MSC 134 Fishing Gear Technology II Gill Nets: Concepts and Design.

MSC 134Fishing Gear Technology II

Gill Nets: Concepts and Design

Gill Net Design: Class Example

We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch square mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a “Sponge” SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately .02(.0163) ounces per foot of rope. We will also consider the “weight” of the webbing in our “design” calculations.

How did we decide what mesh size to use?*The mesh size of gillnets can be calculated as shown in the Fisherman’s Workbook, Pg. 98 or by referring to the handout “Entangling Nets, Gillnets and Seine Nets.”

“There is a ratio between the body girth or the length of a fish one wants to catch, and the gillnet meshsize which will be effective for that fish.”*

Choice of Meshsize (Textbook, Pg. 98)

OM = L (fish) / KOM = mesh opening (mm)

L (fish) = average length (mm) of fish one wants to catchK = coefficient, according to species

K = 5 for long, thin fish

K = 3.5 for average-shaped fish (neither very thick nor thin)

K = 2.5 for very thick, wide or high (shaped) fish

*Fisherman’s Workbook compiled by J. Prado, Fishing News Books, Oxford, England:1990. Pg. 98

OM = L (fish) / KK = 5 for long, thin fish

K = 3.5 for average-shaped fish (neither very thick nor thin)

K = 2.5 for very thick, wide or high (shaped) fish

If asked to design and build a gillnet capable ofcatching a quantity of Bluefish (Pomatomus saltatrix) withan average length of 12 inches,use the formula fromyour text to decide on a suitable mesh size.

Mesh Opening = 12” ÷ 3.5 = 3.42” (3½”)

Examples:

Mesh selection factor based on known values obtained by local sampling experience and/or historical data.

Mo = L / Km (L ÷ Km)

If a gill net with the mesh size M1 has caught fish of the length L1 and a gill net with the mesh size M2 catches fish of the length L2, then:

L2 / L1 = M2 / M1 (L2 ÷ L1=M2 ÷ M1)Therefore, L2 = L1 x M2 ÷ M1 and M2 = M1 x L2 ÷ L1

The numerical value of the ratio L1 ÷ M1 = KmThe Km is specific for each species of fish.Net selectivity data includes the hanging ratios, twine material and diameter, and the ballast and floatation specifications.File the net selectivity data and mesh selection factors for future reference.

MO = L / KmL2 / L1 = M2 / M1 (L2 ÷ L1=M2 ÷ M1)

Therefore, L2 = L1 x M2 ÷ M1 and M2 = M1 x L2 ÷ L1

If asked to design and build a gillnet capable ofcatching a quantity of Bluefish (Pomatomus saltatrix) withan average length of 12 inches,use the formula fromyour handout and refer to historical data from yoursampling area to decide on a suitable mesh size andnet design.

12.2”/10.3” = 3.5”/3”Therefore

L2 = L1 x M2 ÷ M1L2 = 10.3” x 3.5” ÷ 3”

L2 = 10.3”x 1.167

L2 = 12.02” (12”)

Examples:

OR

MO = L / KmL1 ÷ M1 = Km10.3” ÷ 3” = 3.4312.2” ÷ 3.5” = 3.49Therefore average Km = 3.46

So - MO = 12”/3.46 MO = 3.47” (3½”)

L2 / L1 = M2 / M1

12.2”/10.3” = 3.5”/3”Therefore

M2 = M1 x L2 ÷ L1 M2 = 3” x 12.2” ÷ 10.3”

M2 = 3” x 1.185

M2 = 3.56” (3½”)

Examples:

If asked to design and build a gillnet capable ofcatching a quantity of Bluefish (Pomatomus saltatrix) withan average length of:

MO = L / Km

14 inches then MO = 14”/3.46 so the mesh opening = 4.04”(4”)16 inches then MO = 16”/3.46 so the mesh opening = 4.62”(4½-4¾”)

18 inches then MO = 18”/3.46 so the mesh opening = 5.20”(5 -5¼ ”)

10 inches then MO = 10”/3.46 so the mesh opening = 2.89”(2¾-3”)

8 inches then MO = 8”/3.46 so the mesh opening = 2.31”(2¼”)

Remember: •The Km is specific for each species of fish.•Net selectivity data includes the hanging ratios, twine material and diameter, as well as the ballast and floatation specifications.•File the net selectivity data and mesh selection factors for future reference.

