GB2136374A - A Rudder with a Symmetrical Profile Shaped to Give Maximum Lateral Forces with Simultaneous Maximum Directional Stability - Google Patents

Abstract

A rudder with a symmetrical profile has a nose portion (1) and rear edge of a tail portion (5) of particular shape and form. The nose portion (1) forms an obtuse angle alpha of up to 150 DEG and straight sides (2) of the angle alpha form tangents to a convex lateral contour (3) of a front portion (4). The tail portion (5) has a straight, diverging lateral contour with sides (6) having a flank angle beta of up to 15 DEG and a rear edge (7) forming an obtuse angle gamma of up to 150 DEG . Straight sides (8) of the angle gamma each join a respective radius (9) which form sharp separating edges (10) with the sides (6) and obtuse angles delta are formed between tangents (11) to the radii (9) and the sides (6). <IMAGE>

Description

SPECIFICATION A Rudder with a Symmetrical Profile Shaped to Give Maximum Lateral Forces with Simultaneous Maximum Directional Stability The invention relates to rudder profiles and particularly to rudder profiles which will give maximum transverse forces with an adjacent flow at the suction side and an angle of deflection of up to approximately 600, with simultaneous maximum directional stability within the range of +50 angle of deflection, for forward and backward travel.

Symmetrical rudder profiles with streamlined lateral contours and a nose radius have been previously proposed, those with thick profiles giving stronger lateral forces and those with slim profiles giving better directional stability.

A particular drawback is that the thickness of the profile is determined by the necessary thickness of the rudder shank, so that the necessary directional stability cannot be chosen freely.

A disadvantage during backward travel is that the flow is separated even at an angle of deflection of 20 to 300, owing to the acute angle at the rear edge, so that even the maximum lateral forces are not sufficient to keep the ships safely on course. It is only for boats with relatively large rudders, where the ratio of length of ship to length of profile is approximately 30 and less, that directional stability is important under fair weather conditions. Here the acute angled rear edge gives relatively good directional stability.

There is still the serious disadvantage of the maximum lateral forces being too weak.

Single profiles are also known, comprising two slim equilateral triangles with their equal sides at a tangent to the shank of the rudder. The nose of the profile is formed by a flat bar, with the narrow side perpendicular to the central line and the two adjoining right angles. The tail portion comprises an over-dimensioned retaining wedge with approximately twice the width of the profile and a flank angle of approximately 300. The contour of the rear edge forms an acute angle of approximately 1000, with its straight sides each forming an acute angled separating edge of approximately 1000 with the flanks of the wedge.

A disadvantage is that on account of the sharp, perpendicular edges of the nose, the flow at the suction side is separated prematurely, so that strong maximum transverse forces cannot be obtained. A particular disadvantage is that in the propeller race the flow is partly separated in the rhythm of the propeller beat, even in the central position, giving rise to oscillating loads, which increase with the load on the propeller, and finally to cavitation, with the considerable oscillations and cavitation noises being transmitted to the ship. Ability to turn and directional stability in a forward and backward direction are relatively good, particularly owing to the size and shape chosen for the retaining wedge. At the same time, however, a considerable amount of speed is lost when travelling on a straight course, and this is not economically acceptable.

Finally, a construction is known, comprising a streamlined, thicker front portion with convex lateral contours, an adjoining central, parallel portion, one plate thickness wide, and a slim tail portion with straight, diverging lateral contours, the flank angle being approximately 100. A special arrangement for travelling backwards has the rear edge of the tail portion designed as a semi-circle, thereby completely avoiding the harmful separating vortices within the range of small angles of deflection and reducing separation at the suction side up to the maximum range.

This construction gives the strongest maximum lateral forces, combined with good turning power without any loss of speed. Directional stability is similar to that with thicker profiles and is primarily dependent on the shape of the ship.

The problem underlying the invention is to improve symmetrical rudder profiles in such a way that maximum rudder forces are obtained, with an adjacent flow at the suction side and an angle of deflection of up to approximately 600, with simultaneous maximum directional stability within the range of f50 angle of deflection, both for forward and backward travel.

