Download - MSC 134 Fishing Gear Technology II

MSC 134Fishing Gear Technology II

Trawl Plan Conversions

Standard Net Plan Layout: Cutaway View

3P2B

Net Plan with Hidden Lines: Uncommon

3P2B

Full Net Plan Layout: More Detail but not commonly used.

3P2B

3P2B

When converting net plans: Always pay attention to the Do’s

and Don’ts!

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 Exercise

In 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 Ratio

Determine 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 ÷ Lo

E1 = 602.16” (50.18’ x 12”) ÷ 821.25” (438 meshes x 1.875”)

E1 = 0.73322 (.73322 x 100 = 73.322%)

E1 = L ÷ Lo

E1 = 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 TRAWL

1 1/2 INCH STRETCHED MESH, NO. 12 THD.; NYLON

HANGING SPECIFICATIONS:

3 MESHES ON ? TIES

The 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 TRAWL

1 1/2 INCH STRETCHED MESH, NO. 12 THD.; NYLON

HANGING SPECIFICATIONS: 3 MESHES ON ? TIES

E1 = 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 3.299”Of 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”(3¼”) 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” = 0.20

You 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 40.14’

What 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’ ÷ 40.14’

in 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’ = 0.2028

You 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!

3P2B

292 meshes x .7972 = 232.78 meshes92m x .7972 = 73.34 meshes200m x .7972 = 159.44 meshesObviously we can’t have .78 meshes or .34 meshes, etc. so we must discretionarily round off the mesh counts.

You CANNOT Change tapers.

YES

3P2B

CALCULATION OF TAPERSBody Taper = 2 Cut ÷ Depth – Cut

Answer given in Bars/Points

Jib Taper = (2 x Depth) – 2 ÷ Length –Depth + 1Answer given in Bars/Meshes

11. Cut = Top – Bottom ÷ 2

= 292 – 92 ÷ 2 = 100

Body Taper = 2 x 100 ÷ 200 – 100 = 200 ÷ 100 = 2 Bars 1 Point

New 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 Point

Do 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 suppliers

New River Net Company, Sneads Ferry, NCEd’s Net Shop, Brunswick County

Through 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:E

2 =

200

mes

hes

E1 = 2 Stretch Feet per Pound ∞

How 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” ÷ 24”str. in. per pound

So 24 str. in. ÷ 1½ 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:E

2 =

200

mes

hes

E1 = 2 Stretch Feet per Pound

160M

233M80M 80M

36.5M 36.5M73M

56M

56M

12M

12M

176M

176M

80M 80M

56M

56M

78M

78M

19M

19M

54M

54M

73M

54M

73M19M

320M

116M 116M

116M 58M58M

30M

30M

1P2B

1P2B

3P2B

1M2B 1M

2B

WASTE

WA

STE

Calculate the quantity of webbing needed.

200

mes

hes

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. = 32.3125 pounds (33 lbs.)

What is the total twine surface area (not including Codend Bag) of the original net? The plans calls for a #15 Nylon twine (.051”

diameter) with a 1.875” stretch mesh? (see pg. 36 of textbook)

Prepare to calculate the twine surface area by:

Sketch each individual trawl part.Include:

# of meshes at the top of the part (N)# of meshes at the bottom of the part (n)# of meshes in the height of the part (H)

Determine stretched mesh sizeUse appropriate measurement system

Determine diameter of the twine*Use appropriate measurement system

*The twine diameter can be calculated as shown in the Fisherman’s Workbook, Pg. 9 or by referring to a “Twisted Twine Specifications” table found in various net catalogs.

(N+ n x H) 2 1000000

S = x 2 (a x Ø )

Calculating Twine Surface Area: “The drag of a net is proportional to the number and type of meshes in the netting, and to the orientation of the net panel(s) in the water.”

Fisherman’s Workbook, compiled by J. Prado, Fishing News Books, Oxford:1990 - Pgs. 36-37

WhereS = twine surface area (square meters)N = number of meshes at the top of the paneln = number of meshes at the bottom of the panelH = number of meshes in the height of the panela = stretched mesh (mm)Ø = diameter of twine (mm)

For use with the Metric or InternationalSystem of Measurements(ie. Square Meters)

(N+ n x H) 2 144

S =x 2 (a x Ø )

Calculating Twine Surface Area: When the values to be compared are given in “square feet” adjust the formula accordingly.

Fisherman’s Workbook, compiled by J. Prado, Fishing News Books, Oxford:1990 - Pgs. 36-37

WhereS = twine surface area (square feet)N = number of meshes at the top of the paneln = number of meshes at the bottom of the panelH = number of meshes in the height of the panela = stretched mesh (inches)Ø = diameter of twine (inches)

For use with the English or CustomarySystem of Measurement(ie. Square Feet)

292M

92M

200M

(N+ n x H) 2 144S =

x 2 (a x Ø )

Top & Bottom Body Parts:Customary System

S = (292+ 92 x 200) 2 144

x 2(1.875” x .051” )

S = (192 x 200) 144

x 2(.0956” )

S = 38400

144x 0.1913” = 7345.92

144

S = 51.013sq.ft. per part

2 parts = 2 x 51.013 sq.ft.TOTAL = 102.027 sq.ft.

