GB1595242A

GB1595242A - Stabilisers

Info

Publication number: GB1595242A
Application number: GB1107177A
Authority: GB
Original assignee: ANCOM Ltd
Current assignee: ANCOM Ltd
Priority date: 1978-03-14
Filing date: 1978-03-14
Publication date: 1981-08-12

Description

(54) IMPROVEMENTS IN OR RELATING TO STABILIZERS (71) We, ANCOM LIMITED, a British Company, of Devonshire Street, Cheltenham, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to roll stabilizers for boats, and is of particular application to vessels between 30 feet and 80 feet in length.

In sea-going vessels of the displacementhull kind, where rolling is a serious source of discomfort, a major improvement can be obtained by damping out the roll resonance effect, by which effect the boat builds up a large degree of roll when acted upon by a plurality of even quite small waves arriving at the boat's natural resonant frequency.

In practice, sea waves usually contain a large range of frequencies, but the effect of resonance predominates. The primary purpose of a roll stabiliser is to reduce this effect as far as possible.

A known form of roll-reduction system for such vessels comprises a pair of booms, one projecting upwardly and outwardly from each side of the boat, with a suspension line attached to the outer end of each boom.

A paravane in the form of a delta-shaped plate having a weight at its apex and a fixed longitudinal fin perpendicular to the plate is attached to the free end of each suspension line, and is suspended under water at a depth where wave disturbance is relatively minor, for example 12 to 20 feet beneath the surface.

The line of each paravane is secured to a post projecting normally from near the centre of the plate so that, when the line is in tension and the plate is being pulled through the water, the plate orientates itself substantially perpendicular to the line so as to exhibit maximum resistance to movement and thereby maintains considerable tension in the line. However, when the line is slack, the weight causes the plate to tilt and to dive relatively freely. Thus, when the boat rolls, one boom tends to rise against the tension in the corresponding line whilst the other boom falls and allows the corresponding plate to dive. When the direction of roll reverses, tension is transferred from one line to the other, the overall effect being to damp rolling motion in either direction.

This known system was developed primarily for use on fishing boats, which spend a large proportion of their time either with engines stopped whilst handling nets or at a very low speed whilst trawling. Under such conditions, the delta-shaped paravane provides fairly satisfactory damping. However, the system has a considerable disadvantage in that, when set up for optimum roll stabilisation and the boat is travelling at normal cruising speed, each paravane has a high angle of incidence to the water, resulting in large drag forces. In boats equipped with such a system it is commonly found that about 25% to 30% of the engine horsepower is used in towing the paravanes.

In addition, rolling of the boat at cruising speed causes the paravanes to swing in opposite directions fore and aft of the boat in the manner of a pendulum. To operate properly at low speeds, the paravanes must be of heavy constructions and are consequently difficult and dangerous to handle.

It is accordingly an object of the present invention to provide an improved roll stabiliser, suitable for operation at cruising speed as well as at low speed.

According to the invention a roll stabiliser for a boat includes a paravane comprising two spaced vertical fins between which extends at least one aerofoil-shaped member and means for attaching a pair of suspension lines to the paravane in such manner that, in use, the angle of incidence of the paravane to the water can be controlled by said suspension lines.

The paravane may be substantially rectangular in plan view, the two fins being disposed, one at each side of the rectangle. The edge of the paravane which is intended, in use, to be the leading edge mav be weighted to ensure that the paravane dives when the suspension lines become slack, and the paravane is preferably of inverted aerofoil cross section.

The invention also provides a boat having a roll stabilisation system comprising a pair of such stabilisers, each being secured to the free ends of a pair of substantially parallel suspension lines depending from one of a pair of booms projecting outwardly from the sides of the boat.

Preferably the lines are attached to a spreader bar mounted at the end of each boom and extending in the fore-and-aft direction of the boat so as to provide a parallelogram mounting for each stabiliser and thereby to maintain the stabilisers at a predetermined angle of inclination relative to the boat.

