GB2119730A - The reversing wind-sail - Google Patents

Abstract

A reversing wind-sail comprises a double sided sail in which fabric is drawn taut over a number of variable profiled frames. When the boat goes about the frames invert to form their mirror image, bending freely either under pressure of the wind or mechanical action, until they reach a predetermined camber when they become rigid. In its simple form the optimum camber is preset into each frame, and in its fully adjustable form the camber of the whole sail can be controlled. The frames are rigged in a particular way that makes it convenient to raise and lower them with a winch. This sail is compact, rugged, and makes low structural demands on mast and rigging. It is easy to handle and maintain, and the effects of scale are no limit to the size and sail can be made. <IMAGE>

Description

SPECIFICATION The reversing wing-sail TECHNICAL FIELD:-- SAILMAKING BACKGROUND THEORY: An aerofoil can bo considered to be a surface placed in an air stream for the purpose of producing sideways thrust, i.e. a thrust normal to the direction of the wind. Thus when a boat sails into the wind it is using its sail as an aerofoil in distinction from when it is running before the wind when it uses its sail as a parachute, to obtain thrust in the direction of the wind. The faster a boat travels the more the apparent wind moves round before it, thus the development of efficient windward sailing ability is crucial to the evolution of the sailing boat.

The making and understanding of efficient aerofoils is a science that has progressed a great deal with wind-tunnel,tests of aeroplane wings and models of boats' sails.

An aerofoil is esentially a curved surface; for efficiency it does not need to have thickness, though this might be necessary to enclose supporting structure. The depth of curvature can be described as its camber, and its overall length as its chord. The angle of attack a is the angle that oncoming airflow makes with the axis. See Fig. 1.

As air flows over an aerofoil it is deflected, and exerts a force on the aerofoil, and this force is proportional to 0, the angle through which the air has been deflected, and also to v2, where v is the velocity of the air. See Fig. 2.

Drag is also caused by friction between air and the surface of the aerofoil, it is also induced by vortices at the ends of the aerofoil and by inefficient aerofoil shapes.

For the largest lift to be obtained from an aerofoil of a given size the airflow must be deflected through the largest possible angle 0, but if the angle of attack a is too large, or if the camber of the said is too great, the sail will stall which means that the airflow separates from the aerofoil and nearly all lift may be lost.

Approximately 75% of the lift on an aerofoil is imparted to it by an area of reduced pressure over the leeward surface, and this is greatest in the region behind the leading edge.

Thus the resultant force Lasses through a point X which is approximately or the chord but may be as little as 4 of the chord. See Fig. 3.

This large thrust over the lee of the sail is only realised if the airflow follows the surface of the aerofoil, and for this the leading edge must be smooth and continuous with the curve that follows. If the flow is disturbed the lift is reduced and the drag increased, and this is generally the case with sails that follow a mast See Fig. 4.

The degree of camber is very important; if it is too small the aerofoil will not be deflecting as much as it could, and if it is too great the airflow will separate and the aerofoil stall. The greater the wind speed the flatter the aerofoil needs to be if separation is not to occur, since the airflow, containing much more momentum can not so easily be held to the leeside of the aerofoil by a partial vacuum which breaks down into an area of great turbulence. See Fig. 5.

Thus aerodynamics require that for efficiency, an aerofoil should be made flatter in higher winds which is in opposition to the natural effect of the wind which is to make a sail more deeply cambered.

The Reversing Wing-sail is composed of a series of shaped frames between which sail fabric is stretched, and the shape of these frames can be adjusted so as to impart to the sail an efficient aerofoil shape, and they are also able to invert, forming the same shape in its mirror image so that the sale acts with equal efficiency on either tack.

BACKGROUND PRACTICE: At the present time the Bermudan Rig is most commonly used on sailing boats. This is a rig that uses triangular sails supported from one or two masts, and it has a number of disadvantages: 1. The mast causes a great disturbance in the airflow over the lee of the mainsail which reduces the lift as described above, line 31.

2. Being triangular sails the mast needs to be tall in order to carry sufficient sail area. See Fig. 6.

Not only is it more difficult for a taller mast to carry a given compressive load than for a shorter one, but also the taller mast is supported with stays that make smaller angles with the mast which therefore convey greater compressive loads to it than they would need to confer to a shorter mast to convey the same stability.

