# Study on Reducing Towing Drag by Varying the Shape and Arrangement of Floats and Gears
## 1 Introduction  
Fishing with towed gear is a popular fish-catching method worldwide. The bag-shaped net can be towed through the surface, middle, and bottom layers of the sea. Because of its flexibility, this gear can be used on many types of fishing grounds by both small and large vessels for a wide range of target species [1]. This fishing method provides a valuable food source for humans, however, it is one of the most energy-intensive food-production methods [2]. Fishing with towed gear depends heavily on internal combustion engines powered by oil fuels. Fuel costs in the fishing industry have risen substantially over the last 40 years, owing to three major oil price spikes. The rapid increase in fuel costs in recent years has severely affected the profitability of many fisheries. Additionally, the medium-term forecasts for oil prices indicate a high likelihood for further and steady increases [1, 3]. Consequently, the increase in operational costs resulting from high oil prices in the past few years have become a serious concern for the fishing industry [4]. The rise in fuel costs, labor shortages, and increases in other costs have caused a decline in the use of towed gear by fisheries [5]. Since the 1980s, problems such as the decline of fishery resources, strict regulation of international fisheries, and competition have continued to threaten fishery management in this industry [5]. Fishing with towed gear is increasing worldwide, more than other methods, and is responsible for approximately 40% of worldwide production [6]. However, in East Asia, due to limited resources and competition between Korea, Japan, and China, statistics show that the production of fishing with towed gear in Korea and Japan has been decreasing [7; 8]. To reverse this decline in the pair trawl fishery, a deficit reduction in fishery management is necessary. There are several ways to reduce this deficit, such as increasing the benefits of fishing and/or reducing expenditure. All operational expenses, such as fuel and labor, have risen substantially [5]. Fuel expenditure has accounted for 10-20% of total expenditure since 2005 [5]. Therefore, it is necessary to eliminate the deficit by reducing fuel costs. Research and development on various energy-saving technologies in fisheries have been promoted to reduce fuel costs [1; 9; 10; 11; 12], but fuel costs continue to account for the majority of fishery expenditures. Many efforts have been made to reduce fishing's fuel consumption with towed gear, such as using fuel additives to improve the burning efficiency of engines and changing the vessel engine for improved fuel consumption efficiency [5]. However, these efforts were not continued because they imposed a heavy burden on fishermen. In practice, using fuel additives and replacing vessel engines incurred high costs for the fishermen. Therefore, one way to mitigate this issue for towed fishing gear is to reduce gear drag. It has been reported that low gear drag consumes less fuel than does existing gear [1; 11]. Several ways to reduce drag have been proposed. Decreasing the towing speed with a low engine speed using a suitable nozzle and propeller [13] is one suggestion. An appropriate nozzle and propeller could result in fuel savings of 20%. A slight reduction in cruising speed to and from the fishing ground reduces fuel consumption [14]. However, with decreased towing speed, the catch of the target fish may also decrease. Another option is to use modern design trawl doors and nets to reduce drag. A large portion of drag in a towed bottom trawl is due to the resistance of the trawl doors required to spread the trawl [14]. The modern design of trawl doors reduces this resistance, and the use of thinner and stronger twine and an increase in net mesh size can also result in substantial fuel savings [14].