Introduction

Objective measurements and evaluation are now an integral part of the preparation, marketing and processing of many textile goods [1], but for some of them, like sailcloth for example, they are still not employed on a regular basis [2]. Although modern sailcloth belongs to the group of high-performance and high-tech materials, the assessment of its fitness for use is mostly based on the sail-makers’ subjective judgment and experience [3]. In order to implement the concept of objective measurements and evaluation in the sail-making processes (Figure 1), the possibilities of objectively characterising the materials intended for sail manufacture are investigated and defined, with respect to the specific principles of sailing and the requirements of the material for this purpose [4].

Figure 1. Schematic representation of the objective measurements implementation in the sail-making process

A material (cloth, yarn or fibre) should meet a number of requirements for use in sail manufacture. These requirements can, depending upon a particular point of view (that of the naval architecture engineer, sail-maker or sailor [5]) contradict each other to a certain degree, supplement each other, and/or even be identical (Figure 2). From the naval engineer’s point of view, a material to be used in sail manufacture should, besides having a smooth surface, have zero porosity, ensuring effective air flow around the sail [2, 4, 6] and creating an aerodynamic force to push it. To reduce the proportion of the force created which cannot be used (i.e. resistance to airflow), the surface roughness of the sailcloth should be as low as possible. A further requirement, or limitation, to be met by the sailcloth is low surface mass, in order to reduce the moment of swerving, and with it, the tilting of the sailboat. The sailor, like the navy engineer, is the end user of the sail, and so he requires sails of lower mass, to facilitate handling. However, in order to sail more comfortably and safely, the yachtsman also demands strong sails, of a stable shape, which are easy to maintain. The sail strength (the cloth and joining parts and/or seams) should be such that the sails can stand strong wind or sudden squalls, and not be torn or change shape significantly, as this reduces the need to adapt the sail shape instantly to momentary conditions. In addition, the sailcloth must resist water, sunlight and microorganisms, and must also display a good ratio of price and durability. The final decision in selecting the fabric for the manufacture of sails rests on the sail designer. His task is to turn the designed shape into a sail, balancing the forces of wind and strains in the sail. As each particular sail is designed for a precisely defined range of wind strength, the resulting pushing forces of the wind on the sail will determine the limits of the necessary fabric strength. As the push is highest when sailing downwind, breaking force will be a decisive factor in manufacturing spinnakers, while in selecting the fabric for the headsail and the main sail (where stability of form is more important than maximum force), the decisive factor will be the yielding point, or fabric elasticity [2].

Figure 2. Design of sailcloth quality descriptors

Methodology

Bearing the above goals in mind, but also considering the possibility of establishing an expert system that would make the evaluation of material behaviour possible under real end-use conditions, 12 representative samples of modern sailcloth of various constructions were selected (Table 1).

The samples have been used to test six groups of properties (or parameter blocks), that determine in detail the suitability of a particular material for the manufacture of sails with predetermined end-use properties (Figure 2). The parameter blocks are as follows: chemical composition, macro- and micro-construction, sorption properties, resistance to wide variety and intense of forces, and resistance to ageing under natural conditions.

Appropriate standardised testing methods (Table 2) have been used to investigate the above properties, some of them for the first time in this context (KES System). A statistical evaluation of the importance of each quality factor has been implemented to assert the suitability of the parameter blocks selected.

Table 1. Description of the sailcloth samples tested

Table 2. Proposed testing methods and procedures

Objective Measurements And Evaluation Of Sailcloth

The systematic approach to objective material characterisation (Figure 2) has resulted in a sailcloth quality card as a kind of quality assurance, and a key element for production optimisation and improvement of the end-product’s quality. The quality card that includes Figures 3÷6 & Tables 3÷6 is designed in order to permit:

a) b)

Figure 3. Microscopic appearance of sample no. 10: a) 40x magnified in reflected light; b) binary picture 3a

a) b) c) Figure 4. σ-ε diagrams for different testing direction of sample no. 10: a) 0⁰; b) 90⁰; c) 45⁰

Figure 5. Changes of sample (sample no. 10) elongation (εEMT) at cumulative increase of strain in the range from 100 to 2000 [gfcm^-1] (or 0,98 – 19,62 [Ncm^-1]) for a particular direction of testing (0⁰/90⁰/45⁰)

Table 3. Raw material content for sample no. 10

Figure 6. Changes of sample (sample no. 10) tenacity (index σ0-2500^/σ0) during weathering time (0-2500 hours): a) for maximum tenacity (σmax); b) for tenacity at 1% elongation (σ1%ε)

Table 4. Micro- and macro-construction quality descriptors for sample no. 10

Table 5. Mechanical properties of sample no. 10

Table 6. Sorption properties and weathering resistance of sample no. 10

• primary classification and sailcloth evaluation by end-use (manufacture of racing or cruising sails), as well as by the weather-location conditions of the intended use. Although the decision can be taken on the basis of a number of indicators, the most evident is the time of cloth destruction and the trend of the change in tensile properties, as effective aerodynamic flow and optimal maritime sail properties are higher on the scale of importance than durability, at least for racing sails.
• secondary classification and sailcloth evaluation, based on end-use and defined by naval architect requirements for the type of sails adequate for the type and size of the vessel designed. For example, establishing the required size of the sails for the purpose of sail-ship stability sets limits to the maximum sailcloth surface mass. It is also possible, on the basis of surface roughness and the adequate friction coefficient, to precisely define the effectiveness of aerodynamic airflow, which means the efficiency of the sail itself.
• tertiary classification and sailcloth evaluation based on the sail-maker’s requirements for the processability of a 2D fabric into a 3D sail form. For example, quality parameters (determining resistance to tensile forces) in a particular direction of the sailcloth is a key factor in designing sails and selecting the technique of its manufacture (panel or radial cut).
• end-user sailcloth evaluation, as an ideal cloth is considered to be one with as low a ratio of price and durability as possible (raw material content; appearance; weathering resistance), which at the same time offers maximum maritime properties for the sailing ship (i.e. as low a ratio as possible between sail strength and its mass, and as smooth a surface as possible) and security (resistance to high loads and prolonged tearing; weathering resistance), as well as easy handling and maintenance (flexing ability; compressibility; volume mass; hydrophobicity).

Conclusion

The investigations described and the knowledge obtained form a basis for the construction of an expert system to be used to evaluate the behaviour of each sailcloth material in its end-use, starting from objective measurements of each new and potentially applicable sailcloth, and to assess its value, usability and effectiveness for a specific end-use.