3. The tip of a triangular sail is an inefficient shape for the end of an aerofoil and causes induced drag in the form of spiral eddies to a much greater extent than either an elliptical or quadrilateril with upward inclined tip. See Fig. 7.

4. A modern Bermudan mainsaii is supported along its luff and at its clew only; its camber is controlled by tension alone. Thus the greater the tension between the mast and the clew, the flatter the sail, and in a strong wind when the sail needs to be flatter tremendous tension is required to achieve high efficiency. This resuits in very high compressive loads down the mast and in the sails having a short life.

It is not unusual for efficient boats of 50-60 feet in length to carry compressive loads of 30 40 tons down the mast (e.g. the catamaran British Oxygen). Furthermore the support for such a mast is a great difficulty for a catamaran, and in the case of British Oxygen a spreader arm beneath the mast and two straining wires was used; the tension in these wires was 250 tons. Thus the Bermudan Rig is a tension-compression structure involving very high tensile and compressive forces to achieve high efficiency.

5. A triangular sail of more than 300 square feet needs more than one man to handle it. Bermudan ocean racers need relatively large crews to manage the sails which can need frequent reefing and changing.

RECENT DEVELOPMENTS: WING SAILS have been developed using a triangular Bermudan type mainsail behind a large aerofoil mast that can pivot. See Fig. 3.

SEGMENTAL SAILS have been developed by Glauco Corbellini and these are triangular sails cut in strips. Both the wing sail and the segmental sail, though they provide better airflow and lift than bermudan sails share their other disadvantages as described on page 1.

THE JUNK RIG. This is a rig that has been explored and developed commercially, and which has been shown to be a very handy rig, one man being able to manage comparatively large sails with ease.

The Junk Rig, being a quadrilateril shaped sail supported on battens is not a 'tension-compression' structure but a 'bending beam' structure, whereby the force of the wind on the sailcloth is transferred to the battens which are supported at each end and therefore bend under load. See Fig. 9.

This bending confers a more or less aerodynamic shape to the sail, and as the wind flows round it, lift is generated. However, as the stiffness of the battens cannot be adjusted, for a given set of battens there will only be one wind speed at which the junk sail can pull with its full potential efficiency because at a greater wind speed than this the battens will bend more when what is actually required is a flatter sail, while at a lesser wind speed the battens will bend less when what is required for full power is a more deeply curved sail.

Furthermore, with the Junk Rig the mast is an impediment to the flow of air across the sail, and this is especially important when it is on the lee side of the sail since approximately 75% of the available lift is generated here.

GALLANT RIG. A sail has been developed by Manners-Spencer which he calls the Gallant Rig. It is based on the Junk Rig and is double sided, but the battens are rigid and have a fixed camber. See Fig.

10.

This has two drawbacks: 1. To achieve a good camber for medium winds with a ratio of camber to chord of 1:8 a very deep sail is needed; e.g. a 1 6ft. batten would be 4ft. deep at its widest point.

2. Symmetrical aerofoils produce much more drag than suitably curved ones.

THE INVENTION: Thus it can be seen that it would be possible to make a sail that had the Junk Rig advantages of managability and low structural demands with all round efficiency if: 1. The sail were double sided to envelop the mast and provide a good leading edge and smooth airflow on both tacks.

2. The battens were eyolved to become a mechanical device that can adjust the camber of the sail to suit the wind, and which can also invert to form its mirror image in order that the boat can tack to port or starboard equally well.

The new aspects of the Reversing Wing-sail are the development of these controllably flexing battens which we call 'tunny' because of their similarity of appearance and action with these fish. The Reversing Wing-sail also provides an evolved way of raising and lowering these 'tunny'. See Figs. 12 and 13.

In appearance the Reversing Wing-sail relates to a Junk Rig, being best controlled with a multiple sheet in the same way, and reefing in one 'tunny' at a time just as a Junk Rig reefs in one batten at a time.

When at rest, the tunny lie symmetrically about the mast, but they are very flexible and a relatively small force will bend them either to port or starboard, but they will only bend so far, and then beqome in effect an extremely strong rigidly curved beam. When the boat goes about this camber can be released and an equal and opposite camber put in the sale, and this may be done either by the action of the wind alone or in the case of-a very light wind, by throwing a lever or turning a winch to cause all the tunny to reverse. The shape they take up when fully bent can be carefully controlled in order to provide the required aerofoil.