A spread-sheet designed to file your net selectivity data and mesh selection factors may look similar to the example shown below.

Mesh Selection Factors

Species Average Km Twine Hanging Stretch Gear Net Size (inches) Diameter Ratio Mesh Code Depth (ft.)

Atlantic Menhaden 10.40" (6) 3.47 .40mm 61% 3" 210 6'

10.56" (6) 3.52 .40mm 60% 3" 213 7'6.17" (2) 2.24 .40mm 67% 2.75 245 8'

Atlantic Sharpnose Shark 17.4" (1) 4.9 .40mm 60% 3.5" 213 7'

20.6" (2) 5.2 .40mm 60% 4" 213 7'

16.10" (1) 4.6 .40mm 60% 3.5" 213 7'

Bluefish 12.2" (4) 3.49 .40mm 61% 3.5" 213 7'

11.6" (3) 3.87 .40mm 50% 3" 220 7.5'10.3" (3) 3.58 .40mm 61% 2.875" 211 6.5'

11.44" (2) 3.52 .40mm 59% 3.25 245 8'

Harvestfish 4.96" (1) 1.8 .40mm 59% 2.75 245 8'

Ladyfish 18.31" (2) 4.6 .40mm 60% 4" 213 7'

Gear Descriptions

200 - Gill Net Unclassified:Length, mesh size,depth

210-150' floating,drifting,mono. gill net with 3 panelsof varying mesh size

211-150' floating, anchored, mono. Gill net with 3 panels of varying mesh size.

212-mono. Gill net, floating, anchored with variable length panels of variable mesh size.213-experimental gill net.220-monofilament gill net, sink, anchored

222-multifilament gill net, sink,anchored

230-monofilament gill net, floating, drifting

240-monofilament gill net, sink, drifting

241-monofilament gill net, sinking, run-around

242-multifilament gill net, sink, drifting

245- monofilament gill net, floating, anchored

Gill Net Design: Class Example

We would like to construct a gill net 150 yards long. We will be using a #6 (#139) monofilament webbing with a 1¾-inch square mesh. We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung. The primary hanging ratio should be 50%. We will use a “Sponge” SB-2 float with 3/16-inch diameter hollow braided polypropylene rope. The rope will have a buoyancy factor of approximately .02(.0163) ounces per foot of rope. We will also consider the “weight” of the webbing in our “design” calculations.

What is the “twine ratio” selected for this gillnet?

Calculating the Twine Ratio

When selecting a twine diameter for gillnets, the diameter should be proportional to the mesh size.The ratio should be between 0.0025 and 0.01.

.0025 for calm waters and/or low catches..01 for rough waters and/or bottom set nets.

An average ratio is 0.005.

*The twine ratio can be calculated as shown in the Fisherman’s Workbook, Pg. 99 or by referring to the handout “Entangling Nets, Gillnets and Seine Nets.”

Ratio = Twine Diameter ÷ Stretch Mesh Size(always use the same units of measurement to compare the two variables)

Twine Diameter = .40mm

and .40mm = .015748 in.(1mm ÷ 25.4mm = .03937 so .40mm = .03937in. x .40mm = .015748 inches)

Stretch Mesh Size = 3½ in.

and 3½ in. = 88.9mm(1in. = 25.4mm so 3½ in. = 3½ in x 25.4mm =88.9mm)

.40mm ÷ 88.9mm = .0044994 = .005

Or

.015748in. ÷ 3.5in. = .0044994 = .005

Exactly how much rope do I need to complete this net?

935 Feet (920’ to 950’)

Exactly how much rope do I need to complete this net?

How much “Classic” hollow braided polypropylene rope would you order?

What would the rope cost?

935 Feet (920’ to 950’)

One 1200’ reel cost $24.70. Two 600’ reels cost $26.00 ($13.00 ea.) Since we need two lengths of rope - is it worth $1.30 not to separate and measure the ropes, a job that would take approximately 30 to 60 minutes?

1200 Feet

$26.00 (Discretionary Decision – Why?)

How many meshes deep should I order the webbing?

