抄録
80,000 DWT oil tanker having small L/B and large C_b, and being driven by a 5-cylinder long-stroke diesel engine been built and delivered in 1981. This ship suffered from the afterbody vibration caused by the main engine and the propeller exciting forces. Among them, the propeller excited vibration was heavily increased in magnitude, intermittently at the interval of 20-40 seconds. The heavy vibration continued 2-5 seconds. It was considered that such phenomenon might be induced by the unstableness of the flow around afterbody, possibly accompanied by the impulsive surface force due to the collapse of the unsteady cavities or so called PHV cavitation. In order to clalify the above phenomenon and eliminate the impulsive propeller exciting force, the model tests were carried out in our circulating water channel. The detailed investigations of the flow around afterbody have revealed that there exists the unstableness of the flow. And two types of devices were tested to improve the flow. One was the stern tunnel and the other was the horizontal flat fin. The test results showed that the effect of the stern tunnel on the wake field is better than that of the holizontal flat fin, and that, as a result, the flow is made much steadier and stabler. Consequently, it was decided to adopt the stern tunnel for this ship. The effect of it has actually been confirmed by the sea trial successfully. The impulsive, fluctuating vibration has almost eliminated and the ship's vibration level has been improved remarkably.