The Reversing Wing-sail can be made in two forms; simple and fully adjustable:- Simple Form:- in its simple form the tunny are individually adjusted or tuned to take up a particular camber when fully bent. When at sea the speed of the wind increases with height. See Fig.

1 Thus tunny supporting the top of the sail are tuned with a relatively small camber, and those at the bottom with a fuller camber. When the whole sail is set it is ideally suited to lower wind speeds. As the wind speed increases, sail needs to be lowered or the boat will heel too much, or if a catamaran it will risk capsize. As the sail is progressively lowered the deeper cambered tunny are reefed, and that part of the sail best suited to high winds only remains aloft.

The curvature of the tunny is tuned by adjusting the length of the cross-bracing wires along its length. The nose section up to just over T its full length or chord is symmetrical and rigid. The last 32 is made from two flexible strips -- such as thin planks or, ideally, clefts of ashwood, or strips of G.R.P. and these are held apart by rigid spacers at frequent, and preferably diminishing, intervals. The spacers are attached to the flexible strips with a hinging coupling. Inside the cells formed by these spacers are cross wires or cords, and these are anchored either to the hinges, or just near to them. See Fig. 14.

As the tunny bends these cells lozenge until one of the two cross bracing wires becomes taut and it can then bend no more. If the tunny is then bent in the opposite direction this cross wire relaxes and the other eventually becomes taut when the tunny is fully bent in the opposite direction. The length of the cross wires or chords is adjustable, either with a screw device or simply with a suitable knot; and by adjusting their length the depth of camber of the tunny can be altered, and also the position of maximum camber.

The tunny can be made to bend their full extent by pulling the extreme ends of the flexible strips diagonally towards each other. See Fig. 1 5. This can be done either hydraulically or with a cord passing round a sheave mounted in the tail of each flexible strip.

The use of the Reversing Wing-sail in this way requires optimum ca mbers to be found for the tunny, and under certain conditions the sail will not be able to collect all the potential power from the wind. Also if a boat is rigged with two or more such sails, as the wind increases it would be preferable to lower one sail entirely and leave another fully hoisted. Therefore to achieve even greater efficiency, especially when two or more sails are in used, this wing-sail can be made fully adjustable.

The Fully Adjustable Wing-sail:-- To make the tunny fully adjustable the cross wires of each cell are connected to a centre piece, and all the centre pieces of one tunny are connected together with two cords. See rig. 16. As one cord is pulled, the camber is progressively increased to one side, and the other cord controls the camber to the other side. The two control cords from each tunny are rove through pulley blocks to the foot of the mast where they are activated by a lever or winch which thus controls the camber of the whole sail to port or starboard.

THE ORGANISATION OFTHETUNNY The tunny-yard and the tunny-boom need to be made of much deeper section so that as they are hauled apart by the halyard and fore and aft kicking straps (See Fig. 12) the sailcloth is stretched taut between them without causing them to bend unduly.

To raise and lower the tunny in order to reef or furl the sail a winch is mounted horizontally just behind the mast on the tunny-boom. A rope is attached to the tunny-yard via a bridle (See Fig. 13) which then passes over a block at the mast head, and then passes down to the winch round which it takes sufficient turns to get a grip and then passes back to the tunny-yard and attaches to a small windlass mounted on it forward of the effective point of attachment of the halyard so that when the whole system is tensioned, the tunny-yard is held up at an angle even without the influence of the sailcloth. Then by mounting rollers on the tunny-yard it can be run up and down the mast, haufing the sail and the other tunny up and down after it. In this way, and by using a large crank to turn the winch, one man can raise a large sail.Also when reefing or furling, the sail can be positively pulled down by attaching the lowest tunny temporarily to the downward travelling part of the halyard. Thus the said can be lowered in a strong wind.

Advantages: 1. Aerodynamic efficiency.

2. Low structural demands on mast and rigging.

3. The sail will not flog even in high winds because it is well supported.

4. The sail fabric lasts longer because it is not heavily strained.

5. Ease of handling the sails.

6. Ease of access to masthead and ease of maintenance since the tunny can be made so that a man can climb up inside the sail, sheltered from the weather, and look out over the top of the tunny-yard. Also the tunny are accessible to being checked and maintained while in use.

7. Scale is no limit. Extremely large Reversing Wing-sails could be made.

DESCRIPTION OF FIGURES.

