Tom Wray's Gear Talk (As seen in Fishing News International - January 2001)
Comparison of the Drag & Fishing Performance of Two Types of Polyethyene Netting
Written by David Tait, Sr., of Crimond Enterprises. In a joint project with the Canadian Fisheries and the Government of Canada.
Harvesting fish is one of the most energy intensive forms of primary food production. In addition, recent increases in fuel prices and growing concerns over greenhouse gas emissions have further highlighted the need to improve energy efficiency in the fishing industry. For example, the Nova Scotia based trawlers involved in the nearshore Silver Hake fishery are 13 to 20m in length and consume from 11 to 15 gallons per hour. On average, fuel costs for this fleet have increased as much as 60% to 70% over the past 12 months.
Trawling is one type of fishing technology, however, which provides significant opportunities for improvements in energy efficiency. For example, reducing the towing drag or resistance of trawl nets is a very effective approach for improving fuel efficiency in this type of fishing method. One method for achieving such reduced drag is by using smaller diameter twines in the trawl netting. For example
Tricolor Elite High Tenacity Braided Polyethylene is a new netting material which is thinner and stronger per unit weight than the traditional or Regular Braided Polyethylene material.
This project was therefore undertaken on the Scotian Shelf September / October 2000 to evaluate the impacts on energy and fishing efficiency from the use of this new Tricolor Elite netting in Silver Hake trawls in place of the Regular Polyethylene Braided material.
The vessel used in this project was the “Carmelle 3” which operates from West Pubnico, Nova Scotia. This trawler is 19.8m in length, with a 420 horsepower engine driving a 1.6m diameter fixed propeller.
- In comparison with the trawl constructed with Regular Braided Polyethylene twine, the trawl constructed with High Tenacity Braided Polyethylene twine provided the following performance improvements. The gear tension was 8.3% less. The door spread, wingspread, and headline height were increased by 5.4%, 0.8% and 2.7% respectively. The swept area increased by 13.3%. The average hake catch per hour, and the catch per liter of fuel used, increased by 42.9%
- The decrease in the towing resistance of the trawl netting was actually greater than is implied by the that on average, about 25% of the drag after the doors is due to the sweeps, foot-rope and floats, and about 75% is caused by the netting. Since only the netting was changed in the two trawls, and since the gear tension measured after the doors decreased by 8.3%, this suggests that the drag force of the Tricolor Elite net itself was reduced by about 8.3/0.75 = 11%.
- The Tricolor Elite polyethylene trawl also resulted in catch rates increasing from an average of 350 to 500 kg./hour, a 42.9% higher catch rate than with the trawl constructed of regular polyethylene
Project Objectives and Participants
The specific objectives of this project were to compare the towing resistance, fuel consumption, trawl geometry, and fishing performance of two Silver Hake trawls of the same type, shape and size:
a) One constructed of Regular Braided Polyethylene, and
b) The other constructed from Tricolor Elite High Tenacity Braided Polyethylene.
To raise the profile of energy conservation in the fishing industry by making vessel owners and operations more aware of the potential fuel savings from the use of such lower drag trawls. This would be achieved through a variety of approaches including the distribution of reports and videos on this project.
This was a joint project with the following participants
- The Department of Fisheries and Oceans, Responsible Fishing Operations, (Andrew Duthie) supported by PERD which was responsible for the overall co-ordination of the project.
- David Tait Sr. and William Tait were responsible for constructing the trawls, conducting the experiments at sea, analyzing the results, as well as preparation of the project report and video.
- Captain Aubrey D’Entremont of the MV “Carmelle 3” and Inshore Fisheries Ltd. Of West Pubnico, Nova Scotia, the vessel owners.
- The Marine Institute of St. Johns, Newfoundland, (John Foster) supported by the Canadian Center for Fisheries Innovation, were responsible for developing the experimental protocol, construction of a model trawl, flume tank trials.
Both trawls were based on the same two bridle Silver Hake trawl design, with a 504 x 15 cm circumference, and with Rockhopper ground gear. Identical trawl doors and ancillary equipment were also used with both trawls, as well as selectivity grids. A plan of this trawl design is included as Exhibit 2.
Exhibit 2 Silver Hake Trawl
The mesh size in both trawls was 150mm in the wings, square and 1st bellies, and 60mm in the remaining bellies. The wings, square and 1st bellies in one trawl were constructed with 3mm Regular Braided Polyethylene, and in the other trawl with 2mm Tricolor Elite High Tenacity Braided. The remaining bellies in the two trawls were constructed with 1.8mm Regular and 1.3mm Tricolor Polyethylene respectively. A weight and strength comparison of these two materials is shown in Exhibit 3.
Exhibit 3 A Comparison of Twine Characteristics
Runnage Meters per Kg.
1.3 mm Tricolor Elite
1.8 mm Tricolor Elite
2 mm Tricolor Elite
3 mm Tricolor Elite
The additional cost of a trawl made with Tricolor Elite is estimated to be less than $1,000. This similarity in trawl prices is explained by the fact that even though the price of the high tenacity netting is more per kilogram, the netting for such trawl is lighter (331 vs. 368 kg). As well, the twine surface area of the trawl constructed of standard polyethylene was estimated to be 84.7 m2 compared to the 59.3 m2 surface area of the Tricolor Elite trawl, a reduction of over 42%.
The experimental trials were conducted on Emerald Bank, which is located on the Scotian Shelf, as shown in Exhibit 4.
Exhibit 4 Location of Fishing Trials
The trials were carried out during five commercial fishing trips from late September and early October 2000. These were based on a plan of 15 fishing days within a total period of 18 days, including rigging and steaming time to the fishing grounds.