According to one aspect of the invention there is provided a rudder with a symmetrical profile wherein a nose portion of the rudder has, in plan view, a profile with an included obtuse angle a of up to approximately 1 500, with straight sides of the angle a forming tangents to a convex lateral contour of a front portion.

According to another aspect of the invention there is provided a rudder with a symmetrical profile wherein a tail portion of the rudder has a straight, diverging lateral contour with a flank angle P up to approximately 1 50, the contour of a rear edge forms an obtuse angle y of up to approximately 1 500, straight sides thereof each join a respective radius, which forms a sharp separating edge with the respective side of the rear end of the straight, diverging lateral contour, and an obtuse angle S formed between a tangent to the radius and the lateral contour of the tail portion is up to 1 500.

Advantageously a rudder has a nose portion according to said one aspect of the invention and a tail portion according to said another aspect of the invention.

The arrangement of the obtuse angle at the nose-according to the invention means that the dividing point for the flow at the pressure and suction side can be fixed on the centre line with an angle of deflection of +50, and rapid movement of the dividing point over a nose radius towards the direction of approaching flow can be prevented. Owing to the stable position of the dividing point, direction stability can be improved to a particularly high degree when the front profile portions are more full built and thicker and the nose radii larger. Here improvements of well over 500 can be obtained, a figure which is substantially higher than the improvements obtainable with the known tail portions, without there being any unfavourable effect on turning circle behaviour.

The combinatian of the obtuse angle of up to 15QO at the rear edge of the tail portion with the lateral radii and the obtuse angles at the separating edges, according to the invention, can give a considerable improvement in directional stability for backward travel, owing to the stabilisation of the dividing point and a further increase due to the reduction of the boundary vortices, which at the same time produce subsequent separation of the flow at the suction side with an improvement in lateral forces.

Thus the critical main problem of navigation, to increase travelling safety when travelling and meeting in a confined area, can be solved even for single surface helms. At the same time there can be greater economy owing to the reduction in the fuel requirement but with an improved directional stability. Even for turning manoeuvres there can be a degree of improvement.

Preferably given an existing profile with a nose formed by a fairly large radius, and given a straight contour at the rear edge of a tail portion, the particular requisite obtuse angles a, y at the nose and at the rear edge of the tail portion are each formed by a strip of plate pressed at an angle, the angle at which it is pressed being approximately 100 smaller than the necessary obtuse angle a, y and the resultant external bending radius being ground down in such a way that the strip of plate forms the necessary obtuse angle a, y.

Alternatively, given an existing profile with the nose formed by a small radius, and given an already existing narrow, straight contour at the rear edge of a tail portion, the particular requisite obtuse angles a, y at the nose, at the rear edge of the tail portion and for the joining radius are each formed by building up by welding and subsequent appropriate grinding.

If desired, given a slim tail portion known per se, with straight, diverging lateral contours and a length equal to approximately 10 to 200 of the length of the profile, the width of the rear edge is reduced to approximately 60 of the length of the profile.

If desired given a front portion with streamlined lateral contours, its length is approximately 700 and its thickness up to 200 of the length of the profile, while the lengths of the adjoining, central, parallel portion, one plate thickness wide, and of the slim tail portion with straight, diverging lateral contours are each approximately 1 50 of the length of the profile.

The invention is diagrammatically illustrated by way of example in the accompanying drawing, in which: Figure 1 shows a NACA profile rudder with a profile nose according to the invention; Figure 2 shows an IFS profile with the rear edge of the tail portion formed according to the invention; Figure 3 shows a rudder profile in accordance with British Patent Specification 1 426 412, modified to form a rudder according to the invention; Figure 4 shows a profile nose according to the invention on a larger scale; Figure 5 shows the rear edge of a rudder tail portion according to the invention, on a larger scale; Figure 6 shows the mounting of a profile nose according to the invention on an existing rudder profile with a large nose radius;; Figure 7 shows the mounting of a rudder tail portion rear edge according to the invention on a broad, straight rear edge of an existing tail portion; Figure 8 shows the mounting of a profile nose according to the invention on an existing rudder profile with a small nose radius; Figure 9 shows the mounting of a tail portion rear edge according to the invention on a narrow, straight rear edge of an existing tail portion; Figure 10 shows the mounting of a profile nose according to the invention on an existing single profile; Figure 11 shows the mounting of a tail portion rear edge according to the invention on an existing single profile; Figure 12 shows the mounting of a profile nose according to the invention on an existing plate rudder; and Figure 13 shows the mounting of a tail portion rear edge according to the invention on an existing plate rudder.

