On the Sea Keeping Qualities of a Fishing Boat by Field Measurement―II A method to measure the encountered wave heights

摘要

In the study of the sea keeping qualities of a ship, it is considered indispensable to measure the encountered wave heights and to know the relation between the ship's motions and the ocean wave form, simultanously, during the ship's cruising course. From this point of view, the authors devised a step-type wave measuring apparatus which could be fixed on a side of the midship of a fishing boat for the measurement of the relative encountered wave heights vertical to the sea surface at a given point on the ship's body. Then, there is the problem to obtain the exact measurement of the encountered wave heights in the way described above, for the relative wave heights include the components of the ship's motions. In this study, as the relative wave heights were measured, the encountered wave heights were obtained by subtracting the ship's motions from the relative wave heights algebraically. On the other hand, as the pitching angles and heaving accelerations were measured during the ship's motions, it was necessary to convert accelerations into displacements by way of double integration on the basis of the recorded motions. There were some difficulties in the method of double integration of the recorded wave data, but the authors obtained a practically effective method. The following results were obtained from this study. 1) The double integration of the observed vertical accelerations which were recorded as analogous values was carried by the approximate integral method by the use of an active band pass filter with the frequency range from 0.03 Hz to 0.07 Hz. The amplitude gain was ±0.5 dB and the phase shift was less than 15 deg. 2) The error of measurement of the wave heights by this method could be estimated to be within 10% in the range of frequency for the heaving motions of the fishing boat by the experimental way.