E1 = L/Lo.E1 is the Primary Hanging Ratio.L is the length of rope on which a net panel is “hung”.Lo is the length of stretched netting before hanging.

E2 = H/Ho.E2 is the Secondary Hanging Ratio.H is the actual height of a “hung” net.Ho is the height of the stretched netting before hanging.

Meshes Deep is a function of E2

How many meshes deep should I order the webbing?

1 Mesh = 1¾” Square = 3½” Stretch

E1 = 50% (.50) therefore E2 = 87% (.87)**

Finished Gill Net = Approximately 7½ feet deep.

**Refer to Fisherman’s Workbook, pg.38; the handout “Introduction to Netting Theory”or use the formula E2 = 1 – E1²

E2 = H ÷ Ho therefore H = E2 x Ho and Ho = H ÷ E2

The answer can be found by mathematically using the known variables and then referring to the “materials available” in our catalog.

How many meshes deep should I order the webbing? 30 Meshes Deep

1 Mesh = 3½” Stretch …. E2 = 87% (.87)*…. 7½ feet deep.

E2 = H ÷ Ho therefore H = E2 x Ho and Ho = H ÷ E2

H = .87 x 3.5” therefore the “hung” height of one mesh = 3.045”

and 7.5 ft. x 12 in. = 90 inches so 90” ÷ 3.045” = 29.557 Meshes obviously we will look for 30 Meshes Deep in our catalog.

Proof: 30 meshes x 3.5” = 105 str.in. H = E2 x Ho so H = .87 x 105” = 91.35” ÷ 12” = 7.61 ft.E2 = H ÷ Ho so E2 = 91.35” ÷ 105” = .87 (87%)Ho = H ÷ E2 so Ho = 91.35” ÷ .87 = 105”

How many pounds of webbing will I need to complete this net? 7.2 Pounds

E1 = L ÷ Lo150 yd. Net x 3 ft. = 450 foot long Net• 125 stretch feet per pound (Stk. #N221A)• 125 str. ft.(Lo) x .50 (E1) = 62.5 hung Ft./Lb. (L) • 450 ft. net ÷ 62.5 hung Ft./Lb. = 7.2 Pounds

What would the total cost of this webbing be?

How many pounds of webbing would you order?8 Pounds

$70.805 thru 24 Lbs. = $8.85 Lb. therefore 8 Lb. x $8.85 = $70.80

Can’t order partial poundage unless ordering a “hank”. The catalog presents a hank as the “standard” most people would base their gill net calculations with:•A 100 yard net hung on a “half” would require 4.8 pounds of stk. # N221A webbing.• 125 str.ft.(Lo) x .50 (E1) = 62.5 hung ft./lb. (L) • 300 ft. net ÷ 62.5 hung ft./lb. = 4.8 Pounds

How many feet of repair material would you have left if you order 8 pounds of webbing?• .8 lbs. x 62.5 hung ft./lb. = 50 hung feet for repairs.

When hanging the webbing to the headrope and footrope, how many meshes will you insert into every “pickup”?

When hanging the webbing to the headrope and footrope, what will the distances of your “pickups” be?

Options: 1 mesh = 3.5” str.

E1 = .50 or 50%

L = .50 x 3.5” = 1.75” per mesh

2 meshes = 3.5”

3 meshes = 5.25”

4 meshes = 7”

Refer to standards discuss in the handout “Entangling Nets, Gillnets & Seine Nets”

Complete a schematic of the gill net showing a “one unit” section representing the distance and number of pickups between floats and the number of pickups between leads. *Complete the schematic in detail, all answers concerning floats and leads are based on this schematic.

“Unit” Size? “Pickup” Size? Meshes per “Pickup”? Floatation per “Unit”? # of Leads per “Unit”? Ballast per “Unit”?

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Label “Pickup” Size

Label “Unit” Size

Show Lead Placement

Show Float Placement

# of meshes

Gill Net Schematics


“Unit” Size? “Pickup” Size? 7”Meshes per “Pickup”? 4Floatation per “Unit”? # of Leads per “Unit”? 2Ballast per “Unit”?