At the commencement of the work at sea, both trawls were towed for a minimum of two tows of one-hour duration to check that the geometry of the two gears were similar. After any necessary adjustments were made, the experiment proceeded with the standard commercial towing time. Generally, an attempt was made to provide each trawl with equal fishing time, and the same number of tows with and against the tide to minimize any experimental bias. Each type of trawl was switched after a series of eight valid tows. Data on 25 valid tows with each type trawl were compiled.
Trip lengths were restricted to four days, of which two days were for steaming to and from port. This was due to high water temperatures and concern about fish spoilage if trips exceeded four days. All fish were boxed at sea and by-catch rates were extremely low due using the selectivity grid which is mandatory in this fishery.
During trips four and five, large amounts of “slime” marine growth was observed in both types of trawls. This masking of the meshes with marine growth reduced wing and door spreads, and increased trawl resistance and fuel consumption. This phenomenon is common in these fishing grounds and it was considered important to include data from these tows to give a realistic picture of the true fishing conditions in this fishery.
The specific types of data collected during each tow is illustrated in Exhibit 5.
|Haul #||Mean Headline Height|
|Bottom Type||Mean Wingend Spread|
|Wind Speed and Direction||Mean Door Spread|
|Wave Height||Swept Area of Trawl|
|Light Conditions (Surface)||Swept Volume of Trawl|
|Start Towing Time||Swept Area of Gear|
|End Towing Time||Swept Volume of Gear|
|Tow Direction||Distance Towed from Start to End|
|Tide Direction||Fuel Consumption|
|Distance Towed||Engine Load|
|Speed over Ground||Shrimp Catch|
|Mean Log Speed|
The average performance of these two trawls during the 50 fishing tows is shown graphically in Exhibit 6 and in a tabular format in Exhibit 7.Exhibit 6 Graphs of Results
Exhibit 7 Experimental Results
|Type of Polyethylene Netting||Regular Braided||Tricolor Elite High Tenacity Braided||% change|
|Gear Tension (kg)||3150||2,890||-8.3%|
|Door Spread (m)||35.4||37.3||+5.4%|
|Wing Spread (m)||14.66||14.77||+0.8%|
|Headline Height (m)||5.88||6.04||+2.7|
|Swept Area (m3)||278,000||315,000||+13.3|
|Fuel Liters / Hour||
|Hake Catch / hr (kg)||350||500||+42.9%|
|Catch (kg) liter fuel||6.6||9.4||+42.9%|
In comparison with the trawl constructed with Regular Braided Polyethylene twine, the trawl constructed with High Tenacity Braided Polyethylene twine provided the following performance improvements:
- The gear tension was 8.3% less
- The door spread, wingspread, and headline height were increased by 5.4%, 0.8% and 2.7% respectively.
- The swept area increased by 13.3%.
- The average hake catch per hour, and the catch per liter of fuel used, increased by 42.9%
The decrease in the towing resistance of the trawl netting was actually greater than is implied by the measured gear tension. Previous tests have indicated that on average, about 25% of the drag after the doors is due to the sweeps, foot-rope and floats, and about 75% is caused by the netting. Since only the netting was changed in the two trawls, and since the gear tension measured after the doors decreased by 8.3%, this suggests that the drag force of the Tricolor Elite net itself was reduced by about 8.3/0.75 = 11%.
- Although the measured gear tension decreased by 8.3%, there was no corresponding reduction in the fuel consumption rate of the engine. This could have been caused by a variety of factors including:
- The engine power is used both to move the vessel through the water, as well as to tow the trawl. Although most of the power output is absorbed by the trawl gear at towing speeds, significant changes in sea and wind conditions can also cause appreciable variations in the power requirements to simply propel the vessel.
- The towing speed relative to the sea bottom (ie: as measured by Loran) was kept as constant as possible.
- The speed of the vessel and trawls relative to the water, however, is the key determinant of power requirements and fuel consumption. This could have varied significantly during these trials because of changing current velocity and direction.
- Significant marine slime accumulated on the fishing gear during several trips and this had a marked impact on the net geometry and towing drag.
Nevertheless, the theoretical power required to tow a trawl is equal to the towing tension times the towing speed relative to the water. Since the tension measurements were averaged for varying current conditions, one might could reasonably assume that under ideal experimental conditions, the energy requirement to just tow the trawl constructed Tricolor Elite polyethylene trawl could be about 11% less than with the regular polyethylene trawl. This is a substantial energy saving!
The Tricolor Elite polyethylene trawl also resulted in catch rates increasing from an average of 350 to 500 kg. /hour, a 42.9% higher catch rate than with the trawl constructed of regular polyethylene. This is probably because of the larger swept area achieved with the improved trawl geometry provided by this netting material. If such higher catch rates are typical of trawls constructed with this material, then this may be the major source of future energy and economic savings in this fishery. For example, the annual quota for Silver Hake on the Scotian Shelf is currently 20,000 mt. In theory, if all the trawlers in this fishery increased their catch rates from 350 to 500 kgs/hour, then this quota could be caught with about 17,000 less hours of towing. With an average fuel consumption of 50 liters/hour, this would imply a total annual fuel savings for just this fishery of about 850,000 liters.
It should be noted that current stocks and catches in this Silver Hake fishery are at an all time low. For example, catches by foreign vessels in this fishery peaked at 300,000 mt in 1973, however it is not expected that future sustainable catches will be anywhere close to this peak. The truly large potential for fuel savings from the use of modern netting materials such as Tricolor Elite polyethylene and Spectra, however, will be in all of the other Canadian trawl fisheries. Although Atlantic ground-fish stocks are currently in a depleted state, it should be noted that in the 1980’s, Canadian landings were over 500,000mt per year (ie: 25 times current Silver Hake landings). In addition, current catches in the Canadian shrimp fisheries are over 100,000 mt per annum.