WO1990012733A1

WO1990012733A1 - Keel assembly

Info

Publication number: WO1990012733A1
Application number: PCT/CA1990/000134
Authority: WO
Inventors: William J. Unger; Lawrence Alfred Caza
Original assignee: Unger William J; Lawrence Alfred Caza
Priority date: 1989-04-26
Filing date: 1990-04-26
Publication date: 1990-11-01
Also published as: AU5550390A

Abstract

A keel assembly (10) for a water craft is provided. The keel assembly includes a body having an aerodynamic cross-section with a pair of side faces (12a, 12b) and a leading edge section (14). A portion of the leading edge section is moveable relative to the body. A pair of vanes (32) are spaced from and connected to the leading edge section to be disposed within a flow of water moving past the body. Movement of the vanes pivot the portion toward the direction of flow of the water to offset the effects of a negative pressure region occurring at the leading edge section.

Description

KEEL ASSEMBLY

TECHNICAL FIELD

The present invention relates to hydrofoil devices used on water craft and more particularly to those for use on sailboards.

BACKGROUND PRIOR ART

From its inception 20 years ago, boardsailing has grown into a world wide phenomenon practised by millions of people of all ages. The success of boardsailing as a sport has led to extensive development to improve the performance of the equipment to achieve, among other things, improved upwind ability and increased speed.

A current equipment related performance limitation of the sport is due to the skeg, or 'fin' at the back of the board. The purpose of the skeg is to keep the board going in a straight line as does a rudder on a sail boat and, more importantly, to give the board lateral resistance to the wind as does a keel on a sail boat. Since high performance sailboards have no keel to centre board, they must rely on the skeg to act as both rudder and keel. Herein lies the problem. As the sailboard reaches a certain water speed or angle of attack, the skeg loses all effectiveness in maintaining directional control and lateral resistance. This phenomenon is the result of either 'stall' or 'cavitation' of the skeg and in general is known to boardsailors as *spin-out' . It remains a significant limitation on the performance of sailboards. The most pertinent prior art of which the applicant is aware is Canadian Patent 1,062,091 to Haddock and Canadian Patent 1,186,181 to Mistral Windsurfing Ag. Both of these references are directed towards a technique to adjust the shape of a keel cross section. This is done by providing a pair of flexible side walls which extend the distance between the leading and trailing edges of the keel and are deflected in reaction to the prevailing flow conditions across the keel.

While discussion herein is restricted to a sailboard, it will be readily understood that these problems are also to be found in certain other water craft.

It is therefore an object of the present invention to provide a novel technique to improve the performance of water craft.

DISCLOSURE OF INVENTION

Briefly stated, the present invention provides a keel assembly for a sailcraft comprising: a body having a pair of side faces and a leading edge extending the length of said body, said body having a portion adjacent said leading edge; first connecting means connecting said portion with said body for relative movement thereto; a vane means to be disposed within a flow of water relative to said body; said flow having a direction of travel; second connecting means joining said vane means with said portion, said second connecting means constituting means to transmit movements of said vane means to said portion to orient said portion toward said direction of travel to offset the effects of a negative pressure region occurring near said leading edge.

In another aspect of the present invention, there is provided a kit for improving the performance of a keel for a water craft, comprising: leading edge means arranged for attachment to a leading edge of a keel; connection means for attaching the leading edge means to the leading edge for movement relative thereto; vane means for attachment to the leading edge means and to be disposed within a flow of water past the keel; the flow having a direction of travel; coupling means to attach the vane means to the leading edge means so that the vane means follows the direction of flow; the coupling means constituting means to transmit movements of the vane means to the leading edge means to orient the leading edge means toward the direction of travel.

In yet another aspect of the present invention, there is provided a method of improving the performance of a keel for a water craft, comprising the steps of: providing a leading edge means for attachment to a leading edge of a keel; connecting the leading edge means to the leading edge for movement relative thereto; providing a vane means which is attachable to the leading edge means so as to be disposed within a flow of water past the keel, the flow having a direction of travel; coupling the vane means to the leading edge means so that the vane means follows the direction of flow and so that movements of the vane means are transmitted to the leading edge means to orient the leading edge means toward the direction of travel.