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42” Unit

7”

Option 1: 4 meshes @ 7 inches

Units: This net is designed to be 7½’ (90”) deep. 50% of 90”= 45” which we have defined as the maximum distance between floats. 45” ÷ 7” = 6.42 “pickups” between floats. We can’t have partial “pickups” so 6 x 7” = 42”


42”


“Unit” Size? “Pickup” Size? 5.25”Meshes per “Pickup”? 3Floatation per “Unit”? # of Leads per “Unit”? 2Ballast per “Unit”?

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42” Unit

5¼”

Option 2: 3 meshes @ 5¼ inches

Units: This net is designed to be 7½’ (90”) deep. 50% of 90”= 45” which we have defined as the maximum distance between floats. 45” ÷ 5¼” = 8.57 “pickups” between floats. We can’t have partial “pickups” so 8 x 5¼” = 42”


42”


“Unit” Size? 49”“Pickup” Size? 7”Meshes per “Pickup”? 4Floatation per “Unit”? # of Leads per “Unit”? Ballast per “Unit”?

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49” Unit

7”

Option 3 (Discretionary): 4 meshes @ 7 inches

This schematic utilizes a “unit” that exceeds the “50% of hung depth” rulesummarized in the “handout”. However, it does not exceed the general ruleof “75% of the hung depth” and thus becomes a discretionary decision andas such should have an attached explanation.


How many floats will you need to order?

What is the total cost of those floats?

Actual Floats Needed: Floats Ordered:

Total Cost:

128 130

$41.60

Option 1: One float every 42” = 128.57 Floats (128 or 129 Floats) [450’ (length of net) x 12” = 5400” ÷ 42”(units) = 128.57 floats]

Option 2: One float every 42” = 128.57 Floats (128 or 129 Floats)

Option 3 (Discretionary): One float every 49” = 110.2 Floats (110 or 111) [450’ (length of net) x 12” = 5400” ÷ 49”(units) = 110.2 floats]

[Catalog states 50 or more SB2 Floats = $ .32 each = 130 floats x $ .32 = $41.60]

What size lead are you planning to use? Stk. #How many leads per pound?

How many leads per unit?

How many pounds of lead will you need?

• Leads are designated by Hole Diameter, Length and Number of Leads per Pound.• Diameter of the lead hole must be approximately 1/8” larger than rope diameter.

• Stk. LS6 (1/2”) = 6 Leads per Pound = 2.67 ounces each (16 oz. ÷ 6 = 2.6667)• Stk. LS8 (1/2”) = 8 Leads per Pound = 2 ounces each (16 oz. ÷ 8 = 2)• Stk. L8 (3/8”) = 8 Leads per Pound = 2 ounces each (16 oz. ÷ 8 = 2)• Stk. L10 (7/16”) = 10 Leads per Pound = 1.6 ounces each (16 oz. ÷ 10 = 1.6)• Stk. L13 (3/8”) = 13 Leads per Pound = 1.23 ounces each (16 oz. ÷ 13 = 1.2308)• Stk. L16 (9/32”) = 16 Leads per Pound = 1 ounce each (16 oz. ÷ 16 = 1)

• The “handout” specifies a spacing of leads not to exceed 2 to 4 per float.• A “sink gillnet” would need a minimum ¾ to 2 ounce ballast per unit.• A “floating gillnet” would need a minimum ¾ to 2 ounce buoyancy per unit.• 2 #L8 Leads = 4 oz. - - - 2 #L10 Leads = 3.2 oz. - - - 3 #L13 Leads = 3.69 oz.

Number of Leads per unit x # of units in the net ÷ # of leads per pound = Pounds

“Unit” Size? 42”“Pickup” Size? 7”Meshes per “Pickup”? 4Floatation per “Unit”? 2.64 oz.# of Leads per “Unit”? 2 (#L8)Ballast per “Unit”?

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42” Unit

7”

Option 1: 4 meshes @ 7 inchesFloatation per “Unit” = 2.64 ounces

Floats = 2½ oz. (catalog)Rope = .02 oz. per ft. (approximate given)

.0163 oz. per ft.(actual calculation) One unit = 42” therefore 84”(7’) of rope per unit, so 7’ x .02oz. = .14 oz. per unit (approx.)

or 7’ x .0163oz. = .1141 oz. (actual)2.5oz. (float) + .14oz. (rope, approx.) = 2.64 oz. (2.6141, actual)

Rope: The handout “Ropes and Fibers” from the MSC 132 class contains a Fiber Cordage table* which indicates the “typical weights and minimum strengths”.

