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以西底曳網漁船の耐航性に関する研究


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タイトル: 以西底曳網漁船の耐航性に関する研究
その他のタイトル: Studies of the Seakeeping Qualities of Bull Trawlers by Means of Field Measurements
著者: 西ノ首, 英之
著者(別表記) : Nishinokubi, Hideyuki
発行日: 1983年 2月
出版者: 長崎大学水産学部
引用: 長崎大学水産学部研究報告, v.54, pp.61-131; 1983
抄録: 1) The study of seakeeping qualities has remarkably progressed in recent years, accompanied by the great progress in the study of ocean waves. These studies may be considered to form a part of the theoretical background of seamanship. Concerning fishing boats, it is necessary to clarify the characteristics of their behaviors on ocean waves not only for ship's stability but also for safety during fishing operaions at sea. In fact, the motions of fishing boats are affected by their fishing gear during fishing operations, and their motions in response to ocean waves are larger than those of other types of ships, e. g. a general cargo ship, in similar seaway situations. With this viewpoint in mind, researches on the seakeeping qualities of fishing boats were performed by making field measurements during fishing operations. 2) To be concrete, experiments were carried out aboard commercial fishing boats-bull trawlers-in the East China and Yellow Seas. There are several kinds of bull trawlers there, ranging from 50 GT to 300 GT. These bull trawlers operate throughout the year on these fishing grounds. Using a specially-devised wavemeasuring apparatus, the encountered wave heights and the motions of fishing boats were measured on board a bull trawler during fishing operations. The methods of the statistical analysis of stochastic processes were applied to the data thus obtained. 3) Measurement of wave heights (Chapter 4-1) In the study of the seakeeping qualities of a ship, it is indispensable to measure the encountered wave heights, and to obtain the relation between the motion of fishing boats and the form of ocean waves simultaneously, during the cruising course. From this point of view, the author devised a step-type wave measuring apparatus that could be mounted on a midship side of a fishing boat, and that could measure the relative encountered wave heights vertical to the sea surface at a given point of the ship's body. As the observed wave heights in this study include the components of the ship's motions, the true encountered wave heights were obtained by substracting the ship's motions from the observed wave heights algebraically. There were some difficulties in the method of double integration of the recorded data of heaving acceleration, but the author obtained a practically effective method. The error of measurements of the wave heights by this method was estimated to be less than 10% in the range of frequency for the heaving motions of the fishing boat by the experimental way. 4) The wave characteristics of the fishing ground on the East China and Yellow Seas (Chapter 4-2) From the relationship between the wave periods and its height observed in this study, it became clear that on this fishing ground both the wave height and the wave steepness were larger in winter than in autumn. The empirical equation was obtained as follows: Ts=3.29√H₁/₃ where Ts: Wave periods obtained by the zero-crossing method. H₁/₃: Significant wave height. The wave spectra in both seas were approximately similar to the spectra of the modified Pierson-Moskowitz or the I. S. S. C. wave spectra, giving the following numerical spectrum model of the wave as below: S(ω)=0.14H²₁/₃ω₁⁻¹(ω/ω)⁻⁵exp{-0.57(ω/ω₁)⁻⁴} where ω₁=2π/Ts. 5) Short-term maximum distributions of wave heights and the motions of fishing boats (Chapter 5-2) In general the statistical distribution of data is difficult to determine, but when the process has a narrow frequency spectrum, the data are distributed according to the Rayleigh distribution. The distributions of the observed data were considered to be coincident with the theoretical distribution, and these relations were also affirmed by x² test. As for these statistics, the ratios between each average of maxima and the theoretical standard deviations that were calculated by the Gaussian distribution, respectively, were shown as estimated coefficients of maxima. The linear regression equations were obtained between the significant wave heights and the significant values of the motions of fishing boats. Based on these results, it became possible to predict the maximum amplitudes of the ship's response motions in short-term. 6) Long-term maximum distributions of wave heights and the motions of fishing boats (Chapter 5-2) According to order statistics, the root mean squares of maxima on the record of each cruise were statistically analysed. The distributions of wave heights and heaving motions fitted well the logarithmic-normal and Weibull distribution functions, while the rolling and pitching motions fitted well the Weibull distribution functions. By considering the above results, the probabilities of occurrences of maxima for both wave heights and motions of fishing boats in operations could be obtained by the joint distribution of Rayleigh and Weibull. These probabilities can be applied to the problem of the limiting value. 7) Frequency response characteristics of fishing boats at sea (Chapter 5-3) As for all the data obtained by the experiments, the power spectra were calculated by Blackman-Tukey's method, with Akaike's smoothing coefficients. Moerover, for each of the encountered angles between waves and fishing boats, the frequency response functions were calculated by the cross-spectrum analysis method. In this system of ship's motion, input was wave heights and output was motions of fishing boats. The frequency response functions were shown as values of amplitude gain, phase shift, coherency and reliability for motions of fishing boats. The forced rolling period corresponded to the free rolling period, but as for the pitching and heaving motions, the peaks of power coincided with those in the wave spectra of their frequencies. As for the pitching and heaving motions, it can be said that fairly good estimations of frequency response functions were obtained. For example, it can be seen that the coherencies were above 0.7 and the reliabilities were below 0.3 in the frequency range from 0.25 Hz to 0.05 Hz in the heaving motions for beam and quartering seas, and that in case of pitching motions, coherencies were above 0.6 or 0.7 and the reliabilities were below 0.4 in the frequency range from 0.2Hz to 0.1Hz. Therefore when the frequency response functions were calculated by the strip theory, the so-called O. S. M., it was found that the amplitude gains in both cases were similar, especially in that of the heaving motions. The effects of fishing gear during fishing operations were not considered in the calculations by the strip theory, and it may be assumed that the difference mentioned above would be affected by them. Based on these results, pitching and heaving motions were considered to be satisfied with linear for the ship's motion system. 8) Estimation of numerical spectrum model of ship's motions (Chapter 6) The estimation of the motions of the fishing boats during fishing operation is the most important for the investigation of the safety and the fishing plan of the boat. The author tried to establish an empirical equation to estimate ship's motion spectra, based on the ship's response spectra obtained by field measurements. Then the average spectra were deduced for the rolling, pitching and heaving motions for each of the encountered angles between waves and fishing boats. The average spectra obtained by the above-mentioned method were nearly the same over the whole range of sea states experienced and all the heading angles, and these spectra were calculated by the empirical equation of the Gaussian form. The non-dimensional spectra for each motion were obtained by the least squares method, and given as follows: For rolling motion:S(ω)/(H²₁/₃/ω₁)=0.0830exp{-21.3258(ω/ω₁-0.9855)} For pitching motion: S(ω)/(H²₁/₃/ω₁)=0.0634exp{-11.8376(ω/ω1-0.9761)} For heaving motion: S(ω)/(H²₁/₃/ω1)=0.0647exp{-11.6779(ω/ω₁-0.9756)} where H₁/₃: The significant value of the motions of the fishing boats which would be deduced by the significant wave height or the r. m. s. values of wave heights. ω₁: The average circular frequency of the encountered wave to the ship in the pitching and heaving motions, and the natural frequency in the rolling motion.
URI: http://hdl.handle.net/10069/30424
ISSN: 05471427
資料タイプ: Departmental Bulletin Paper
原稿種類: publisher
出現コレクション:第54号

引用URI : http://hdl.handle.net/10069/30424

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