MSC 134 Fishing Gear Technology II
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MSC 134Fishing Gear Technology II
Trawl Plan Conversions
Standard Net Plan Layout: Cutaway View3P2B
Net Plan with Hidden Lines: Uncommon 3P2B
Full Net Plan Layout: More Detail but not commonly used.3P2B3P2B
When converting net plans: Always pay attention to the Dos and Donts!You CAN!Change Mesh Sizes.Change Mesh Counts.You CANNOT!Change Hanging Ratios. (except within 2.5%)Change tapers.Change Net Orientations. (E1 & E2 alignment)Double Check all work Before Beginning Construction
Trawl Plan Conversion: Class ExerciseIn the handout Traditional Trawls you have a set of plans for a 50 ft., 4-seam flat shrimp trawl which specifies a #15 nylon twine and a 1.875 inch stretch mesh. We will convert the original trawl plans to facilitate building a 32 foot, 4-seam flat trawl. Our new trawl will be built with a #15 nylon twine and a 1.5 inch stretch mesh. We will do the calculations and answer the following questions before completing a revised trawl schematic. The first thing we should do is calculate the hanging ratio (E 1) used in the original 50 ft. trawl plans?
Calculating the Hanging RatioDetermine which variables are known and utilize the equation E1 = L/Lo.E1 is the Primary Hanging Ratio.L is the hung distance along the top or bottom of the net.Lo is the number of meshes multiplied by the stretch size of the mesh in a given distance.
E1 = 4.125 5.625 (3 meshes x 1.875)E1 = 0.73333 (.73333 x 100 = 73.333%)E1 = L LoE1 = 602.16 (50.18 x 12) 821.25 (438 meshes x 1.875)E1 = 0.73322 (.73322 x 100 = 73.322%)
E1 = L LoE1 = 668.16 (55.68 x 12) 911.25 (486 meshes x 1.875)E1 = 0.73323 (.73323 x 100 = 73.323%)
What hanging ratio (E 1) should you plan to use in the new 32 ft. trawl? 73.3% or as close as physically possible (Calculate the new hanging specifications)32 ft., 4-SEAM FLAT TRAWL1 1/2 INCH STRETCHED MESH, NO. 12 THD.; NYLONHANGING SPECIFICATIONS:3 MESHES ON ? TIESThe hanging ratio (E 1) used in the original 50 ft. trawl plans is 73.3%.
You CANNOT Change Hanging Ratios. (except within 2.5%)
32 ft., 4-SEAM FLAT TRAWL1 1/2 INCH STRETCHED MESH, NO. 12 THD.; NYLONHANGING SPECIFICATIONS: 3 MESHES ON ? TIESE1 = L Lo and the original plans indicate .7333 = 4.125 5.625 Therefore the new Lo is 4.5 (3 meshes x 1.5) and the desired E1 is 73.3% (.7333). The L value in this problem would represent the measurement of the ties.Solve the problem by using the equation with the known variables E1 and Lo: L = E1 x Lo.L = .7333 x 4.5 which mathematically indicates an L value of 3.299Of course a measurement of 3.3 is not necessarily practical when using a standard tape measure to lay out the distance of the ties (pick-ups).
What could a practical discretionary answer to the questions be?
Instead of 3 MESHES ON 3.299 TIES = E1 of 73.3% A more practical approach may be:3 MESHES ON 3.25(3) TIES = E1 of 72.22% (within 2.5%)E1 = 3.25 4.5 (3 meshes x 1.5)E1 = 0.7222 (.7222 x 100 = 72.22%)
3 MESHES ON 3.3125(35/16) TIES = E1 of 73.61% (within 2.5%)E1 = 3.3125 4.5 (3 meshes x 1.5)E1 = 0.7361 (.7361 x 100 = 73.61%)
The original 50 ft. trawl plans utilized a 1.875 inch stretch mesh. The new 32 ft. trawl plans will be using a 1.5 inch stretch mesh. What percentage of decrease does this change in mesh size represent?
Percentage is a part of a whole expressed in hundredths.The equations we use for finding E1 and E2 values are basic percentage problems. Percentage gives us a method of comparing quantities. It helps us make a comparison where the relationship is not easy to see at once. Percentages will be used in every aspect of fisheries related work.
In this case you want to find what per cent of 1.875 (17/8) the measurement 1.5 (11/2) represents. First, write the problem in the form 1.5 = ? x 1.875. You can find the missing factor by dividing 1.5 by 1.875.1.5 1.875 = 0.80 therefore 1.5 = .80 x 1.875 Generally speaking the relationship we have just calculated shows us that a 1.5 stretch mesh is 80% of the size of a 1.875 stretch mesh. This represents a 20% decrease in mesh size. Another way we could have accomplished this is:1.875 1.5 = .375 therefore .375 = ? x 1.875 so .375 1.875 = 0.20You CAN Change Mesh Sizes.
If you left all the mesh counts the same as on the original 50 ft. trawl plans but utilized the 1.5 inch stretch mesh size, how big would the trawl be? We just determined the new mesh size is 20% smaller than the original mesh size therefore we can safely assume the new trawl will be 20% smaller than the original trawl. Original Trawl Plans = 50.18 Headrope New Trawl Plans = 20% smaller or 80% of the original Headrope Therefore ? = .80 x 50.18 which means the new trawl would be 40.14What percentage will you have to decrease all the mesh counts from the smaller net in order to yield a 32 ft. trawl using a 1.5 inch stretch mesh?