Nominal Size (inches) Polyolefins (polypropylene/polyethylene)Dia. Circ. Net Wt. 100' Ft. per lb. Breaking Strength3/16 5/8 .73 137 7501/4 3/4 1.24 80 1,250

*Similar information is shown in the table “Synthetic Fiber Rope” in Fisherman’s Workbook, Pg.16.

1 Pound = 16 ounces16 oz. ÷ 137 ft. = 0.116789 oz. per ft. of rope

or16 oz. X .73 lbs. = 11.68 oz. ÷ 100 ft. = 0.1168 oz. per foot

HoweverTo obtain the weight in water of a certain quantity of material, you must multiply its weight in air by the factor given in the “Density of Materials” table found in your textbook on page 3. Obviously, polypropylene(PP) is listed as a “Floating Material”. Polypropylene(PP) = 0.14- (Multiplication factor in sea water)

Therefore0.1168 oz. X .14-= 0.016352 oz. per foot of rope (approx. 0.02 oz.)

“Unit” Size? 42”“Pickup” Size? 7”Meshes per “Pickup”? 4Floatation per “Unit”? 2.64 oz.# of Leads per “Unit”? 2 (#L8)Ballast per “Unit”? 3.73 oz.

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42” Unit

7”

Option 1: 4 meshes @ 7 inches (2 #L8’s)Ballast per “Unit” = 3.73 oz.

Leads = 2 oz. each (catalog)1.82 oz. (actual weight in seawater)

Webbing = .09 oz. per “unit” (approximate) .089600 oz. Per”unit”(actual calculation)

4 oz. (leads approx.) + .9oz. (webbing approx.) = 4.9 oz. (Air Weight)

3.64oz. (leads approx.) + .09oz. (webbing approx.) = 3.73 oz. (3.7296 actual in seawater)

“The weight in water, with examples for materials and for a rigged gillnet” is shown in the Fisherman’s Workbook, Pg. 04.

Ballast: To obtain the weight in water of a certain quantity of material, you must multiply its weight in air by the factor given in the “Density of Materials” table found on page 3 of your textbook. Obviously, lead is listed as a “Sinking Material”. Nylon or polyamides(PA) are also listed as a “Sinking Material”. Lead = 0.91+ (Multiplication factor in sea water)

Webbing, Polyamide(PA) = 0.10+ (Multiplication factor in sea water)Each #L8 lead has an air weight of 2 ounces (given in catalog)

Therefore2.0 oz. X .91+ = 1.82 oz. per lead of actual ballast in seawater

AndThis gillnet contains 62½ hung feet of webbing per pound (16 oz.)

SoOne “unit” on this net is 42” (3½’) which means: 62½’ ÷ 3½’ = 17.857 units per pound of material. 16 ounces ÷ 17.857 units = .896 oz. per unit of webbing. Therefore

.896 oz. X .10+ = .0896 oz. of actual ballast per “unit” of webbing in seawater.

What size lead are you planning to use? Stk. #L8 How many leads per pound? 8

How many leads per unit? 2

How many pounds of lead will you need?

• Leads are designated by Hole Diameter, Length and Number of Leads per Pound.• Diameter of the lead hole must be approximately 1/8” larger than rope diameter.

•Stk. L8 (3/8”) = 8 Leads per Pound = 2 ounces each (16 oz. ÷ 8 = 2)

• The “handout” specifies a spacing of leads not to exceed 2 to 4 per float.• A “sink gillnet” would need a minimum ¾ to 2 ounce ballast per unit.•2 #L8 Leads = 4 oz. (Air Weight)

Number of Leads per unit x # of units in the net ÷ # of leads per pound = Pounds(2) x (5400” ÷ 42” = 128.6) ÷ (8)

2 x 128.6 = 257.2 leads ÷ 8 leads per lb. = 32.15 Pounds

Option 1: 4 meshes @ 7 inches (2 #L8’s)

32.15

Is this net designed to sink or float?

What is the average buoyancy per “unit”?

What is the average ballast per “unit”?

What is the average “positive” (floats) or “negative” (sinks) factor you have calculated for each “unit”?

