Fig。 4: Magnetic Sensor Installation at Dock Yard

Characteristics  of  Magnetic Sensor

The subminiature pressure sensor was utilized for mag- netic sensor originally designed by HMRI and its char- acteristics were shown in Table 3。

Table 3: Characteristics of Subminiature Pressure Sensor

Capacity 2 kgf/cm2

Rated Output 0。892 mV/V Safe Excitation 3 V

  Input & Output Resistance 120 ohm

For attaching this subminiature pressure sensor to the hull surface uses the magnet as shown in Fig。 5。 On this occasion, the most careful point is the interference phe- nomenon caused by the effect of a magnetic field when the pressure transducer and the magnet are joined。

surface because of high magnetic capacity above 100 Gauss。

Fig。 6:  Magnetic Sensor

The capacity of pressure transducer was determined in consideration of the maximum speed and draft of the subjected ship and the result of model test。 From this examination, the capacity of pressure transducer was determined as 2 kgf/cm2 in considering safety factor for the measurement of fluctuating pressure of the LNG carrier。

Calibration

The linearity of the signal obtained from subminiature pressure sensor attached on the magnet is verified by the calibration procedure after the completion of magnetic sensor。 The pneumatic dead weight tester as shown in Fig。 7 was used for calibrating the magnetic sensor。 From this test, the linearity and hysterisis of magnetic sensor was confirmed as shown in Fig。 8。

Fig。 7:  Pneumatic Dead Weight Tester

Fig。 5:  Sketch of Magnetic Sensor

Also, for preventing the drop of response ability from the sensor the adhesive spread to magnetic face was selected carefully。

The magnetic sensor in Fig。 6 is almost impossible to artificially detach from hull after attaching it on the hull论文网

Reference  pressure (psi)

Fig。 8:  Calibration Result for Magnetic Sensor

For certifying the measurement possibility of the actual vessel’s fluctuation pressure by the complete magnetic sensor, the fluctuating pressure was measured in cavita- tion tunnel through setting up the subminiature pressure sensor and the general pressure sensor (Galtec) used at cavitation tunnel on acryl panel as shown in Fig。 10。

Fig。 9:  Cavitation Tunnel

Fig。 10: Comparison Test Set-up for Hull Pressure Meas- urement Sensor in Cavitation Tunnel

According to the experimental result, the sensitivity and signal characteristics of two different sensors appeared nearly same level and the harmonic component of the fluctuating pressure gives good agreement as shown in Fig。 11。

Table 4:Comparison of Test Results for Pressure Trans- ducers

Fig。 11:  Hull Pressure Measurement Results in Cavitation

Tunnel for 5/6 Blades

From the experiment, it was confirmed that the mag- netic sensor was reasonable for propeller fluctuating pressure measurement of the actual ship。

Pressure Measurement for LNG Carriers

Synopsis for Measurement

The measurements of fluctuating pressures induced by propeller were carried out for 3 LNG carriers。 The main particulars of the LNG carriers were shown in Table 5。 The purposes of measurements are to identify the pres- sure characteristics, to investigate the effect on vibration and to confirm the effectiveness of new method on pressure measurement。

Table 5:  Main Characteristics of Subjected Ship

Ship A