Fig. 1 Nomenclature A -- onflowing airstream a - angle of attack B-the aerofoil C - the camber D - the chord.

Fig. 2 Deflection of the airstream producing lift.

A -- onflowing airstream D - the drag force B -- off-flowing airstream R - the resultant force L-the lift force 0-the angle of deflection Fig. 3 Reduced pressure on leeward surface produces 75% of total available lift.

A -- onflowing airstream B-- arrows represent lift vectors, length is proportional to partial vacuum at that point on the surface.

L -the lift force as a resultant of all the small arrows 'B'.

Fig. 4 Airflow round a mast.

A - onflowing airstream B - mast C-sail Fig. 5 Separation caused by increased air speed.

A -- airstream flowing over an aerofoil and being held to it by the partial vacuum (shaded area) that it is creating.

B - a faster airstream flowing over the same aerofoil. It has too much momentum to be held to the surface of the aerofoil and separates leaving a zone of turbulence beneath.

C - a faster airstream flowing over a flatter aerofoil. Being deflected to a lesser extent, the partial vacuum beneath is sufficient to hold it down and the flow is smooth and lift is generated.

Fig. 6 Comparison of length of mast needed to carry: A - a triangular sail. B - a quadrilateral sail a-40foot d - 30 foot b20 foot e-20foot c -400 square feet f -- 400 square feet.

Fig. 7 Comparison of induced drag produced by different shapes.

A -- triangular tip produces a large vortex.

B -- elliptical tip is the mathematically preferred shape with little leakage of air from high pressur windward side to low pressure leeward side.

C - upward slope as on birds wing, the flow lines are parallel to the end of the aerofoil causing minimum leakage.

Fig. 8 Wing Sail Principle. A -- starboard tack B-port tack W - airstream Fig. 9 Principle of the Junk Rig.

A - mast supporting the first section of each batten.

B -- multiple sheet supporting each batten at the leech.

Fig. 10 Plan of one batten of the 'Gallant' rig to show symmetrical rigid aerofoil.

A-the mast B - the spacer C - the battens Fig. 11 Typical Wind Gradient at sea in relation to the Reversing V8irïg-sail showirig In'crease or wind speed between the bottom of the sail and the top of 25%.

Fig. 12 Elevation of Reversing Wing-sail as made forTaulua showing the tunny to which the sailcloth is attached.

A - the tunny-yard # both are double the depth B -the tunny-boom # of the tunny.

C - are the tunny D - control levers that put the camber into the whole sail.

E - smaller control levers that control the camber of each tunny and which are connected by control lines to the control lever D.

F - the winch to raise and lower the sail.

G - the main sheet which supports the leech ends of each tunny.

N.b. Sailcloth removed from nearside to reveal the tunny.

Fig. 13 A. Diagrammatic drawing to show mechanism of supporting the sail and of raising and lowering it.

Halyard 'a' attaches to the tunny-yard via the bridle and is then rove through mast head block 'b' down to the winch 'd' mounted on the tunny boom. From the winch it passes back up to the tunny-yard to which it attaches via a small windlass at 'c' to enable the whole system to be kept in tension.

There is a single forward kicking strap 'e' and a pair of aft ones 'f'.

B. Pictorial representation to show the tunny-yard and tunny-boom with particular regard to the sail hoisting mechanism.

'a' and 'b' are rollers mounted on the tunny-yard.

'e' and 'f' are the kicking straps.

'g' is the four part bridle.

'h' is a temporary attachment of a tunny to the down traveiling part of the halyard in order to forcibly lower it, and maintain sailcloth tension between it and the tunny-yard.

Fig. 14 Plan of tunny No. 4 from the Reversing Wing-sail of Taulua.

A - dgid head section.

B -flexing tail section.

C - mast.

D - levers that control the camber.

E - pully blocks that take the control lines from the levers D to the tail blocks F.

F-tail blocks.

G - spacers H - cross bracing cords.

Fig. 1 5 Bending mechanism for tunny which could be used with the simple Reversing Wing-sail and with the fully adjustable version in which case the control line 'e' also attaches to the rotating centre of each cell.

A - mast E - pulley blocks taking B - rigid section of tunny control lines from levers D Cflexible section of tunny to the tunny-boom.

D -levers that control the camber.

F-Leech. G-Luff a - spacer b -- flexible strips 'c' - camber control lines attached to flexible strips.