Sink

Option 1: 4 meshes @ 7 inches

Original Specifications: “We would like to fish the net on the bottom (sink net) and would like the net to be approximately 7½ feet deep when hung.”

2.64 oz.

Floatation Calculations: 2.5oz. (float) + .14oz. (rope, approx.) = 2.64 oz. (2.6141 oz. actual)

3.73 oz.

Ballast Calculations: 3.64oz. (leads approx.) + .09oz. (webbing approx.) = 3.73 oz. (3.7296 actual in seawater)

1.09 oz. “negative”Calculation: Average ballast per unit – average buoyancy per unit.3.73 oz. – 2.64 oz. = 1.09 oz.

Other Design Options and Discretionary Decision Making

• Gill Nets: Concepts and Design


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42” Unit

7”

Option 1: 4 meshes @ 7 inches (2 #L10’s)Floatation per “Unit” = 2.64 ouncesBallast per “Unit” = 2.99 oz.

Leads = 1.6 oz. each (catalog)1.456 oz. (actual weight in seawater)


2.9 oz. (leads approx.) + .09oz. (webbing approx.) = 2.99 oz. (3.0016 actual in seawater)



What is the average ballast per “unit”?What is the average “positive” (floats) or “negative” (sinks) factor you have calculated for each “unit”?

Sink



2.64 oz.Floatation Calculations: 2.5oz. (float) + .14oz. (rope, approx.) = 2.64 oz. (2.6141 oz. actual)

2.99 oz.Ballast Calculations: 2.99oz. (leads approx.) + .09oz. (webbing approx.) = 2.99 oz. (3.0016 actual in seawater)

.35 oz. “negative”Calculation: Average ballast per unit – average buoyancy per unit.2.99 oz. – 2.64 oz. =.35 oz.

Will this Design Work?No – Not enough ballast!


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42” Unit

7”

Option 1: 4 meshes @ 7 inches (3 #L13’s)Floatation per “Unit” = 2.64 ouncesBallast per “Unit” = 3.49 oz.

Leads = 1.2308 oz. each (catalog)1.12 oz. (actual weight in seawater) 3 x 1.12oz. = 3.36 oz. (3.4 oz.)






Sink




3.49 oz.Ballast Calculations: 3.4 oz. (leads approx.) + .09oz. (webbing approx.) = 3.49 oz. (3.0016 actual in seawater)

.85 oz. “negative”Calculation: Average ballast per unit – average buoyancy per unit.3.49 oz. – 2.64 oz. =.85 oz.

Will this Design Work?YES?? – Discretionary ballast!

“Unit” Size? 42”“Pickup” Size? 7”Meshes per “Pickup”? 4Floatation per “Unit”? 2.64 oz.# of Leads per “Unit”? 2½(#L8) Ballast per “Unit”? 4.69 oz.

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42” Unit

7”

Option 1: 4 meshes @ 7 inches (2½ #L8’s)

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Floatation per “Unit” = 2.64 ouncesBallast per “Unit” = 4.69 oz.

Leads = 2 oz. each (catalog)1.82 oz. (actual weight in seawater) 2.5 x 1.82oz. = 4.55 oz.

(4.6 oz.)Webbing = .09 oz. per “unit” (approximate)

.089600 oz. Per”unit”(actual calculation)4.6 oz. (leads approx.) + .09oz. (webbing approx.) = 4.69 oz. (4.6396 actual in seawater)




Sink

Option 1: 4 meshes @ 7 inches (2½ #L8’s)





Will this Design Work?NO?? – Discretionary ballast!

“Unit” Size? 42”“Pickup” Size? 5.25”Meshes per “Pickup”? 3Floatation per “Unit”? 2.64 oz.# of Leads per “Unit”? 2½(#L10)Ballast per “Unit”? 3.69 oz.

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42” Unit

5¼”

Option 2: 3 meshes @ 5¼ inches (2½ #L10’s)Floatation per “Unit” = 2.64 ouncesBallast per “Unit” = 3.69 oz.

Leads = 1.6 oz. each (catalog)1.456 oz. (actual weight in seawater) 2.5 x 1.456oz. = 3.64 oz.