Two embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a monoplane paravane and its means of attachment to the side of a boat, Figure 2 is a side view of the paravane, partly in section, indicating the relative water velocity under differing roll conditions, and Figure 3 is a perspective view of a biplane paravane.

Shown in Figure 1 is a monoplane paravane 10 which includes a rectangular body portion 11 of inverted aerofoil cross-section having a leading edge 12 and a trailing edge 13, in accordance with aerodynamic practice. Two fins 14 extend perpendicularly to the body portion 11, one mounted along each of the two shorter sides of the rectangle, and project somewhat beyond the trailing edge 13 of the body portion 11 in order to locate the lateral centre of pressure of the paravane 10 as far back as possible. This effect can alternatively be produced by making the fins 14 of increasing height towards their trailing edges. The fins 14 also improve the efficiency of the paravane by reducing span-wise flow of water, which would otherwise give rise to vortices at the sides of the body portion 11.

The effective aspect ratio of the paravane 10 is thereby increased, resulting in reduced drag.

The body portion 11 is of composite construction, as shown in Figure 2, and comprises a hollow shell 15, the leading part 16 of the interior thereof being weighted to move the centre of gravity of the paravane as far forward as possible. The trailing part 17 of the shell interior is filled with a rigid light-weight plastics foam to prevent the ingress of water. The combination of forward centre of gravity and rearward centre of lateral pressure gives the paravane 10 good directional stability, ensuring that it tows without tendency to wander sideways or up towards the surface.

The suspension line 18 is attached to the paravane 10 adjacent the leading edge 12 and mid-way between the fins 14 whilst a bridle 19 is attached adjacent the trailing edge 13 at two points 20 equidistantly spaced from the mid point, a second suspension line 21 being secured to the centre of the bridle 19. Alternatively the bridle 19, which controls the lateral attitude of the paravane 10, may be provided at the leading edge 12 and carry the first suspension line 18, the second suspension line 21 being attached at the centre of the trailing edge 13, or bridles 19 may be provided at both the leading edge 12 and the trailing edge 13.

The free ends of the two suspension lines 18 and 21 are fastened one to each end of a spreader bar 22 mounted at the end of a boom 23 projecting upwardly and outwardly from one side 24 of a boat. The spreader bar 22 extends in the fore-and-aft direction of the boat, the lengths of the lines 18 and 21 being such that, in use, the paravane 10 hangs substantially parallel to the spreader bar 22 and approximately 15 feet below the surface of the water. The distances along the spreader bar 22 and along the paravane 10 between the line attachment points are equal, providing a parallelogram mounting for the paravane 10 which maintains the paravane 10 at a predetermined angle of inclination relative to the boat regardless of lozenging of the parallelogram due to drag.This angle of inclination can be adjusted by rotating the spreader bar 22 about a transverse axis, by rotating the boom 23 about its longitudinal axis or by varying the length of one of the suspension lines 18 and 21. However, it may be advantageous in particular cases to vary the positions of the attachment points of the lines 18 and 21 to the spreader bar 22 to give a trapezoidal geometry.

A corresponding boom and suspended paravane arrangement is provided on the other side of the boat, to provide a rollstabilisation system. The system is set up whilst travelling in calm water by adjusting the attitudes of the paravanes 10 to provide a maximum ratio of downward thrust to drag, giving a large downward thrust with much less drag than is the case with the conventional variable-inclination delta arrangement. As the resultant of the drag force and the downward thrust acting on each paravane 10 is resisted solely by the total of the tension forces in the corresponding lines 18 and 21, it follows that the resultant force will be equal and opposite to the tension forces. Thus the maximum ratio of downward thrust to drag is obtained when the lines 18 and 21 are most nearly vertical. In practice, there is a range of angles of paravane inclination over which the suspension lines 18 and 21 are as near vertical as possible, and the angle of inclination is fixed at the lower limit of this range.

Roll stabilising action results from the fact that the effective angles of incidence of or the paravanes 10 to the water vary as the boat rolls. When the tip of one of the booms 23 rises due to roll, the corresponding paravane 10 is pulled upwardly through the water and the water has a downward component of velocity 24 relative to the paravane, effectively increasing the angle of incidence a and therefore the downward thrust. The reverse happens on the other side of the boat, so that a couple is set up which opposes the rolling motion.