In still yet another aspect of the present invention, there is provided a keel assembly for a sail craft comprising: a keel body having a leading edge and a trailing edge; said keel body having a leading section on which said leading edge is located and a trailing section on which said trailing edge is located; hinge means joining said leading and trailing sections; a support means attachable to said sail craft to support said keel body in position below said sail craft; said support means having an axis parallel to a central axis extending the length of said craft; a pivot member joining said keel body to said support means, said pivot member being of construction sufficient to transfer substantially the entire bending load to be generated by said leading section and said trailing section to said craft; restraining means coupled to said leading section, said restraining means constituting means to confine displacement of said leading edge to a region centered on said axis; said pivot member being positioned on said keel body so that a center of pressure to be exerted by a flow of water past said keel body is located between said pivot member and said leading edge, thereby to cause a rotation of said trailing section about said pivot member and an opposite rotation of said leading section relative to said hinge.

The term 'keel' used herein represents a generic term to cover both the keel of a sail boat and a skeg of a sailboard as well as any stabilizing member used in a similar manner below the hull of other water crafts.

BRIEF DESCRIPTION OF DRAWINGS

Preferred embodiments of the present invention will now be described by way of example only with reference to the appended drawings in which:

Figure 1 is a side view of a conventional prior art skeg;

Figure 2 is a schematic sectional view taken on line 2-2 of Figure 1;

Figure 3 is a schematic cross sectional representation of a conventional prior art skeg illustrated in Figure 1;

Figure 4 is schematic cross sectional representation of the present skeg;

Figure 5 is an exploded fragmentary perspective view of the skeg assembly illustrated in Figure 4;

Figure 6 is a perspective view of the skeg assembly illustrated in Figure 4;

Figure 7 is a plan view of the skeg assembly illustrated in Figure 4; - 6 -

Figure 8 is a side view of the skeg assembly illustrated in Figure 4;

Figure 9 is a front view of the skeg assembly illustrated in Figure 4;

Figure 10 is a schematic cross sectional representation of the skeg assembly illustrated in Figure 4;

Figure 11 is a side view an alternative skeg assembly;

Figure 12 is a front view of the skeg assembly illustrated in Figure 11.

Figure 13 is a prospective assembly view of a keel assembly;

Figure 14 is a cross sectional view of the keel assembly of figure 13 taken on line 14-14 thereof;

Figure 15 is another cross sectional view according to figure 14 with the keel assembly in an alternative orientation.

BEST MODE FOR CARRYING OUT THE INVENTION

Before discussing the preferred embodiments of the present keel assembly, a discussion of the onset of stall or cavitation as it occurs on conventional sailboard skegs is provided for clarity and with reference to Figures 1 to 3. The cross section of a conventional sailboard skeg is shown at 'b' in Figure 2 in a 'flow' of water. The term 'flow' here refers to the relative movement of the water relative to the sailboard. The direction of flow is shown by the arrows ¹c' . When the skeg is aligned with the direction of flow, the water will pass the skeg on either side thereof. The skeg is rotated relative to the direction of flow (as shown in Figure 3), to allow the skeg to generate 'lift' and thereby provide a counter force to the sailboard against the lateral force generated by the wind on the sail. In this orientation, water passes by the skeg in a manner causing a high pressure region 'd' on the side face 'e' of the board facing the oncoming water and a region of relatively lower pressure 'f' on the opposite side face 'g' of the skeg. By virtue of the pressures thus formed, the skeg generates lift 'h' and drag 'i'. This angle, alpha, at which the skeg is oriented with respect to the oncoming flow is known as the 'angle of attack' .

The 'lift' is a transverse force which counters the transverse component of the wind force on the sail. If the 'lift' is reduced to the point where it no longer balances the transverse component of the wind force, the sailboard will tend to follow the direction of the wind, which means that the boardsailor's ability to control the direction of the sailboard, particularly when heading into the wind, is substantially reduced.

Within this low pressure region 'f' is a region of minimum pressure 'j' located immediately behind the skeg's leading edge illustrated at 'k'. This region of minimum pressure can induce two phenomena which limit the performance of the skeg and, in turn, the sailboard. At relatively low speeds and high angles of attack, the region of minimum pressure results in an adverse pressure gradient across the chord of the skeg. This can cause the flow to separate from the side face *g' resulting in the loss of lift. This phenomenon is known as 'stall' . - 8

At relatively high speeds, this region of minimum pressure may be sufficiently extreme to cause the water flow to vaporize. This effect is known as 'cavitation' and results in a drastic increase in drag with an associated decrease in lift.

