bladed propellers, hull with and without stern 

 fins) have been analysed and compared. 



Included in the comparison are results of theoretical 

 calculations of the extent of cavitation over the pro- 

 peller blades, different kinds of shaft vibrations and 

 propeller inducedhull pressure fluctuations from ca- 

 vitating propellers. 



I. INTRODUCTION 



The ships included in the present investigation are the first 

 two in a series of tankers of about 230 000 tons deadweight, which are 

 to be delivered from Uddevallavarvet AB(UV) within the next few years. 

 The first ship, T/T "Thor shammer", was delivered in December 1969, 

 being equipped with a five-bladed propeller. 



The model tests in the towing tank indicated very good propul- 

 sive performance, which was confirmed by the results of the speed 

 trials with the first ship. 



At the trials with this ship extensive measurements of vibra- 

 tory response and pressure fluctuations on the hull near the stern- 

 post were carried out by Det norske Veritas (DnV) together with re- 

 cordings of different kinds of shaft vibrations [l] . The results of 

 these measurements indicated that the levels of the pressure fluctua- 

 tions registered by some of the transducers were very high ; in addition 

 a loud pattering noise could be heard in the aft part of the ship. The 

 vibration levels recorded were, however, not annoying. 



At the docking of the ship, shortly after the trials, erosion was 

 detected on four of five blades. The blades were modified but after one 

 voyage to the Persian Gulf and back, erosion was again observed. 



The high level of the pressure fluctuations was attributed to 

 significant development of various forms of unstable cavitation and it 

 was felt that there was a risk that the structure of the stern might be 

 damaged. A proposal was made to fit the ship with streamline fins on 

 the afterbody above the propeller and after extensive model testing of 

 different alternatives in the towing tank the ship was fitted with fins, 



582 