For a given stabilising capability, the drag forces are estimated at one-quarter to onetenth of those obtaining with a variableinclination stabiliser, for which the angle of incidence of the paravane to the water remains substantially constant.

For an upwardly moving paravane 10, the increased angle of incidence a will naturally result in increased drag which, in turn, will tend to swing the paravane 10 and the suspension lines 18 and 21 rearwardly about the corresponding spreader bar 23. However, this tendency will at least partially be resisted by the increased downward thrust, so that pendulum action will not be so marked as with the delta-shaped paravane.

If the boom-tip velocity due to rolling exceeds a certain value in a downward direction, the effective angle of incidence becomes negative, and might be expected to capsize the paravane 10. In practice, however, this does not happen, because the weighted leading edge 12 of the paravane 10 tips the paravane 10 downwards, allowing it to dive and keep the front suspension line 18 fairly taut, whilst drag maintains some degree of tension in the rear line 21. Thus a negative downthrust condition never actually develops and the lines 18 and 21 are maintained in tension, thereby avoiding snatch when the direction of roll reverses.

There is a possibility that the suspension lines 18 and 21 will become twisted. It has been found, however, that one or two twists do not seriously impair operation so long as the lines can slide smoothly against each other.

It can be of advantage for the paravanes 10 to tow slightly outwardly from the boat, so that the suspension lines 18 and 21 pull on the booms 23 at a more efficient angle, and for this purpose at least the trailing edge portions of the fins 14 can be bent slightly away from the boat, in the manner of a rudder. It has been found appropriate for the lines 18 and 21 to be inclined outwardly from the booms 23 at an angle of about 15 to the vertical.

Figure 3 shows an alternative form of paravane, in which two members 25 and 26 of inverted aerofoil cross-section are arranged one above the other in biplane configuration. The adjacent sides of the members 25 and 26 are interconnected by vertically extending fins 27. A bridle 28 is attached to the leading edges of the fins 27, a second bridle 29 being attached to the trailing edges of the fins 27. Suspension lines 18 and 21 are attached to the bridles 28 and 29, respectively, and are mounted on the boat in the manner described above.

Although structurally more complicated than the monoplane configuration, the biplane paravane can more conveniently be made with aerofoil surfaces having higher aspect ratios. In addition, the biplane assembly cannot plane along the surface of the water as can happen when launching a monoplane paravane. Other multiplane structures are, of course, also within the scope of the invention.

WHAT WE CLAIM IS: 1. A roll stabiliser for a boat, including a paravane comprising two spaced vertical fins between which extends at least one aerofoil-shaped member, and means for attaching a pair of suspension lines to the paravane in such manner that, in use, the angle of incidence of the paravane to the water can be controlled by said suspension lines.

2. A roll stabiliser according to claim 1, wherein the paravane is substantially rectangular in plan view.

3. A roll stabiliser according to claim 1 or 2, wherein the paravane comprises a plurality of aerofoil-shaped members etxending between the vertical fins, the aerofoil-shaped members being vertically spaced from one another.

4. A roll stabiliser according to any preceding claim, wherein the edge of the paravane which is intended, in use, to be the leading edge is weighted to ensure that the paravane dives when the suspension lines become slack.

5. A roll stabiliser according to any preceding claim, wherein the spaced vertical fins are parallel.

6. A roll stabiliser according to any preceding claim, wherein at least the trailing edge portion of each fin is angled laterally with respect to the fore-and-aft axis of the paravane, to give the paravane a directional bias when in use.

7. A roll stabiliser according to any preceding claim, wherein the means for attaching one of the suspension lines is provided adjacent that edge of the paravane which is intended to be the leading edge, and the means for attaching the other suspension line is provided adjacent that edge which is intended to be the trailing edge.

