A study of roll motion and roll damping for biased small fishing vessels

Wang, Zhiguo (1992) A study of roll motion and roll damping for biased small fishing vessels. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (13MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Three sets of experiments with small fishing vessel models in biased condition were conducted at the towing tank of Memorial University of Newfoundland. A simulation of the asymmetric rolling motion of one of the models in beam waves without forward speed was also performed. -- The first set of experiments was carried out with three fishing vessel models denoted by M363, M365 and M366 to measure the roll damping. Hundreds of decay curves were obtained from free roll tests for the analysis of biased roll damping coefficient. The Energy Approach developed by Haddara [10] was applied in the analysis. The results show that the nondimensional equivalent linear damping coefficient ζΕ increases linearly with average amplitude of roll angle. The effect of bias angle on roll damping varies from model to model. In the case of M366, ζΕ increases linearly with bias angle. The same tendency is seen for M365 when its roll amplitude is larger than ten degrees. However, ζΕ decreases with bias angle when the roll amplitude is lower than ten degrees. In the case of M363, the damping coefficient ζΕ shows a nonlinear relationship with bias angle for low roll amplitude, i.e. ζΕ decreases with bias angle first, then increases. For the large roll amplitude, ζΕ increase with bias angle almost linearly. -- The second set of experiments was carried out with M363 in regular beam waves without forward speed. The model was tethered in drift and yaw but all other modes were free. The model was set under two different bias conditions and was tested both biased towards the wave source (wave maker) and away from the wave source. The results indicate that the effect of bias angle is pretty slight. -- The last set of experiments was performed also with M363 in beam waves without forward speed but the model was restrained through a pivot in every mode except for roll. The pivot was placed at the model's roll center in the first group of tests; for the second group of tests, the pivot was transversely moved to a new position whose transverse coordinate is the model's actual center of gravity and vertical coordinate was unchanged. The results can be summarized as follows. -- When the model was biased towards the wave source the roll amplitude was much larger, compared with when the model was biased away from the wave source or under no bias condition. -- In the frequency domain, the roll amplitudes show a strong resonant phenomenon when the model is biased towards the wave source or under no bias condition. On the other hand, the curve of roll amplitude with respect to the wave frequency is fairly flat when the model is biased away from the wave source. -- The results do not reflect any significant effect of pivot position. This is believed to be due to the very small distance between two pivot positions. -- The results from the simulation of the rolling motion of the biased M363 under restrained conditions show reasonable agreement with experimental results, especially for the tendency of roll amplitudes with respect to the wave frequency. It is also indicated by the simulation that not only bias and wave direction but restraints are factors which make the roll amplitude large when the model is biased towards wave source.

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