When these conditions occur, since the 'lift' is suddenly reduced, the transverse component of the wind force on the sail suddenly overwhelms the opposing force of the keel and the sailboard shifts sideways in the direction of the wind. This causes the boardsailor to lose control in a condition called 'spin-out' .

In contrast to the prior art, a first embodiment of a skeg according to the present invention is schematically illustrated at 10 in Figure 4 having a body of aerodynamic cross section with side faces 12a and 12b. The body has a trailing portion 13 and a leading portion 14 in the form of a leading edge section pivotally attached thereto. As schematically represented by the rotation 'theta' in Figure 4, a mechanism is provided whereby the leading edge section 14 is turned toward the direction of flow 'c' so that the region of minimum low pressure is substantially reduced, thereby reducing the likelihood of the onset of both stall and cavitation. This is shown in Figure 4 wherein the region of minimum pressure (shown in dashed lines) is replaced by a continuation of the region of relatively less extreme low pressure 16 extending along substantially the entire length of the surface 12a.

The skeg assembly 10 is shown in more detail in Figures 5 to 9. The body 12 has a lower region 18 and an upper region 20, the latter of which is coupled with a mounting bracket 22 of suitable construction. The body 12 also has a front face 12c and a rear face 12d which converge toward the lower region 18. The leading edge section 14 is pivoted at the front face 12c to the upper region 20 by first connecting means in the form of a pivot pin 24 which its into a matching hole in the manner illustrated by the arrow 'A' . The leading edge section 14 tapers toward its lower end 14a and is held to the lower region 18 by another first connecting means in the form of a pivot pin 25 which fits into a matching hole. The above-mentioned mechanism to turn the leading edge section 14 toward the flow direction is embodied in a trailing vane section 28 which is attached to the upper end 14b of the leading edge section 14.

Second connecting means are provided to join the trailing vane section 28 with the leading edge section and are in the form of arms 30 which extend laterally outwardly from side faces 12a, 12b and rearwardly of the leading edge section 14. An aerofoil 32 having an aerodynamic cross section and a wing-like platform depends from each arm 30. In conjunction with the arms 30, each of these aerofoils 32 constitutes a vane which acts to turn the pivoted leading edge section 14 toward the direction of flow of oncoming water.

The body 12, the leading edge section 14 and the trailing vane section 28 may be formed of metal, plastic or composite materials or any other suitable material as desired.

To assemble the skeg assembly 10, the leading edge section 14 with the attached trailing vane section 28 is mounted on the body 12 by inserting the pivot pins - 10 -

24 and 25 in the holes as shown by arrow A and B respectively. It will thus be seen that the skeg assembly 10 is simple to assemble. -

With the appropriate mounting hardware, the skeg may then be mounted on the sailboard.

During use, the skeg assembly 10 functions in the same manner as a conventional skeg only when its central axis is in line with the direction of flow of the water. However, when the angle of attack, alpha, is increased as shown in Figure 10, the trailing vane section 28 is maintained in alignment with the direction of flow as it is free to pivot about the pivot pins 24 and 25. This causes the leading edge section 14 to remain oriented toward the flow direction thereby to counter the onset of those phenomena leading to stall or cavitation as would occur in similar circumstances with a skeg of conventional construction.

Figures 11 to 12 illustrate an alternative skeg assembly 50. For clarity, in these figures, elements equivalent in function with those in the previous embodiment are identified with like numerals. A significant feature of the assembly 50 is the use of a pair of lower trailing vanes 54 which are attached to the lower end 14a and which extend laterally outwardly from a respective side face of the body 12 and are angled upward and rearward of the leading edge section 14. Similarly, a pair of upper trailing vanes 56 are attached to the upper end 14b and extend laterally outwardly from a respective side face but in a downward and rearward direction from the leading edge section 14. The skeg assembly operates in identical fashion to the previous skeg assembly. However, the elimination of the arms 30 simplifies manufacture while the addition of the additional pair of trailing vanes improves performance in rough water where the uppermost vanes may momentarily leave the water and be rendered ineffective in orienting the leading edge toward the flow direction.

The following discussion concerns another embodiment of the present invention as illustrated in those drawings identified hereinbelow.