8. A roll stabiliser according to any pre

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. vertical as possible, and the angle of inclination is fixed at the lower limit of this range. Roll stabilising action results from the fact that the effective angles of incidence of or the paravanes 10 to the water vary as the boat rolls. When the tip of one of the booms 23 rises due to roll, the corresponding paravane 10 is pulled upwardly through the water and the water has a downward component of velocity 24 relative to the paravane, effectively increasing the angle of incidence a and therefore the downward thrust. The reverse happens on the other side of the boat, so that a couple is set up which opposes the rolling motion. For a given stabilising capability, the drag forces are estimated at one-quarter to onetenth of those obtaining with a variableinclination stabiliser, for which the angle of incidence of the paravane to the water remains substantially constant. For an upwardly moving paravane 10, the increased angle of incidence a will naturally result in increased drag which, in turn, will tend to swing the paravane 10 and the suspension lines 18 and 21 rearwardly about the corresponding spreader bar 23. However, this tendency will at least partially be resisted by the increased downward thrust, so that pendulum action will not be so marked as with the delta-shaped paravane. If the boom-tip velocity due to rolling exceeds a certain value in a downward direction, the effective angle of incidence becomes negative, and might be expected to capsize the paravane 10. In practice, however, this does not happen, because the weighted leading edge 12 of the paravane 10 tips the paravane 10 downwards, allowing it to dive and keep the front suspension line 18 fairly taut, whilst drag maintains some degree of tension in the rear line 21. Thus a negative downthrust condition never actually develops and the lines 18 and 21 are maintained in tension, thereby avoiding snatch when the direction of roll reverses. There is a possibility that the suspension lines 18 and 21 will become twisted. It has been found, however, that one or two twists do not seriously impair operation so long as the lines can slide smoothly against each other. It can be of advantage for the paravanes 10 to tow slightly outwardly from the boat, so that the suspension lines 18 and 21 pull on the booms 23 at a more efficient angle, and for this purpose at least the trailing edge portions of the fins 14 can be bent slightly away from the boat, in the manner of a rudder. It has been found appropriate for the lines 18 and 21 to be inclined outwardly from the booms 23 at an angle of about 15 to the vertical. Figure 3 shows an alternative form of paravane, in which two members 25 and 26 of inverted aerofoil cross-section are arranged one above the other in biplane configuration. The adjacent sides of the members 25 and 26 are interconnected by vertically extending fins 27. A bridle 28 is attached to the leading edges of the fins 27, a second bridle 29 being attached to the trailing edges of the fins 27. Suspension lines 18 and 21 are attached to the bridles 28 and 29, respectively, and are mounted on the boat in the manner described above. Although structurally more complicated than the monoplane configuration, the biplane paravane can more conveniently be made with aerofoil surfaces having higher aspect ratios. In addition, the biplane assembly cannot plane along the surface of the water as can happen when launching a monoplane paravane. Other multiplane structures are, of course, also within the scope of the invention. WHAT WE CLAIM IS:

1. A roll stabiliser for a boat, including a paravane comprising two spaced vertical fins between which extends at least one aerofoil-shaped member, and means for attaching a pair of suspension lines to the paravane in such manner that, in use, the angle of incidence of the paravane to the water can be controlled by said suspension lines.

8. A roll stabiliser according to any pre

ceding claim, wherein the means for attaching at least one of said suspension lines to the paravane comprises a bridle for connecting the suspension line to two points on the paravane spaced laterally on opposite sides of the fore-and-aft axis thereof.

9. A boat having a roll stabilisation system comprising a pair of roll stabilisers according to any preceding claim, each paravane being secured to the free ends of a pair of substantially parallel suspension lines depending from one of a pair of booms projecting outwardly from the sides of the boat.

10. A boat according to claim 9, wherein the suspension lines are attached to a spreader bar mounted at the end of each boom and extending in the fore-and-aft direction of the boat so as to provide a parallelogram mounting for each stabiliser and thereby to maintain the stabilisers at a predetermined angle of inclination relative to the boat.

11. A roll stabiliser according to claim 1 and substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 or Figure 3 of the accompanying drawings.

12. A boat having a roll stabilisation system according to claim 9 and substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.