(3.6 oz.)Webbing = .09 oz. per “unit” (approximate)

.089600 oz. Per”unit”(actual calculation)3.6 oz. (leads approx.) + .09oz. (webbing approx.) = 3.69 oz. (3.7296 actual in seawater)

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Sink

Option 2: 3 meshes @ 5¼ inches (2½ #L10’s)




1.05 oz. “negative”Calculation: Average ballast per unit – average buoyancy per unit.3.69 oz. – 2.64 oz. =1.05 oz.

Will this Design Work? YES!

“Unit” Size? 49”“Pickup” Size? 7”Meshes per “Pickup”? 4Floatation per “Unit”? 2.663oz.# of Leads per “Unit”? 3 (#L10)Ballast per “Unit”? 4.6 oz.

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49” Unit

7”

Option 3 (Discretionary): 4 meshes @ 7 inchesFloatation per “Unit” = 2.66 ouncesBallast per “Unit” = 4.6 oz.

Leads = 1.6 oz. each (catalog)1.456 oz. (actual weight in seawater)






Sink

Option 3 (Discretionary): 4 meshes @ 7 inches



4.6 oz.Ballast Calculations: 4.5oz. (leads approx.) + .10 oz. (webbing approx.) = 4.6 oz. (4.50166 actual in seawater)


Will this Design Work? Yes

Weight in Water – Overview of OptionsOption 1:

4 meshes @ 7”

42” unit,2x #L8

Option 1:4 meshes @ 7”

42” unit, 2x #L10


42”unit, 3x#L13


42” unit,2.5x #L8Rope & Floats

2.64 oz. / unit

Leads & Webbing3.73 oz. / unit

1.09 oz. Ballast/unit

150 yard Gillnet128.6 “units”

21.22 lbs. Buoyancy29.97 lbs. Ballast

8.75 lbs. Ballast (not including anchors)

Rope & Floats2.64 oz. / unit


.35 oz. Ballast/unit





Leads & Webbing3.49oz. / unit










18.97 lbs.Ballast (not including anchors)

Weight in Water – Overview of OptionsOption 1:

4 meshes @ 7”

42” unit, 2x #L8


42” unit, 3x #L13

Option 2:3 meshes @

5.25” 42” unit, 2½x

#L10

Option 3: Discretionary

4 meshes @ 7” 49” unit, 3x

#L10Rope & Floats2.64 oz. / unit
























The average buoyancy and ballast values will change depending on local experience and gear applications. How do our net designs compare with examples given in our text and why might they differ?

ComparisonsOption 1:4 meshes @7”,42” unit, 2x #L821.22 lbs. Buoyancy 29.97 lbs. Ballast8.75 lbs. Ballast Ratio 1 to 1.41Option 2:3 meshes @5.25”,42” unit, 2½x

L1021.22 lbs. Buoyancy 32.15 lbs. Ballast10.93 lbs. Ballast Ratio 1 to 1.52Option 3:4 meshes @ 7” 49” unit, 3x

#L1018.32 lbs. Buoyancy 33.06 lbs. Ballast14.74 lbs. Ballast Ratio 1 to 1.81

The text generally recommends a buoyancy to ballast ratio for a bottom set gillnet at approx. 1 to 2.

RationaleThe weights do not include anchors

which add considerably to our overall weight and brings our ratios in line with the text.

Both our ballast and buoyancy values were based on sea water – we generally are working in brackish waters.

We generally use shorter length nets with smaller catches than commercial gear.

Our nets are generally used in shallow waters without excessive currents.

We can add ballast easier than subtracting ballast.

Discretionary Decision Making

“Average buoyancy and ballast of gillnets and trammel nets” is shown in the Fisherman’s Workbook, Pg. 103.

E1 = L ÷ Lo .50 = 50 ÷ 100.16 (320mm x 313)

Pickups: 2 meshes @ 320mm Units:1 Float @ 1600mm (5 pickups x 320mm)32 floats: 50 M ÷ 1.6 M (31.25)

156 leads: 50 M ÷ .32 M (156.25)

Buoyancy: 1600gf (floats only)Ballast: 7098g-(leads only-seawater weight)5498 grams ballast (12.14 lbs.)Ratio 1 to 4.44

MSC 134 Fishing Gear Technology II Gill Nets: Concepts and Design.

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Transcript of MSC 134 Fishing Gear Technology II Gill Nets: Concepts and Design.