Trawl Doors: How They Work and How to Choose the Right One

1. What Is a Trawl Door?

A trawl door is a specially designed board installed between the towing cables and the trawl net. A pair of doors is normally used, with one on each side of the net.

When the fishing vessel moves forward, water flows around the doors and generates hydrodynamic force. This force pushes the two doors outward, helping keep the net wings separated and maintaining the horizontal opening of the trawl.

The trawl door therefore acts as an important link between the vessel and the net, helping the complete fishing system operate effectively.

2. How Do Trawl Doors Work?

The working principle is mainly based on water flow and hydrodynamics.

As the vessel tows the fishing gear, water passes over the surface of the doors. The shape, surface area, and angle of the door determine how much spreading force is generated.

The outward force produced by the two doors helps pull the trawl net open horizontally. At the same time, the doors must remain stable during towing.

A suitable design needs to provide enough spreading force without creating unnecessary drag. If the door is too small for the fishing system, it may not provide sufficient opening force. If the design is unsuitable, it may also affect towing stability and fuel efficiency.

3. Main Types of Trawl Doors

Different fishing methods require different trawl door designs.

Bottom Trawl Doors

Bottom trawl doors are used for fishing close to the seabed. Their design needs to provide effective spreading force while adapting to the working conditions near the bottom.

Strength, stability, and resistance to demanding operating conditions are important considerations.

Pelagic Trawl Doors

Pelagic trawl doors are designed for fishing in the water column. Since they normally operate away from the seabed, hydrodynamic performance and depth control are particularly important.

Semi-Pelagic Trawl Doors

Semi-pelagic trawl doors are used for operations between bottom and pelagic fishing conditions. The appropriate design depends on the vessel, net configuration, target depth, and fishing method.

4. What Should Be Considered When Choosing a Trawl Door?

There is no single trawl door suitable for every fishing operation. Several factors should be considered before making a selection.

Vessel size and engine power: The door should match the vessel's towing capability.

Trawl net size and design: A larger or heavier trawl system may require greater spreading performance.

Towing speed: Hydrodynamic force changes with towing speed, so the door should work effectively within the vessel's normal operating range.

Fishing depth: Bottom, semi-pelagic, and pelagic operations have different requirements.

Fishing conditions: For bottom trawling, seabed conditions can also influence the suitable door design.

Door size, weight and structure: These factors affect spreading force, stability, drag, and durability.

5. Why Is Trawl Door Stability Important?

A trawl door does more than simply spread the net. It also needs to maintain a stable position while the vessel is towing.

If the doors move excessively or become unstable, the distance between the doors may change, which can affect the shape and opening of the trawl net.

Good structural design and suitable weight distribution can help the doors maintain stable performance under normal towing conditions.

6. Trawl Doors Are Part of a Complete Fishing System

Trawl doors should not be selected only according to their dimensions or weight. They work together with towing wires, bridles, sweeps, net wings, and other components.

Changing the net size, towing speed, or fishing depth can affect the requirements for the doors. Therefore, the most suitable choice should be based on the complete trawling system rather than one individual parameter.

For this reason, when selecting trawl doors, it is useful to provide information such as vessel size, engine power, trawl type, net dimensions, towing speed, and target fishing depth.

Conclusion

Trawl doors are an important part of modern commercial trawling equipment. Their main function is to generate hydrodynamic spreading force and help maintain the horizontal opening of the trawl net.

The right door depends on the vessel, net, fishing method, towing speed, depth, and operating conditions. A properly matched trawl door can contribute to better stability and more efficient operation of the complete fishing system.

QinHo provides different types of trawl doors and water-flow diverter boards for various commercial fishing applications. Based on the vessel and trawling requirements, the door design can be matched to the specific fishing system.