Still yet another alternative keel assembly is shown at 70 in figures 13 to 15. The keel assembly 70 has an aerodynamically shaped keel body 72 with a leading edge 72a and a trailing edge 72b. The leading edge is formed on a leading section 74 and the trailing edge is formed on a trailing section 76. Both sections 74, 76 have surfaces which together form the faces of the keel body. One of the faces is formed by surfaces 74a and 76a while the other face is formed by surfaces 74b and 76b. The leading and trailing sections 74 and 76 are articulated by a hinge assembly 78 permitting the leading section 74 to rotate relative to the trailing section 76 about a hinge axis 78a. The hinge assembly 78a makes use of a pair of upper mounting holes 78b, one on each of the leading and trailing sections 74,76 and a pair of lower mounting holes 78c, each also mounted on one of the trailing and leading sections 74, 76. An upper hinge pin 78d and a lower hinge pin 78e are provided to fit through the upper and lower pairs 78b, 78c respectively to complete the hinge assembly 78. 12 -

A pivot member 80 joins the keel body 72 with a support assembly 82 permitting the keel body 72, in use, to rotate about a pivot axis 80a through the pivot member 80. The support assembly is provided with a suitable bearing assembly 82a to receive the pivot member 80. The support assembly 82 is arranged to be mounted, with the appropriate hardware, to the hull of the sail craft. For those sail craft with a keel slot, the support assembly 82 is arranged to fit within the keel slot.

The support assembly 82 has an axis 82a which, in use, is substantially parallel to the longitudinal _axis of the hull of the sail craft.

The leading and trailing sections 74 and 76 are formed from relatively rigid materials. The pivot member 80 is located in the mid-region of the trailing section 76. The dimensions of the pivot member 80 and the keel body are selected so that essentially the entire bending load exerted on the keel body 72 is transferred by the pivot member to the support assembly 82 and thus to the sail craft. This of course means that the pivot member 80 must have a sufficient diameter to be capable of transferring this bending load.

The use of rigid materials to form the articulated keel body and a large diameter pivot member means that the external support for the keel body does not extend through the keel body and beyond to a lower frame member, which can in some cases constrain the design of the shape of the keel body. Rather, the primary load bearing structure of the support assembly 82 terminates at the upper edge of the keel body which enables the shape of the keel body to be tailored to complement the shape of the hull to optimize the performance of the craft.

A restraining assembly 84 restrains the displacement of the leading edge within a region 'R' centered on the axis 82a. The restraining assembly 84 includes a bracket 86 which is attached to the upper end of the leading section 74 adjacent the leading edge 72a. The bracket 86 extends outwardly from the leading edge 72a and has a free end 86a. A pin 88 extends upwardly from the bracket 86 and fits in a key slot 90 formed in the front end of the support assembly 82. The restraining mechanism 84 permits the leading section 74 to articulate relative to the trailing section 76 when the keel body rotates about the pivot axis. As with the support assembly, the restraining assembly is arranged so that it will not substantially interfere with the shape of the keel body.

It will be seen that the distance of the pin from the leading edge 72a may be adjusted to increase the lateral dimension of the region R, that is, the degree of displacement of the leading edge away from a neutral position on the axis through the support assembly 82 when the leading and trailing sections articulate.

A particular feature of the keel assembly lies in the fact that the keel body 72 has a pair of articulated sections and is rotatable relative to the support assembly 82 in a manner which permits the camber of the keel body 72 to be varied automatically and continuously according to the pressures exerted on the keel body 72 by the flow of water past the keel body 72.

When the sail craft is oriented at an angle of attack, alpha, relative to the direction of flow 'C as shown in figure 15, the keel body 72 is subjected to a relatively high pressure at face 74a, 76a and a relatively low pressure at face 74b, 76b thereby forming a pressure differential. As occurs on an aerodynamic shaped body when presented to a flow of fluid past it, the relatively high and low pressures peak in a region adjacent the leading edge.

As shall be explained, the keel body 72 is arranged to take the pressure differential between the faces thereby causing a resultant force 'F¹ to be directed on the keel body 72 at location known by those skilled in the art as the 'centre of pressure' and this force causes the keel body 72 to rotate about the pivot axis 80a. Meanwhile, the bending load generated by the resultant force is transferred by the pivot member 80 through to the support assembly 82.

As the keel body 72 rotates about the pivot member 80, the leading section, due to the restraining assembly 84, rotates in the opposite direction relative to the trailing section 76. As a result, the leading section is oriented toward the direction of flow which causes the negative pressure peak appearing adjacent the leading edge on face 74b, 76b to be reduced. This of course offsets those conditions which bring on cavitation and stall as described previously. This means that the keel assembly operates in a more efficient manner and over a greater operating range thereby improving its performance.