You CAN Change Mesh Counts.
Based on our previous use of percentages to make comparisons of differentials in mesh size, why not use percentages to make comparisons to mesh counts.New Mesh Size = 40.14 Headrope with a 1.5 stretch mesh.We Want a 32 Headrope with a 1.5 stretch mesh.32 = ? x 40.14 or ? = 32 40.14 therefore .7972 = 32 40.14in order to yield a 32 ft. trawl using a 1.5 inch stretch mesh? Generally speaking the relationship we have just calculated shows us that a 32 trawl is 79.7% of the size of a 40.14 trawl. This will represent a need to decrease the mesh count by 20.3%. Another way we could have accomplished this is:40.14 32 = 8.14 therefore 8.14 = ? x 40.14 so 8.14 40.14 = 0.2028You CAN Change Mesh Counts.
Decrease several of your key mesh counts by the percentage you just calculated and check your plans to see if the body tapers will remain the same when the new mesh counts are used. Do the new mesh count tapers match the original mesh count tapers? If your answer is no, recalculate your answers and make corrections before continuing!
3P2B292 meshes x .7972 = 232.78 meshes92m x .7972 = 73.34 meshes200m x .7972 = 159.44 meshesObviously we cant have .78 meshes or .34 meshes, etc. so we must discretionarily round off the mesh counts. You CANNOT Change tapers.YES
3P2BCALCULATION OF TAPERSBody Taper = 2 Cut Depth CutAnswer given in Bars/PointsJib Taper = (2 x Depth) 2 Length Depth + 1Answer given in Bars/Meshes1Cut = Top Bottom 2 = 292 92 2 = 100Body Taper = 2 x 100 200 100 = 200 100 = 2 Bars 1 PointNew Calculations: 292 meshes x .7972 = 232.78 meshes = 232 or 233 meshes 92m x .7972 = 73.34 meshes = 73 or 74 meshes 200m x .7972 = 159.44 meshes = 159 or 160 meshes Therefore: Cut = Top Bottom 2 so 233 73 2 = 80 Body Taper = 2 x 80 160 80 = 160 80 = 2 Bars 1 PointDo all the new mesh count tapers match all the original mesh count tapers?
The size of a trawl is defined by its headrope. Check the newly calculated mesh count along your headrope and by using your new mesh size (1.5) and the intended primary hanging ratio, verify whether or not you will yield a 32 trawl. Do your calculations check out correctly?E1 = 384.24 (32.02 x 12) 522 (348 meshes x 1.5)E1 = 0.7361 (.7361 x 100 = 73.61%)
3 MESHES ON 3.3125 TIES = E1 of 73.61%E1 = 3.3125 4.5 (3 meshes x 1.5)E1 = 0.7361 (.7361 x 100 = 73.61%)
E1 = L Lo116 meshes x 3 = 348 meshes along the headrope.348M x 1.5 = 522 and 522 12 = 43.5 stretch feet (Lo) 43.5 stretch feet x .7361 (E1) = 32.02 feet (L)
Convert the plans: Calculate all the new mesh counts by applying the percentage indicated previously (.203 or .7972) and label the schematic accordingly. (note: use only whole numbers for mesh counts, round as necessary) Label the Top, Bottom & Stretched Mesh Length dimensions with the new linear measurements. Double check all work before beginning construction!
How many pounds of nylon webbing would you need to order so you could build the 32 four seam flat trawl you just converted/designed? Supplies can be purchased:Through local suppliersNew River Net Company, Sneads Ferry, NCEds Net Shop, Brunswick CountyThrough Catalogs and/or InternetMemphis Net & Twine - CatalogsNylon Net Company Catalogshttp://www.nylonnet.com/
Ordering:You will be ordering#15 Nylon Twine with a 1-1/2 Stretch MeshMaterial is priced by the poundCheck your catalog for the stretch feet to the pound
Ordering:E2 = 200 meshesE1 = 2 Stretch Feet per PoundHow many meshes would we have if we ordered 1 pound of the webbing we plan to use? (E1 & E2)E1 = L Lo therefore 100% = 24 24str. in. per poundSo 24 str. in. 1 str. in. = 16 meshes per poundE2 is given as 200 meshes deep.The values given in the catalog for the depth in feet of a particular mesh size of webbing used in the construction of hydro-dynamic gear is generally based on an E1 value of 71%.
Configure the parts of the trawl to be cut:E2 = 200 meshes E1 = 2 Stretch Feet per Pound160M233M80M80M36.5M36.5M73M56M56M12M12M176M176M80M80M56M56M78M78M19M19M54M54M73M54M73M19M320M116M116M116M58M58M30M30M1P2B1P2B3P2B1M2B1M2BWASTEWASTE
Calculate the quantity of webbing needed.200 meshes 2 Stretch Feet per Pound Based on our previous work we decided we would need a section of webbing that was 517 meshes long (E1) by 200 meshes deep (E2). The webbing cited above would not allow for waste and/or future repairsNor does it include the material needed for the codend bag.Therefore 517 meshes 16 meshes per pound = 32.3125 pounds (33 lbs.) or 517M x 1.5 = 775.5 st.in. 24 st.in. per lb. = 32.3