In the embodiments shown in Figures 1, 3, 4, 6, 8, 10 and 12 a profile nose 1 on a rudder with a symmetrical profile is designed to form an obtuse angle a of up to approximately 1 500.

Straight sides 2 form tangents to a convex lateral contour 3 of a front portion 4. For forward travel this brings striking improvements in travelling on course as explained above, particularly for thick rudder profiles with large maximum lateral forces.

The embodiments shown in Figures 2, 3, 5, 7, 9, 11 and 13 show a tail portion 5 arranged on a rudder with a symmetrical profile, with straight, diverging lateral contours 6 which have a flank angle p of up to approximatsiy 1 50. The contour of a rear edge 7 is designed to form an obtuse angle y of up to approximately 1 500, and straight sides 8 forming the angle y each join a radius 9, which forms a sharp separating edge 10 with the rear end of the straight diverging lateral contour 6. An obtuse angle a of up to 1 500 is formed by a tangent 11 to the radius 9 at the edge 10 of the lateral contour 6 of the tail portion 5.

The arrangement of the obtuse angle y at the rear edge 7 provides an improvement in direction stability during backward travel. This is considerably increased as a result of the radius 9 and the appreciably more obtuse angle 8 at the separating edge. At the same time the lateral forces are increased, particularly in the region of the separating flow on the suction side.

A simple construction of the arrangement of the obtuse angle a on an existing profile with a nose 1 formed by a large radius 12 is shown in Figure 6, and the arrangement of the obtuse angle y on the existing broad, straight contour of the rear edge 7 of a tail portion 5 is shown in Figure 7. The particular requisite obtuse angle a, y at the nose 1 or the rear edge 7 of the tail portion 5 respectively can be formed by a strip of plate 13 which is pressed so that it is bent at the desired angle. The impressed angle is approximately 10 smaller than the necessary angle a, y, and the resultant external bending radius 14 is ground down in such a way that the strip of plate 1 3 forms the necessary obtuse angle there.

It is thus clear that the shaping of existing rudder profiles to convert them into rudders according to the invention can be achieved by very simple means, with ail the full advantages of the invention thereby being obtained. A further simplification is provided by the arrangement according to the invention of the obtuse angle a on an existing profile with the nose 1 formed by a small radius 12, and on an already existing, narrow, straight contour of the rear edge 7 of a tail portion 5, the particular requisite angles a, y of the nose 1 and rear edge 7 respectively and the joining radius 9 being formed by an appropriately post ground build-up weld, so that the additional improvement which can be obtained in the already superior directional stability is also worthwhile.

Finally, the modified arrangement according to the invention of the obtuse angle a and y are shown, respectively, in Figure 10 on the nose 1 and Figure 11 at the rear edge 7 of the tail portion 5 of an existing single profile and, respectively, in Figure 12 on the nose 1 and Figure 13 at the rear edge 7 of the tail portion 5 of an existing plate profile.

The amount of work is so small that this arrangement is worthwhile, even when directional stability is already relatively good and can only be improved to a degree. Since the arrangements and constructions of the nose 1 and rear edge 7 according to the invention each have special advantages of their own, which are sum mated, they may be applied either separately or together.

If cavitation is to be avoided at the sharp tip 1 of the obtuse angle a at the nose 1 in conjunction with a heavily loaded propeller and a fast ship, the sharp tip of the nose 1 may be rounded slightly. It should be borne in mind that directional stability is reduced as the radius becomes larger.

Figure 3 shows a modified profile from British Patent Specification 1 426412 with the arrangement according to the invention of a slim tail portion 5 with straight, diverging lateral contours 6 and with a length 20 equal to approximately 10 to 200 of the overall length 1 6 of the profile; the width 1 5 of the rear edge 7 is reduced to approximately 60 of the length of the profile.

