Cell design for a trawl system and methods

Details Cell design for a trawl system and methods Cell design for a trawl system and methods

2000-02-23

2002-03-19

Ark, Darren W. (Department: 3643)

Fishing, trapping, and vermin destroying

Fishing

Nets

C043S009950

Reexamination Certificate

active

06357164

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an improved mesh cell design for a trawl system (that by definition is iterated or cloned in varying geometric patterns) providing improved shaping and performance, especially when incorporated in mid-water or bottom trawls of such systems.
BACKGROUND ART
It is well understood that the basic cell of a selected portion of every trawl system is the unit cell (called mesh cell hereinafter). The selected portions of the trawl system is then built by repeating the shape of the basic mesh cell.
It is axiomatic that the ability to predict the overall shape and performance of the finished product depends entirely on the shape and structural integrity of that single basic mesh cell. Heretofore, proper trawl making was a two-step process that involved initial construction of undersized mesh cells, and setting the knots and mesh sizes by the substeps of depth stretching and heat setting involving turning the finished mesh in direction opposite to its natural bent and applying first pressure, and then heat to set the knots.
Materials used in mesh cell construction can be plastics such as nylon and polyethylene but other types of natural occurring fibers also can be (and have been) used. Single, double (or more) strands make up a thread or twine composed of, say, nylon, polyethylene and/or cotton. Additionally, in making the mesh portion of conventional trawls particularly mid-water trawls especially the forward section mesh portion thereof, braided cords and twisted ropes of natural and synthetic materials, bonded and unbonded, and cables have been used. However, the pitch of any braided or twisted thread, such as a twine, cord and/or rope (distance between corresponding points along one of the strands constituting one turn thereof which is analogous to the pitch between corresponding screw threads) either has usually been small, or has produced shallow or narrow depressions. Conventional trawl making practices balance the towing force generatable by a vessel against the largest possible trawl for a particular fishing condition, i.e. a trawl having the minimum possible drag. Thus, conventional trawl makers are taught to use the smallest possible diameter twine to reduce drag. Accordingly, meshes in conventional trawls, and especially the mesh of the forward sections of mid-water trawls, have been made of twines, including conventional three strand twisted twines of any pitch including loose pitch, that have relatively shallow or narrow and uniform spiral depressions, or smaller diameter braided twines having an equivalent breaking strength. Moreover, modern manufacturing processes using threads, such as twines, cords, cables or ropes to form mesh cells, have always been designed to produce mesh cells in which twist direction of the individual bars comprising each mesh cell, if any, is always the same. None have proposed the systematic and regular use of differently oriented twist for individual mesh bars of the mesh cell in the manner of the present invention.
Even though various Japanese Patent Applications superficially describe mesh cells for nets in which mesh bars have differing lay directions, (see for example, Jap. Pat. Apps. 57-13660, 60-39782 and 61-386), the mesh bars employ conventional, essentially smooth twine or rope. The patent applications disclose differing lay directions of conventional, essentially smooth twine or rope for balancing residual torque within the net structure during its deployment and use, not for generating lift that enhances of trawl system performance. The first-mention Application, for example, states that its purpose is to provide “net legs with different twist directions according to a fixed regular pattern so that torsion and torque of said net legs are mutually canceled.” The use of conventional, essentially smooth twine or rope will not yield substantial lift any different from conventional nets.
As set forth in published Patent Cooperation Treaty (“PCT”) International Patent Application, International Publication Number WO 97/13407, International Publication Date Apr. 17 1997, (“the PCT patent application”) it has been recently discovered that threads, such as twines, cords, braided cords, cables, ropes or straps, may be advantageously twisted, during assembly of trawl net meshes into a loose, corkscrew-shaped pitch establishing helical grooves that are deeper and/or broader than the depressions in conventional tightly or loosely twisted multi-strand ropes or cables making up conventional mesh bars. During field operations in a water entrained environment, properly orienting mesh bars having the loose, corkscrew-shaped pitch produces lift that increases a performance characteristic of a trawl system such as increased trawl volume (particularly in shallow water) in comparison with a trawl made from conventional mesh, improved trawl shape, and reduced vibration, noise, and drag. Trawl performance improves even though, contrary to conventional trawl design, mesh bars having the loose, corkscrew-shaped pitch have a diameter (or shadow area) larger than corresponding mesh bars of a conventional trawl.
DISCLOSURE OF INVENTION
An object of the present invention is to provide further improved trawl systems.
Yet another object of the present invention is to provide trawl systems having improved performance characteristics.
Briefly, the present invention improves upon the basic discovery disclosed in the PCT patent application that individual bars of a mesh cell can be formed to act as mini-hydrofoils in field operations. During field operations in a water entrained environment, the trawl disclosed in the PCT patent application becomes disposed symmetrically about a central axis. The disclosed trawl includes a plurality of mesh cells, each mesh cell having at least three mesh bars. Each mesh bar in the trawl intersects with at least one other mesh bar. During field operations with the trawl in a water entrained environment, at least a portion of at least one of the mesh bars of at least one of the mesh cells in the trawl exhibits a substantial hydrofoil-like effect that aids in increasing a performance characteristic of a trawl system. That portion of mesh bars in accordance with the present invention which generate substantial hydrodynamic lift is preferably formed from a material that has a substantially incompressible cross-sectional shape, is offset from the central axis of the trawl, and is formed with a hydrofoil shape that:
1. has a lay with a loose, corkscrew-shaped pitch establishing a corkscrewing groove that provides cambered sections; and
2. is oriented to establish leading and trailing edges for that portion of mesh bars which generate hydrodynamic lift.
The lay of that portion of mesh bars which generate hydrodynamic lift has an orientation relative to a receding direction, and the leading edge for that portion of mesh bars which generate hydrodynamic lift, when normalized to the receding direction relative to the central axis, resides at a side of the mesh bar. Pairs of lay and the leading edge are selected for mesh bars from a group consisting of:
1. a left-hand lay, and the leading edge being a right side of the mesh bar as viewed in the receding direction; and
2. a right-hand lay, and the leading edge being a left side of the mesh bar as viewed in the receding direction.
If the lay and leading edge are selected from the preceding group, then movement of the mesh bar in accordance with the present invention through the water entrained environment relative to a water flow vector that is neither parallel nor perpendicular to the mesh bar creates a pressure differential across that portion of mesh bars which generate hydrodynamic lift. The pressure differential thus created across such mesh bars establishes a lift vector relative to the central axis of the trawl, most commonly directed away from the central axis of the trawl. Consequently, the lift vector created by movement of the mesh bars which have a portion that generate hydrodynamic lift increases the performance characteristic of the tr

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