Hidrodinamika Luas Jaring

Summary

This video on hydrodynamics explores the calculation of net mesh area in fishing gear, focusing on three key terms: pseudo mesh area, working mesh area, and net cross-sectional area. It explains how to compute these areas using different methods, including rectangular and trapezoidal shapes for nets. The video demonstrates practical examples of calculating mesh area for different net shapes, such as Gillnets and trawl nets, highlighting the impact of mesh size, hanging ratio, and net configuration. The content is aimed at helping students understand the hydrodynamic properties of fishing nets and their effect on catch efficiency.

Takeaways

• 😀 **Understanding Net Areas**: The video discusses different types of areas used to calculate fishing net surfaces, such as pseudo area (luas semu jaring), operational area (luas kerja jaring), and cross-sectional area (luas penampang jaring).
• 😀 **Pseudo Area (Luas Semu Jaring)**: This is an approximation using the formula for a rectangle (length × width) to calculate the surface area before the net is deployed.
• 😀 **Operational Area (Luas Kerja Jaring)**: Similar to pseudo area, but the formula adjusts based on the length and height when the net is in operation.
• 😀 **Cross-Sectional Area (Luas Penampang Jaring)**: This is the area covered by the net's threads and requires calculating based on thread coverage, eliminating empty spaces between threads.
• 😀 **Changes in Net Shape**: When deployed, the shape of a net, initially designed as a rectangle, can change to a trapezoidal or even inverted trapezoidal form due to the forces of buoyancy and weight.
• 😀 **Buoyancy and Weight Forces**: These forces are critical in shaping the net, with buoyancy lifting the net and weight pulling it down, creating the shape change.
• 😀 **Calculation of Cross-Sectional Area**: The cross-sectional area of a net can be calculated for both rectangular and trapezoidal nets, depending on the net's deployed shape.
• 😀 **Formula for Rectangular Net Cross-Sectional Area**: For rectangular nets, the formula incorporates hanging ratios and the dimensions of the net, including mesh size and thread diameter.
• 😀 **Trapezoidal Net Calculation**: For nets with trapezoidal shapes, the area is calculated using the trapezoid area formula, adjusting for thread diameter and mesh size.
• 😀 **Example Calculation**: The video provides an example calculation for a Gillnet, demonstrating how to calculate its cross-sectional area using the appropriate formula.
• 😀 **Partitioning Irregular Nets**: When dealing with nets made up of multiple components (like trawler nets), the net is partitioned into sections, each calculated separately based on its shape and mesh size to find the total area.

Q & A

What is the apparent area (luas semu jaring) of a net and how is it calculated?

The apparent area is the calculated surface area of the net before it is deployed. It is calculated by multiplying the length (L0) and width (H0) of the net before installation, essentially approximating the net's shape as a rectangle.

What is the working area (luas kerja jaring) of a net and how does it differ from the apparent area?

The working area refers to the net's surface area when it is deployed and functioning in the water. Unlike the apparent area, the working area accounts for the net's dimensions when it is stretched or adjusted, and it is calculated by multiplying the installed length and height of the net.

What is the cross-sectional area (luas penampang jaring) of a net and how is it determined?

The cross-sectional area is the actual area covered by the net's threads, excluding the empty spaces between the threads. It is calculated by considering the sections of the net that are covered by the net material (the threads or knots) and eliminating the empty areas in between.

Why does the shape of the net change when it is deployed in water?

When a net is deployed in water, the net's shape changes due to two forces: the buoyant force from the floats that pull the net upward and the weight from the sinkers that pull it downward. This causes the net, originally designed as a rectangle, to form a trapezoidal or inverted trapezoidal shape.

How are nets with a trapezoidal shape calculated differently from rectangular nets?

For nets with a trapezoidal shape, the cross-sectional area is calculated using the formula for the area of a trapezoid. This involves adding the lengths of the two parallel sides, dividing by two, and multiplying by the height. Additionally, other factors such as mesh size and net material are considered.

What is the hanging ratio (E) in net calculation and why is it important?

The hanging ratio (E) refers to the ratio of the net's height in the water compared to its height when dry. It affects the net's shape and surface area, and it is crucial in determining the net's effective surface area when deployed in water.

How does the calculation change if a net is used for a bottom-set operation as compared to a surface operation?

In bottom-set operations, the net will typically have a different shape and behavior due to the forces of sinking and water resistance. This can cause the net to deform differently than in surface operations, which affects how the area is calculated and requires adjustments to the calculation formulas.

What formula is used to calculate the cross-sectional area of a rectangular net?

For a rectangular net, the cross-sectional area is calculated by multiplying the primary hanging ratio (E1) by the square root of (1 - E1²), and then multiplying by the net's length, height, and the square of the mesh size. This gives an estimate of the net's effective surface area.

How is the cross-sectional area calculated for a trapezoidal net?

For a trapezoidal net, the cross-sectional area is calculated by finding the average length of the two parallel sides of the trapezoid, multiplying by the height, and then adjusting for the net's mesh size and thread diameter using a specific formula that accounts for the geometry.

What is the method for calculating the surface area of a trawl net?

For trawl nets, which consist of multiple sections with different net designs, the net is partitioned into smaller sections. Each section is calculated separately based on its shape and mesh size. These individual areas are then summed up to get the total surface area of the trawl net.