Reduction of Propeller Induced Pressure Fluctuations by Systematic Air Injection

摘要

Competitve ship designs are characterized by low vibration levels on one hand as well as by low fuel costs. The market demands clearly indicate a trend for higher speed, leading to higher propeller loadings. Low fuel demand requires high propeller diameter which then results in lower tip clearance, which can be increased by tunnel shaped stern designs to a certain degree. Low pressure pulses, which are a condi-tio sine qua non for some ship types such as RoPax vessels require sufficient tip clearance to keep the pressure pulses at acceptable level. This becomes most important for ships which operate most of the time on shallow waters, as it is typical for North Sea or Baltic routes. Then, vibration levels can restrict the optimum service speed. To solve this problem, a system was developed that produces a stable air layer above the propeller. This air layer has a damping effect on the pressure pulses and during model tests it was verified that not only blate rate, but also the higher orders could be drastically reduced. During the model test, special testing techniques to cope with the scale effect had to be applied. It was found most crucial to generate a stable air layer, because pressure pulse reduction could only be achieved when the air layer was sufficiently stable. From the investigations and some theoretical considerations, an air generating device was optimised and later applied to the single screw RoRo- newbuildings of FSG. The operating of the device was thoroughly tested during the Trial Trip. Besides cavitation observations pressure fluctuation measurements have been performed. The paper describes the model test procedures as well as the Trial Trip results and gives an overview about the achieved results.