Abstract The fluctuating hull pressures induced by propeller have been a dominant excitation source of ship vibration。 In this study, a new technology has been introduced to effectively measure the hull pressures without trans- ducer installation holes。 It was applied to the hull pres- sure measurements of the 3 LNG carriers。 The meas- urements have been carried out in both ballast and loaded conditions。 From the measurements, the 1st, 2nd and 3rd harmonic pressure components of propeller blade frequency were dominant and the propeller fluc- tuating pressures were comparatively favorable。 It was confirmed that the new technology was very useful for obtaining the characteristics of propeller fluctuating pressures for actual ship。81935

Keywords Propeller fluctuating pressure; LNG carriers; measure- ment;  ballast/loaded;  magnetic sensor

Introduction Since the end of 1990’s, the ship vibrations have been continuously occurred together with the changes of ship structures and propulsion system for the larger and the faster ship。 To meet more comfortable habitability and severe vibration requirement by ship owner, the fluctu- ating pressures induced by propeller become prime importance。 They have been generally estimated by three methods such as a theoretical analysis, an experi- mental test at cavitation tunnel and a direct measure- ment by pressure transducer on ship。 Among these methods the measurement by pressure transducer is conventional method and provides more accurate and reliable results for propeller pressures on ship hull, on the other hand the holes for transducer installation is necessary。 The experimental test has been performed for reliable evaluation of propeller excitation by many insti- tutes and universities over the world however the   pres-

sure measurement data by transducer on actual ship are insufficient due to hull hole for transducer installation。文献综述

A new technology was developed to effectively measure the propeller fluctuating pressures, which uses magnetic sensors without transducer installation holes on ship hull。 It was applied to hull pressure measurements for actual ships during official sea trial。 The propeller pres- sure measurements for 3 LNG (Liquefied Natural Gas) carriers were uniquely carried out in both ballast and loaded conditions by new method in order to get the characteristics of propeller excitation force and the effect on ship vibration。

Propeller Fluctuating Pressure

The propeller, main engine and wave are main excita- tion sources of ship vibration。 The propeller excitation forces are pided into surface force and bearing force。 The surface force is the propeller fluctuating pressure on ship hull above propeller and is major source for deck- house vibration including ship aftbody structures。 For propeller with good performance and minimum cavita- tion, the excitation forces are evaluated theoretically at propeller design stage and tested by model propeller at cavitation tunnel and occasionally measured by pressure transducer at sea。

Standard Measurement Method for ITTC

The fluctuating pressures induced by propeller are typi- cally distributed longitudinally and transversely on ship bottom hull as shown in Fig。 1 and 2。 Based on charac- teristics of pressure distribution, ITTC recommend that for pressure measurement of actual ship, 10 points at least be carried out simultaneously in range of 60 ~ 80% of propeller diameter as shown in Fig。 3。 Because high cost and some holes for transducer installation are re- quired for ITTC (International Towing Tank Commit- tee) recommendation, on the whole ITTC proposal  is not applied for pressure measurement of general ship, therefore pressure measurements on 3 points as  shown in Fig。 3 are carried out, practically。

Fig。 1: Transversal Pressure Distribution

Fig。 2: Longitudinal Pressure Distribution