Economic framework

Laboratorio de Ingeniería Mecánica ─ Universidade da Coruña Escuela Politécnica Superior, Mendizábal s/n, 15403 Ferrol, Spain E-mail address: [email protected], [email protected]

* Corresponding author. E-mail address: [email protected] Tel.: (+34) 981337400 fax: (+34) 981337410

2

Abstract

The increased mesh resistance to opening of netting panels manufactured with thick and stiff twines has a notable impact in the structural response and selective performance of the fishing gears. The only available method to quantify the mesh resistance to opening of netting panels was described in (Sala et al., 2007b). We present an alternative method with a similar methodology: we attempt to estimate the mechanical and geometrical properties of a netting material that best fits the experimental measurements of a netting panel. We introduce three major contributions: (i) a considerably simpler uniaxial experimental setup, which stretches a netting sample in the normal direction of the meshes while leaving free its deformation in the transverse direction; (ii) more accurate theoretical models for mesh resistance to opening; (iii) new strategies to estimate the parameters of the models. We present the results of the analysis of polyethylene (PE), compacted polyethylene (CPE), single-twine and double-twine netting. Some of the assessed combinations of estimation strategies and theoretical models have an excellent goodness of fit with experimental data. The method proved to be a simple yet accurate way to quantify the mesh resistance to opening of netting panels.

Keywords: bending stiffness; flexural rigidity; mesh resistance to opening; twine; regression

3

Introduction

In recent years, there is a tendency in some sectors of the fishing industry towards the use of thicker and stiffer twines in the manufacture of netting materials for the codend of trawls. The increased mesh resistance to opening of such materials has a notable impact in the structural response and performance of the fishing gears. For example, an increased mesh resistance to opening hinders mesh opening in the cod-end (O’Neill, 2004), which affects the escapement of small fish. Theoretical and experimental studies demonstrate that mesh resistance to opening plays as major role in the reduction of selective performance of trawls (Herrmann and O’Neill, 2006; Herrmann et al., 2013; Lowry and Robertson, 1996; Sala et al., 2007a). Theoretical models for mesh resistance to opening are generally based on the beam theory of solid mechanics. In diamond mesh panels, the predominant netting in towed fishing gears, the resistance to opening is mainly characterized by the bending stiffness of the netting twine. An increased twine bending stiffness also changes the overall shape of the fishing gear during fishing operations (O’Neill, 2004; Priour, 2001). Therefore, methods to quantify the mesh resistance to opening and to incorporate this property in theoretical models of netting materials are necessary to accurately predict the selective performance of fishing gears by simulation. Despite this, research about this topic is still scarce (Priour and Cognard, 2011).

The only available method to quantify mesh resistance to opening of netting panels is described in (Sala et al., 2007b). The method uses a specially designed instrument that applies normal and transversal displacements to a netting sample and measures the generated reaction forces. Then, twine bending stiffness and geometric parameters of the netting are estimated through nonlinear regression analysis of the obtained

4

2002) is used as model in the regression analysis. Although the method proved to be robust and useful to estimate mesh resistance to opening, the authors reported several problems: inconsistencies between normal and transversal forces and displacements, occasional unrealistic estimates of geometrical parameter values and systematic lack of fit of the model to the experimental data. Another concern is that the authors carried out the regression analysis using the force as independent variable and the displacement as dependent variable, despite the force was an effect caused by an applied displacement in the experimental setup. The complexity of the experimental setup required by this method is another important drawback.

A method to estimate twine bending stiffness was proposed by (Priour and Cognard, 2011). The method measures the out-of-plane bending deformation of a netting sample and then adjusts a theoretical model of a cantilever beam to the experimental data, in order to estimate the twine bending stiffness EI. The method is very simple, but it has some drawbacks. It does not take into account the knot size, which can have an important effect on the shape of the codends (PREMECS: Development of predictive model of cod-end selectivity, 2000). In addition, and it cannot estimate the slope angle between twines and knots at the insertion points.

The goal of this article is to describe a simple but accurate experimental method to quantify the mesh resistance to opening of netting panels. We follow a methodology similar to (Sala et al., 2007b), that is, we attempt to estimate the mechanical and

geometrical properties of a netting material that best fit the experimental measurements of a netting panel. This research introduces three original contributions:

(i) The biaxial experimental setup used in (Sala et al., 2007b) requires a very complex measurement instrument which is not commercially available. In contrast, this work

5

proposes a new uniaxial experimental setup that notably simplifies the required measurement instrument.

(ii) The regression model used in (Sala et al., 2007b) is the asymptotic model for a bending twine described in (O’Neill, 2002). This model is an approximate solution. This work uses more accurate models to describe mesh resistance to opening: the exact model described in (O’Neill, 2002) and two recently developed models based on finite element analysis (de la Prada and González, 2014a).

(iii) The parameter estimation strategy used in (Sala et al., 2007b) fixed one of the geometrical properties of the netting (the slope angle between the twine and the knots) and leaved the remaining parameters unconstrained. As results, the estimates were sometimes out of physical limits. This work assesses other estimation strategies that avoid that problem.