Model Experiments on Small Trawls Feasibility Study on the Double Rigged Versus Single Rigged Type

This paper provides a general feasibility study on double rigged versus single rigged trawls. This study used model experimental nets tested in a circulating water tank. Three kinds of trawls were tested, i.e. the Mexican semi ballon double rigged shrimp trawl designed by Nichimo (hereafter called net A), the traditional two seam single rigged Malaysian shrimp trawl (net B) and the four seam trawl modified from the above Malaysian trawl (net C). The head rope length of net A is about half that of net B and net C. Primary objectives are to determine physical characteristics, working performance and to estimate the main engine output necessary to pull these nets. To provide a double rigged shrimp trawl in the Malaysian shrimp fisheries, results of experiments are summarised as follows:1) At 2 knots, which is a normal towing speed of a Malaysian shrimp trawler, net mouth height of net A is 1.2 meter, net B is 2 meter and net C is 1.8 meter. In accordance to the behaviour of shrimp, these net mouth height are sufficient.2) In order to ensure the security of the catch, the most efficient average mesh opening was determined to be five vertical cross-sections. The mesh opening is bigger at the square panel and becomes smaller towards the cod end. The opening is about 3 centimeter for 5.7 centimeter mesh size.3) The wing tip spread of net A is 11 meter, which is 80% of its head rope length. Net B is 14 meter (55%) and net C is 14.4 meter (60%).4) The working performance of a shrimp trawl is determined by its sweeping area. The obtained experimental equations of the sweeping area of the three net are slimplified as follows:2 net A: A's=2.18Vnet B: A"s=1.57Vnet C: A‴s=1.58V5) The estimation of suitable main engine output necessary to pull the net is calculated from the conversion of total gear drag force values to horse power values. Equations of the above power of the three nets are:2 net A: EHP'=12.19V^<2.79>net B: EHP"=11.01V^<2.68>net C: EHP‴=10.89V