In order for proper operation, the pivot member 80 must be located aft of the center of the pressure and the hinge axis 78a must be located between the pivot member 80 and the leading edge 72a.

The relative dimensions of the leading and trailing sections 74, 76 may be adjusted as desired, as can the position of the pivot member 80 relative to the support assembly 82 and the keel body 72, as long as the above relative positions of the leading edge 72a, the hinge axis 78a and the pivot member 80 are maintained.

The pivot member 80 must have sufficient dimensions to transfer substantially the entire bending load generated by the resultant force 'F' to the support assembly 82 and thus to the sail craft. This means that the mid-region of the trailing section 76 should have a width at least equal to the width of the pivot member 80. Alternatively, the pivot member 80 may have a width greater than the mid region of the trailing section 76, although this will cause the surfaces 76a, 76b of trailing section to bulge in the region of the pivot member 80 which will reduce the efficiency of the keel assembly.

Thus, the keel assembly permits automatic adjustment of keel body camber by permitting the keel body 72 to pivot and articulate relative to the support assembly 82, while doing so in a simple and economic manner. - 16 -

The keel assembly is equally simple to construct. The leading section 74 is mounted on the trailing section 76 by aligning the upper and lower mounting holes 78b, 78c in both and inserting the upper and lower mounting pins 78d, 78e respectively. Next, the bracket is fastened to the leading edge section and the pivot member 80 and pin 88 are inserted in the support assembly 82. The support assembly 82 may then be mounted on the sail craft as discussed above.

It is to be understood that, while the keel assembly makes use of one pivot member 80 which transfers essentially the entire bending load generated by the pressure on the keel body 72, the pin 88 may transfer a small part of the bending load depending on the play present between the pin 88 and its associated key slot. However, it will be recognized that, for all intents and purposes, the bending loads arising from forces exerted on the keel body will be transferred by the pivot member 80.

While discussion hereinbefore has been restricted to a keel assembly being fully automatic, it will be understood that a manual override system, such as that shown schematically at 90 in figure 14 may be provided so that manual adjustment of the keel body camber may be made in special circumstances. In this instance, the system 90 has a solenoid actuated piston 92 which is activated by a controller 94 along cable 96. It will be readily understood to those skilled in the art that a number of alternative configurations could be used to provide the manual override system. Thus, the present invention provides a skeg which is able to adjust automatically its 'camber', that is the curvature of the median line of the profile of the keel, to that best suited for the angle of attack of the keel relative to the flow direction. It is able to do this since it has a separate leading edge section hinged to the skeg body. By employing a trailing vane linked to the leading edge which causes the leading edge section to be turned toward the flow direction, the skeg takes on a different cambered configuration and this in turn counters the physical processes herein above described that cause stall and cavitation. This allows the skeg to achieve a greater angle of attack and thereby generate more 'lift' prior to stalling as well as to achieve a higher speed prior to cavitation. By virtue of this enhancement, the performance of the present skeg exceeds those of conventional skegs.

While the embodiments discussed hereinabove have been directed at sailboards, it will become readily seen that the present invention is equally applicable to other sail craft.

In addition, the leading edge section, the vanes and the associated coupling hardware to couple them to the body of the keel can conveniently form a kit to improve existing keels. In this case, the leading edge section would be designed to compliment the shape of the body of the keel. The coupling hardware should have a relatively low profile to minimize the interruption of the flow of water past the keel.

If desired, the function of the vane and the leading section may be performed by a single leading - 18 -

edge component. In this instance, the leading edge component would be attached to the leading edge of a keel assembly so that it can move to follow the direction of travel of the flow of water past the keel. This movement will in turn counter the peak negative pressure appearing at the leading edge of the keel.

It will also be understood that the vanes may not necessarily trail the leading edge as described hereinabove but may be disposed in front of the leading edge, provided a suitable mechanism joins the vane with the leading edge section so that the latter will be turned toward the flow direction. The ability of the vane means to follow the direction will depend to a great extent on the range of rotation given to the leading edge section by the connecting mechanism joining it to the body of the keel. If the range is relatively small, the ability of the vane means to follow the flow will be correspondingly reduced. In addition, the connecting mechanism should be selected so that the friction therein is relatively low to permit the force of the water against the vanes to dictate their orientation.

In addition, the leading edge need not extend the entire length of the body of the skeg. Instead, a leading edge section with a length which is merely a fraction of the length of the body may only be necessary to counter the onset of stall or cavitation in some instances.