1P2B

1P2B

Original 50’ Trawl

92M

68M

24M

(N+ n x H) 2 144S =

x 2 (a x Ø )

Top & Bottom Body Extension Parts: Customary System

S = (92+ 68 x 24) 2 144

x 2(1.875” x .051” )

S = (80 x 24) 144

x .1913

S = 1920

144x 0.1913” = 367.296

144



1P2B

1P2B


68M

68M

24M

(N+ n x H) 2 144S =

x 2 (a x Ø )

Remaining Parts: Customary System

Top Extension Part:

1 Part = 2.17 sq.ft

70M

70M

121M

Front Side Extension Part: (11.25sq.ft.)

2 Parts = 22.5 sq.ft.

70M

14M

200M

Side Tapered Part: (11.16sq.ft.)


3P2B

146M

36½

M

Jib Corner Part: (3.56sq.ft.)


1M2B1M2B

CODEND BAG NOT INCLUDED


What is the total twine surface area (not including Codend Bag) of the original 50 ft. net which uses a #15 Nylon twine with a 1.875” stretch mesh?

Top & Bottom Body Parts: 102.027 sq.ft.

Top & Bottom Body Extension Parts: 5.101 sq.ft.

Remaining Parts: 61.23 sq.ft.

TOTAL TWINE SURFACE AREA: 168.358 sq.ft. (Not including Codend Bag or Knot correction factor)


Prepare to calculate the twine surface area of the 32’

trawl by: Sketch each individual trawl part.

Include:# of meshes at the top of the part (N)# of meshes at the bottom of the part (n)# of meshes in the height of the part (H)

Determine stretched mesh sizeUse appropriate measurement system

Determine diameter of the twine*Use appropriate measurement system

*The twine diameter can be calculated as shown in the Fisherman’s Workbook, Pg. 9 or by referring to a “Twisted Twine Specifications” table found in various net catalogs.

233M

73M

160M

(N+ n x H) 2 144S =

x 2 (a x Ø )

Top & Bottom Body Parts:Customary System

S = (233+ 73 x 160) 2 144

x 2(1.5” x .051” )

S = (153 x 160) 144

x 2(.0765” )

S = 24480

144x 0.153” = 3745.44

144



1P2B

1P2B

32’ Trawl

73M

54M

19M

(N+ n x H) 2 144S =

x 2 (a x Ø )

Top & Bottom Body Extension Parts: Customary System

S = (73+ 54 x 19) 2 144

x 2(1.5” x .051” )

S = (63.5 x 19) 144

x .153

S = 1206.5

144x 0.153” = 184.595

144



1P2B

1P2B

32’ Trawl

54M

54M

19M

(N+ n x H) 2 144S =

x 2 (a x Ø )

Remaining Parts: Customary System

Top Extension Part:

1 Part = 1.0901 sq.ft

56M

56M

78M

Front Side Extension Part: (4.641sq.ft.)


56M

12M

178M

Side Tapered Part: (6.4303sq.ft.)

2 Parts = 12.8606 sq.ft.

3P2B

116M

30 M

Jib Corner Part: (1.849sq.ft.)


1M2B1M2B

CODEND BAG NOT INCLUDED

32’ Trawl

What is the total twine surface area (not including Codend Bag) of the new 32 ft. net which uses a #15 Nylon twine with a 1.5” stretch mesh?

Top & Bottom Body Parts: 52.02 sq.ft.

Top & Bottom Body Extension Parts: 2.56388 sq.ft.

Remaining Parts: 30.6277 sq.ft.

TOTAL TWINE SURFACE AREA: 85.21158 sq.ft. (Not including Codend Bag or Knot correction factor)

32’ Trawl

How does the total twine surface area of the two nets compare?What, if any, adjustments to the projected plans may be necessary based on your calculations?

ComparisonsOriginal Net: 50 ft. 1.875”str.mesh,#15 PA Twine

Original Net = 168 sq.ft.

Converted Net: 32 ft. 1.5”str.mesh,#15 PA Twine

Converted Net = 85 sq.ft.

32’ net is 36% smaller than 50’ net

1.5” mesh is 20% smaller than the original 1.875” mesh.

The square footage of the 32’ net is 49.4% less than the 50’ net

AdjustmentsThe trawl door size should be reduced by approx. 50%.A reduction in twine diameter (#15 to #12) could reduce the square footage (54.5% less than original) and the drag even more – allowing less H.P.Flotation and bridle lengths could be reduced.Winch requirements not as great as the original net.