the sea condition was very smooth in which the significant wave height was about 7cm and the swell from open sea could not be observed in the experimental sea area.
Photo. I Displacement type boat
The other experimental ship is a training ship which is a high speed type craft of Hiroshima College of Mercantile Marine, her displacement is 20 tonnage, her length is 14m, her breadth is 3.7m, her draft is 1.1 m, her one shaft horse power of twin screw propeller is 265 SHW and her navigational ship speed is 16 knots. Photo.2 shows the high speed type boat This ship has been used for experiments of ship maneuvering for students. The measurement of ship waves by this small vessel has been carried out at the Oosaki Kamisima Island in Hiroshima Prefecture. During the wave experiment, the westerly wind blew slightly but the sea condition was relatively smooth for the reason of the closed sea bounded by many small islands in Seto Inland Sea.
Photo 2 High speed type boat
The Water depths at these observed sea area are about 20m and the ship waves are sufficient deep water wave. The ship waves were measured by the wave gauge of ultrasonic-type setting on the quay and the wave profiles were recorded on charts by using a multi-pens recorder. The maximum wave height and wave period of the ship waves were obtained from these wave profiles.
As the main characteristics of ship waves in the deep water depend on the ship speed and the distance from the sailing line, the measurements of ship waves were carried out in two kinds of experimental condition. One experiment was made under the condition varying the ship speed in the constant distance from the sailing line, and the other experiment was made under the condition varying the distance at the constant ship speed4). Fig. 1 shows the schematic measuring condition.
The ship speeds were determined from the speed trial. The distances from the sailing line were determined by reading the height of the pole using the reticle of binocular with a dial calculator on board. The pole is installed near the wave gauge and its height is 1m.
Fig 1 Schematic measuring condition of ship waves
3. TWO SIMPLIFIED ESTIMATION METHODS OF SHIP WAVES
Recently, the estimations of ship waves utilize the numerical computational method of Navier-Stokes equations or the panel method for the purpose of the correct calculation of the wave or frictional ship resistance5)6). However, these methods are very complicated and many efforts to calculate the ship waves are required. And the results obtained from these method are restricted only information of ship waves adjacent to the ship hull and wave profile along the hull surface, and it is very difficult to obtain information of ship waves propagated far from the ship hull for the reason of the narrow computational region restricted by memories of the computer.
Our present interest is focused on to obtain information of ship waves relatively simply, easily and reasonably. As we do not need the more detailed information about the ship waves under the present conditions, the simplified estimation methods of the ship waves as shown below are sufficient.
