船舶乗組員による風観測の特性

Abstract

Nowadays, the wind reports from Japanese ships are believed to be comparatively correct because almost of them have a wind vane and an anemometer. The present method of the wind observation that is actually carried on ships for the weather report is as follows ; (1) The observation is put into practice in a few minutes before the appointed time. (2) The observer looks at the apparent wind indicator for 10 seconds or so. (3) The average wind speed and direction for a looking period are determined by intuition. (4) The true wind speed and direction are lead from the apparent wind speed and direction and ship's speed and course. (5) The true wind speed and direction are reported at every 10 degree units and at every 1 knot units, respectively. According to the report data with reference to the automatically measured data, we found out that the most of data have some difference between them. Therefore we tried to find the cause of these differences. In the result, the error of the reported data is caused at above each process, and it is amplified by the reading error of apparent wind indicator. The fluctuation of indicated wind in the following wind is greater than in the head wind. So, the readings of indicator in the following wind have to be carried more carefully. Generally speaking, the period for reading indicator should be recommended to be more than 30 seconds. Furthermore it is desirable to be more than 100 seconds for the indicated wind with a sharp fluctuation.