Improvement of the 'Soft Door', a newly developed mouth opening device for trawls considering hydraulic dynamics

摘要

A new mouth opening device for trawls called "Soft Door", which prevents the damage that results from touching the sea bottom, has been developed. The "Soft Door" is constructed from canvas cloth and an iron frame, and floats and chains are attached to the door so that if raises and the canvas part expands by itself. A model test for clarifying the characteristics of the hydraulic dynamics of this opening device and actual towing tests of trawls with this door were carried out. From this model test, it became clear that though the lift force of the new door was similar to that of a flat board, its drag force was too large and its characteristics of lift to drag ratio was poor. These poor characteristics were attributed to the attachment of floats and sinkers for the purpose of maintaining its posture. In this study, a newly improved model door was made, in which the floats and sinkers were integrated into the canvas part. A further model test was carried out, in which the attack angles were changed and lift and drag force were measured. The characteristics of lift and drag were clarified and the effects of the improvement are discussed.