Claims

We claim:

1. A keel assembly for a water craft comprising: a body having a pair of side faces and a leading edge extending the length of said body, said body having a portion adjacent said leading edge; first connecting means connecting said portion with said body for relative movement thereto; a vane means to be disposed within a flow of water relative to said body; said flow having a direction of travel; second connecting means joining said vane means with said portion, said second connecting means constituting means to transmit movements of said vane means to said portion to orient said portion toward said direction of travel to offset the effects of a negative pressure region occurring near said leading edge.

2. A keel assembly as defined in claim 1 wherein said vane means is disposed rearwardly of said leading edge.

3. A keel assembly as defined in claim 2 wherein said portion defines the entire length of said leading edge.

4. A keel assembly as defined in claim 3 wherein said portion is separate from said body, said second connecting means including a pair of pivot pins joining said portion with said body.

5. A keel assembly as defined in claim 2 wherein said vane means includes a pair of trailing vanes, one adjacent each side face of said body, said first - 20 -

connecting means includes a pair of arm portions each of which is fixed at one end to and extends laterally outwardly from said portion and is fixed at an opposite end ^*to one of said vanes.

6. * A keel assembly as defined in claim 3 wherein said vane means includes a pair of upper trailing vanes, one dispcteed adjacent on each side face of said body and attached to an upper end of said portion, each of said upper trailing vanes extending laterally outwardly relative to an associated side face and downwardly and rearwardly relative to said upper end.

7. A keel assembly as defined in claim 6 wherein said Vane means further comprises a pair of lower trailing vanes, one disposed adjacent each side face of said body and attached to a lower end of said portion, each of said lower trailing vanes extending laterally outwardly relative to an associated said side face and upwardly and rearwardly relative to said lower end.

8. A kit for improving the performance of a keel for a water craft, comprising: leading edge means arranged for attachment to a leading edge of a keel; connection means for attaching said leading edge means to said leading edge for movement relative the±"eto; vane means for attachment to said leading edge means and to be disposed within a flow of water past said keel; said flow having a direction of travel; coupling means to attach said vane means to said leading edge means so that said vane means follows said direction of flow; said coupling means constituting means to transmit movements of said vane means to said leading edge means to orient said leading edge means toward said direction of travel.

9. A method of improving the performance of a keel for a water craft, comprising the steps of: providing a leading edge means for attachment to a leading edge of a keel; connecting said leading edge means to said leading edge for movement relative thereto; providing a vane means which is attachable to said leading edge means so as to be disposed within a flow of water past said keel, said flow having a direction of travel; coupling said vane means to said leading edge means so that said vane means follows said direction of flow and so that movements of said vane means are transmitted to said leading edge means to orient said leading edge means toward said direction of travel.

10. A keel assembly for a sail craft comprising: a keel body having a leading edge and a trailing edge; said keel body having a leading section on which said leading edge is located and a trailing section on which said trailing edge is located; hinge means joining said leading and trailing sections; a support means attachable to said sail craft to support said keel body in position below said sail craft; said support means having an axis parallel to a central axis extending the length of said craft; a pivot member joining said keel body to said support means, said pivot member being of construction sufficient to transfer substantially the entire bending load to be generated by said leading section and said trailing section to said craft; restraining means coupled to said leading section, said restraining means constituting means to confine displacement of said leading edge to a region centered on said axis; said pivot member being positioned on said keel body so that a center of pressure to be exerted by a flow of water past said keel body is located between said pivot member and said leading edge, thereby to cause a rotation of said trailing section about said pivot member and an opposite rotation of said leading section relative to said hinge.

11. A keel assembly comprising a keel body with a leading section defining a leading edge and a trailing section defining a trailing edge; said leading and trailing sections being articulated for relative rotation therebetween; a support means; a pivot means disposed in said trailing section for pivotally mounting said keel body to said support means, restraining means for limiting displacement of said leading edge to within a range centered on a neutral position, said leading and trailing sections being formed from relatively rigid materials and having dimensions sufficient to transfer to said support means through said pivot means, substantially all bending moments being exerted thereon by a pressure differential across said keel body in a flow of water past said keel body, said pivot means constituting means to permit said trailing section to rotate relative to said support means under the action of said pressure differential, said restraining means constituting means to cause said leading section to articulate relative to said trailing section to orient said leading edge toward said flow.

12. A keel assembly as defined in claim 11 further comprising override means for adjusting the orientation of said keel body independent of said pressure differential.