As a result of the narrow rear edge 7 the accelerated flow at the pressure side and the delayed flow at the suction side are closer together, and, owing to the smaller flank angle p of the tail portion 5, are directed approximately parallel with one another, so that the boundary vortices of the faster flow at the pressure side pick up and entrain the flow at the suction side more intensively. The maximum lateral force is thereby increased with larger angles of lock, and the reduction in lateral force after the maximum takes place more slowly, over a wider deflection range.

The rotary speed that can be obtained and the ship's abiiity to turn are consequently greatly improved.

The symmetrical, modified profile according to the invention, based on British Patent Specifications 1 426412 and 1 561 281 and illustrated in Figure 3, for an adjacent suction side flow within the maximum range, is characterised in that, given a front portion 4 with streamlined lateral contours 3, its length 1 7 is approximately 700 and its thickness 18 up to 200 of the overall length 1 6 of the profile, while the lengths 1 9 and 20 of the adjoining, central, parallel portion 21, one plate thickness wide, and of the slim tail portion 5 with straight lateral contours 6 are each approximately 1 50 of the length 1 6 of the profile.

Owing to the front portion 4 according to the invention, the flow at the suction side can be turned round more gently and remain longer in contact; a further increase can be obtained through the reduction in the width 15 of the rear edge 7 to approximately 60 of the length 1 6 of the profile in accordance with the invention. At the same time the shank arrangement is provided regardless of the length of the front portion 4, and depends on whether the rudder is positioned by hånd or e.g. by power hydraulics.

Thus the construction according to the invention is universally applicable, both for sailing and motor ships of any size, in all areas of inland and sea navigation to give good directional stability and maximum ability to turn.

Claims (8)

1. A rudder with a symmetrical profile, wherein a nose portion of the rudder has, in plan view, a profile with an included obtuse angle a of up to approximately 1500, with straight sides of the angle a forming tangents to a convex lateral contour of a front portion.

2. A rudder with a symmetrical profile, wherein a tail portion of the rudder has a straight, diverging lateral contour with a flank angle jB up to approximately 1 50, the contour of a rear edge forms an obtuse angle y of up to approximately 1500, straight sides thereof each join a respective radius, which forms a sharp separating edge with the respective side of the rear end of the straight, diverging lateral contour, and an obtuse angle a formed between a tangent to the radius and the lateral contour of the tail portion is up to 1 500.

3. A rudder with a symmetrical profile having a nose portion according to claim 1 and a tail portion according to claim 2.

4. A rudder with a symmetrical profile according to claim 3, wherein given an existing profile with a nose formed by a fairly large radius, and given a straight contour at the rear edge of a tail portion, the particular requisite obtuse angles a, y at the nose and at the rear edge of the tail portion are each formed by a strip of plate pressed at an angle, the angle at which it is pressed being approximately 100 smaller than the necessary obtuse angle a, y and the resultant external bending radius being ground down in such a way that the strip of plate forms the necessary obtuse angle cg, y.

5. A rudder with a symmetrical profile according to claim 3, wherein given an existing profile with the nose formed by a small radius, and given an already existing narrow, straight contour at the rear edge of a tail portion, the particular requisite obtuse angles a, y at the nose, at the rear edge of the tail portion and for the joining radius are each formed by building up by welding and subsequent appropriate grinding.

6. A rudder with a symmetrical profile according to claim 3, wherein given a slim tail portion known per se, with straight, diverging lateral contours and a length equal to approximately 10 to 200 of the length of the profile, the width of the rear edge is reduced to approximately 60 of the length of the profile.

7. A rudder with a symmetrical profile according to any one of claims 3 to 6, wherein given a front portion with streamlined lateral contours, its length is approximately 700 and its thickness up to 200 of the length of the profile, while the lengths of the adjoining, central, parallel portion, one plate thickness wide, and of the slim tail portion with straight, diverging lateral contours are each approximately 1 50 of the length of the profile.

8. A rudder with a symmetrical profile substantially as hereinbefore described and illustrated with reference to the accompanying drawing.