Effect Of Propeller Tip Clearance On Ship Hull Excited Force Induced By Propeller

Since the 70's in 20th century, the excited force induced by propeller on ship hull and the problem of violent vibration of the ship come from that force were being subjected to extensive concern in ship research field. On the later of few decades, the researcher concentrated on publishing mechanism and regulations for the propeller induced excited force, developed related theories and experimental techniques, and paid great effort in order to reduce the propeller induced excited force on ship hull. The ship hull excitation force produced by propeller, specially the pressure fluctuation on ship hull induced by cavitating propeller is one of the main causes of ship serious vibration. The magnitude of excitation force is related with●propeller geometry,●the loading,●cavitation extent on the propeller blade surface and●wake distribution, moreover it also mainly related with●propeller tip clearance.The propeller tip clearance, i.e. the distance between propeller tip and ship hull, is an important factor related to the magnitude of the excitation force on ship hull surface. Generally speaking, increasing tip clearance ratio can reduce the propeller fluctuating pressure on ship hull. However increasing tip clearance ratio would change the stern shape of the ship hull or reduce the propeller diameter, this certainly will affect the ship powering performance.This dissertation introduce starting from the basic concept of excitation force that propeller working in simulated non - uniform flow field and explain the bearing force and surface force. Bearing forces are the excitation forces that are transmitted to the ship hull via shaft line by unsteady propeller loading in a non - uniform flow field. Propeller induces a pressure field around it and also pressure fluctuations on ship hull surface, and the integrated fluctuating pressure is called surface force. Pressure fluctuations are more significant than bearing forces. And then the main sources of ship vibration are pointed out. There are theoretical and experimental ways in forecasting the excited level induced by propeller on ship hull. In theoretical way, the key of accurate forecasting of excited level is how to estimate the cavitation volume on propeller blade and its variation with time. Current advanced CFD technique has acquired great advancement in propeller cavitating flow calculation. But at present stage, related calculation results are not accurate enough for practice and need experimental validation. So, using experimental way to forecast propeller excited level is still very practical and dependable method.This dissertation discuss about the effect of propeller tip clearance on ship hull excited force induced by propeller by using experimental way. About the experimental method of measuring pressure fluctuation induced by propeller, this dissertation introduce concerning the experimental method include cavitation tunnel, wake field simulation, measuring system, similarity rules between model and real ship, etc.. When experiment performed in the cavitation tunnel, the signals of flow speed, fluctuating pressure, thrust signals and torque signals were amplified by an amplifier, then were converted and managed with A/D converter and transmitted to micro computer and at the same time the propeller shaft revolution signals were also sent to the computer through a frequency meter. After processing the measured pressure fluctuation signals by FFT (Fast Fourier Transformation), the values of pressure fluctuation coefficient of each blade can be obtained. In the cavitation tunnel, the simulation of non - uniform wake field behind ship stern is also the important fact while researching the pressure fluctuation induced by propeller. So this dissertation introduces the concept of wake field simulation and how to simulate wake field in cavitation tunnel.For container ship propellers their power densities are higher than that of other type ships, there are more possibilities of serious ship vibration caused by propeller induced excitation force. To reduce propeller power density and to improve propulsion efficiency propeller diameter can be adopted as big as possible, but this will reduce the propeller tip clearance, and increase the influence of propeller excited level on ship hull. In this dissertation, to analyze the effect of propeller tip clearance on ship hull excited force induced by propeller, a container ship propeller is taken as an example, the model propeller is a five blade propeller, made of Babbitt alloy, with a variable radial pitch distribution, a skew angle of 28.7°and a diameter of 240 mm. And the test for total of 27 combinations of the measurements of pressure fluctuation induced by the propeller working in a simulated non- uniform flow field under the conditions of (1) three different tip clearance ratios gt/D=0.25, 0.35, 0.45, (2) three different propeller loadings K_T =0.10, 0.15, 0.20 and (3) three different cavitations numbersσ_n =1.5,2.5, Atm are carried out in the cavitation tunnel lab of SSSRI. The main purpose is to find the range of propeller tip clearance that can make the level of ship vibration caused by propeller induced pressure fluctuation acceptable.The pressure fluctuation survey is arranged at five measuring points. Five pressure sensors were installed in a plate right above propeller model. The distance between the plate and the center line of propeller shaft can be changed for different tip clearance ratios. The measured pressure fluctuation data at different tip clearance ratios are processed with FFT (Fast Fourier Transformation). The measurement results and analysis such as pressure fluctuations, simulated wake fields and cavitation patterns are illustrated in tables, graphs and figures. The pressure fluctuation coefficients K_PL of the first to fifth blade frequency harmonic components are taken to be analyzed.The analysis of the experimental results indicate that(1) The effect of propeller tip clearance ratio on induced pressure fluctuation mainly depends on the combination of propeller loading (thrust coefficient K_T) and cavitation level (cavitation numberσ_n).(2) In the case of higher propeller cavitation induced pressure fluctuation, for instance when K_T= 0.15 andσ_n = 1.5, the increase of tip clearance ratio from the value of 0.25 to 0.35 or bigger can reduce the pressure fluctuation more effectively.(3) In the case of medium propeller cavitation induced pressure fluctuation, for instance when the pressure fluctuation coefficient K_P1 is less than 0.05, the smaller tip clearance ratio of about 0.25 is recommended.