'd' - sheaves mounted in tail blocks.

'e' - camber control lines.

Fig. 16 Mechanism of fully adjustable tunny.

A: Plan of section of tunny showing first three cells going back from the rigid head.

B: the above section of tunny flexed to port tack.

'a' - central disc to which the cross bracing wires 'b' are connected 'e' and to which the control lines 'g; and 'g2, are also connected at 'f'. 'd' are the flexible strips.

'c' are spacers with elongated holes cut in the centre 'i' to take control lines 'g; and 'g2 . - As control line 'g; is pulled, the discs rotate, causing the effective length of the diagonals of each cell to alter so that the cells lozenge and the tunny bends to one side. If control line 'g; is released and '92 is pulled instead the tunny will bend to the other side.

Fig. 17 Detail showing the attachment of the sail leech and sheet to the end of a tunny.

A - tablings at leech B - tail blocks C - running bullseye D - one part of multipart sheet Fig. 18 Detail of hingeing joint: A - flexible strips B -stainless steel split pin C - stainless steel pin that fits the eye of the split pin.

E - legs of split pin bent over and countersunk in completed assembled hinge.

EXAMPLE OF REVERSING WING-SAIL: We have developed and made a Reversing Wing-sail for 'Taulua', a 34 foot long overall length catamaran with 16 foot beam: Displacement - 5+ tons Wetted surface area - 260 square feet Area of Reversing Wing-sail -- 420 square feet Mast height - 32 feet

Position Chord Radius of Max. of No. of Max. thickness Tunny No. Chord Camber Curvature Spacers thickness from Nose (BOom3 1 20 6.6 18' 31/31' 7 2 45/8" 59 9" 2 18 6 7 18 1 7 2 5n 5Z 5 3 16' 8 7.3 16' 11" 6 1 9'4" 4' 10% 4 14' 6 7.6 15 4" 6 1 574 4 2n 5 12 6" 8 13 9'/2" 5 1 2, 31' 8" (Yard) 6 10 7H 8.3 12'1" 4 1'21/8" 2'10"

See Fig. 12:-Elevation of Reversing Wing-sail as made for Taulua.

Fig. 14:-Plan of tunny No. 4 from Reversing Wing-sail of Taulua.

Fig. 17:-Detail showing the attachment of the sail leech and sheet to the end of a tunny.

Fig. 1 8:- Detail of a hingeing joint.

Uses: The Reversing Wing-sail is practicable on all sizes of sail but the main benefits become apparent on larger sails. Sails that have their camber controlled by tension suffer from the effects of scale since each time their linear dimension is doubled, their area is squared, and the force on the sail is proportional to its area. So, while it is easy to control the camber of a dinghy sail with tension, the forces involved can get disproportionately large with a sail of 400 square feet; and this is why the sail supported on the masts of very big boats like the square riggers was set out in several managable courses.

The Reversing Wing-sail does not suffer from this effect of scale since managable sized panels of sailcloth are supported between the very strong tunny, and in square rigger terms, these tunny relate to the yards on a mast, and one mast could carry one huge sail that could be reefed in panels.

Thus the Reversing Wing-sail would be of particular advantage not only to yachts but to all trading vessels, particularly as the cost of fuel increases.

The Reversing Wing-sail is of particular use to catamarans where capsize is a particular danger, because this rig is low and compact. Also the compressive loading of the mast is very much less with this rig than with the Bermudan type, for example, and therefore much more easily supported by the cross beams of a catamaran.

Claims (2)

1. The Reversing Wing-sail is a double sided sail in which the fabric is drawn taut over a number of special variable profile frames so as to give it an efficient aerofoil shape similar to that of a low speed glider. The special feature of this sail lies in these variable profile frames which are able to perform the following functions when required: a. to bend with ease until a predetermined camber is attained.

b. this above camber can be adjusted in the 'simple Reversing Wing-sail' frame by frame, but in the 'fully adjustable Reversing Wing-sail' the camber of the entire sail can be rapidly adjusted.

c. these frames can be inverted with ease to produce their mirror image.

d. this inversion or bending can be caused by the pressure of the wind alone or it can be caused to happen by mechanical means by operating a control either on each individual frame or one control that operates all the frames at once.

2. The variable profile frames can be both hoisted and lowered using the power of a winch and not relying on gravity.