The invention relates to a wire according to the preamble of claim 1.

The invention thus pertains to a wire of the kind used for transmission of tensile forces between a first object and a second, whereby the wire has a longitudinal extent and travels through a fluid such as water in a direction transverse to the longitudinal direction of the wire, the wire has an elongate cross-section preferably shaped to minimise resistance to movement through the fluid, and the wire also comprises material of high rigidity and high strength and is dimensioned to undergo substantially no elongation in its longitudinal direction in response to the intended tensile forces in the longitudinal direction of the wire.

Such a wire may for example be used as a paravane wire, i.e. a wire which extends up from a paravane travelling through water to the region of the water surface in order to be connected, possibly via a further wire joined to it, to a sailcraft, e.g. to a mast on th sailcraft in order to stabilise the sailcraft laterally so that it can more effectively convert wind forces to propulsion.

However, the wire also has a broad range of applications in other contexts where a wire which transmits tensile forces in a longitudinal direction is to be propelled through, for example, a liquid in a direction transverse to the longitudinal direction of the wire.

Experiments with a wire of generally teardrop-shaped cross-section and straight longitudinal extent have shown that the wire tends to be hydrodynamically unstable.

An object of the invention is therefore to indicate a wire which not only offers low resistance to propulsion through a fluid but also has good hydrodynamic stability during propulsion through the fluid, e.g. water.

This object is achieved by the invention. The invention is defined in the attached claim 1.

Embodiments of the invention are indicated in the attached dependent claims. An essential feature of the invention is that the wire is preformed so that it curves in a plane which is parallel with the longest dimension of the oblong wire cross-section. In particularly preferred embodiments of the invention, the precurvature of the wire is adapted to correspond to the curvature to which the wire will be subjected by hydrodynamic forces acting upon it. In its preformed curved state, the wire is preferably substantially stress-free in the unloaded state.

In one embodiment of the invention, the wire according to the invention may in the leading front portion of its oblong cross-section comprise material of higher rigidity with respect to tensile load than the material in the other portions of the wire's oblong cross-section.

Instability of the wire when it travels through water gives rise to an uncontrolled varying lifting force acting upon at least some parts of the wire and also gives rise to greater resistance to the wire's movement through the fluid. As the wire's resistance to propulsion through the fluid depends also substantially on the area of the cross- section, in the case of a cross-sectional shape which results in minimised flow resistance it is of course important to minimise the wire's cross-sectional area by adopting high-strength material of great rigidity.

The invention is described below in the form of examples with reference to the attached drawings.

Fig. 1 illustrates a wire according to the invention which is used as a water-shearing element in flexible tensile force transmission between a paravane and a sailboat.

Figs. 2, 3 and 4 illustrate plan views of the water-shearing paravane wire.

Fig. 5 depicts a cross-section through the wire according to any one of Figs. 2 to 4.

Fig. 1 depicts a sailboat 1 which when, for example, beating up against the wind direction 8 is stabilised laterally by, for example, a wire 3 fastened to an upper portion of the mast 2 and connected in the vicinity of the water surface to a water-shearing wire 4 which is connected to a submerged part of the paravane 5. The paravane 5 is stabilised transversely to the direction in which it travels through the water by a wire 6 which connects to a portion of the paravane situ-ated at or above the water surface and is connected to the wire 3, advantageously at the connection point between the wire 3 and the wire 4.

The paravane lies just below the water surface and is usually designed to be pulled outwards from the windward side of the boat by a force approximately proportional to the square of the boat's velocity through the water. It is advan-tageous if the wire which has to absorb the tensile force of the paravane can be fastened to the lower portion of the paravane, as depicted, but this does of course involve a great need for low water resistance for the water-shearing wire 4, as the wire will extend transversely to the direction of the wire's travel through the water.

For a slender wire which is of streamlined oblong cross-section and is stressed by a tensile force S transverse to the direction of flow of a liquid to be hydrodynamically stable, the stressing force S has to act upon the cross-section forward of the point where the wire's lifting force L acts, as a result of a small disturbance to the angle of incidence of the wire's cross-section relative to the direction of flow. For a wire with a symmetrical and generally teardrop-shaped cross-section, the point L is usually situated at a distance from the front edge of the cross-section which corresponds to 22 to 25% of the width of the wire cross-section.

A straight wire which is at rest relative to a surrounding medium is stressed by a force S acting at a distance from the front edge of the cross-section which corresponds to about 20% of the width of the wire cross-section. If the centre of gravity of the tensile rigidity of the wire cross-section is also situated at a distance from the front edge of the cross-section which corresponds to 20% of the width of the cross-section, the wire will remain free from bending moment and be straight even when it is stressed, see Fig. 2.

But if a liquid flows at a velocity V towards and along the longitudinal axis of the wire cross-section (Fig. 5) so that the wire is subject to a hydrodynamic resistance D (N), this resistance D will cause in the cross-section A a bending moment which is balanced by the fact that the stressing force S (N) acts at a point further back in the wire cross-section, thereby causing hydrodynamic instability, see Fig. 3. This may also be described as the wire endeavouring to orientate itself so that it deflects in the direction in which it has least flexural rigidity.

The invention means that an oblong wire with resistance-minimised oblong cross- section is manufactured in accordance with Fig. 4, with a prebending corresponding to or somewhat greater than the deflection of the wire which the water flow resistance would impart to a moment-free wire according to Fig. 4, with the result that the prebending radius R < S 1/D, in which R is the radius of curvature of the wire (in metres) and 1 is the length of the wire (in metres) in the water. With such prebending of the wire, S will act at a sufficiently small distance from the front edge of the wire cross-section for the wire to be hydrodynamically stable.

The material of the front portion of the wire's cross-section may be selected to have a greater tensile rigidity than the material of the other portions of the wire's cross- section, so that the position of the centre of gravity of the wire cross-section's tensile rigidity may be offset relative to the geometrical centre of gravity of the wire's cross- sectional area.

The wire according to the invention is advantageous for use as a water-shearing paravane wire, as exemplified above, but may of course also be used in the general case where an elongate wire with an oblong cross-section is required to move through a fluid, e.g. a liquid, transversely to its longitudinal direction with simultaneous tensile loading in its longitudinal direction.

By way of example, a wire according to the invention may have a water-shearing length of 2 m, in which case the wire has a width of about 40 mm, i.e. a length/width ratio of about 50. The invention relates to wires which are slender and flexible and have a length/width ratio preferably exceeding 20.