| The tuna purse seine should sink fast after completing the setting around the fish school by the fishing vessel due to the characteristic of the main targeting specie, skipjack tuna (Katsuwonus pelamis) with high swimming speed, swift reaction to objects. According to the fishing principle of tuna purse seine, soon after the targeted fish school is surrounded by the netting, the low edge of the wall shaped netting is closed to form a purse and the catch is detained inside the bunt by hauling in the purse line and gathering the purse rings. So whether or not the purse seine is rapidly sinking and the bottom rings are quickly gathered together are the keys to determine successful operation and fishing efficiency. The sinking performances have been usually treated as the main research contents in the tuna purse-seine fishing operations.The sinking performances of a purse seine can be studied through full-scale net measurement at sea, by numerical simulation on computers or the model net test in specially designed tank. Results from full scale net measurements at sea can better reflect the sinking performance of the purse seine in real fishing operation; however, it is hard to conduct the measurements following statistical experimental design because of the high cost. Various factors affecting the sinking performance of the purse seine can be tested by modeling net test. But, the error could be very big when interpreting the full scale net sinking performance from the modeling test results because the dimension of model net test facility limits the scale used. Numerical simulation can theoretically simulate the sinking process of the purse seine at the visualized interface, but its effectiveness will depend on if there are enough data from full scale measurements or model test. Model testing with large scale model net can increase the reliability in interpreting the performance of the prototype from the model net test by reducing the error from different sources.In the paper, study on the tuna purse seine performance by model test. The sinking performance of a 1:20 scale model purse seine has been measured by DR-1050 minitype depth meter recorder in QianDao Lake (thousands Island Lake),Zhejiang Province between October 24 and 26, 2009. The prototype net, the 1618.7m×311.1mtuna purse seine, is the typical tuna purse seine currently used by Chinese tuna purse seine Vessels at Western and Central Pacific Ocean. The model purse seine is made following the Tauti's modeling law withλ=20 andλ′=1. The main dimension of the model purse seine are: float line length 80.9 m, the maximum height of the net 15.6m, leadline length 98m, basic weight of the leadline 0.663kg/m,buoyancy of the floatline 25N/m, the same hanging ratio between the model net and prototype is kept. Three boats have been used during the model net test, with one boat as the net boat and the other two as the auxiliary boats. The average depth of the lake is 31m, the model net test were conducted on the open waters with the water depth over 25m and without current. The results are summarized as follows:(1) The sinking depth The maximum sinking depth of different parts of leadline is obvious in consistentwith the net design of the purse seine as a whole, i.e, with the maximum sinking depth in the middle part of leadline and decreasing towards the two ends; The bigger fluctuations of sinking depth was found at the back lead line end than at the front lead line;After 100s, the lead line sinking depth had little change; The relationship between the sinking depth (h) and time (t) at the middle part of leadline can be described as h = ?0.0016t 2 +0.3124t?0.77(1 R2=0.999)(2) The sinking speedDifferent parts of the lead line sinking speed increased with the increase of sinking time decreases, but the sinking speed of the aft part lead line was the fastest decrease, while the middle part lead line under the relatively uniform down, the sinking speed of the forepart lean line decreased rapidly in the 0-40s, after 40s then decreased slowly ; in the 0-20s, the sinking speed of the aft part lead line was the fastest, followed by the middle and fore part lead line; after 20s, the sinking speed of the middle part lead line exceeded the aft part lead line; and the sinking speed of the fore part lead line less than aft part lead line and middle part lead line early, but in the 46s and 58s, respectively exceeded aft part lead line and middle part lead line。(3) the effect of the leadline weights and the net setting speed on the sinking speed of the model purse seineThe lead line weight and the net setting speed had significant effect on sinking speed of lead line, but there was no significant interaction between the lead line weight and the net setting speed to sinking speed of lead line.The sinking speed of the forepart, middle part aft part and of the whole leadline was significantly higher at 0.795kg/m of the leadline weight than at other level of leadline weights used. But no significant effect was found when using both 0.531kg/m and 0.597kg/m of the leadline weight. The relationship between the sinking speed of the lead line and lead line weight can be described as the following function: V = 0.1796w 2 ?0.2877w+0.1526(R2=0.858)The sinking speed of lead line increased with the setting speed. various setting speeds planed for the model net test has significantly affected the sinking speed of both the middle part of the leadline and the lead line in whole. However, when the netting was shot at the speed of 0.75m/s and 0.85m/s, the net shooting speed had no significant effect on the sinking speed of both the fore and aft part of the leadline. The relationship between the sinking speed of the lead line and the net setting speed can be described as the following function: V = ?0.0375ν2 ?0.0118ν+0.1491(R2=0.998)... |
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