GB2239842A - Device to reduce turbulence in marine craft - Google Patents

Abstract

An opening to a cavity (23) is formed in the bottom of a hull (24) of a marine craft. A hydrofoil (20) is pivotable between a first position in which the hydrofoil projects from the hull and a second position in which the hydrofoil is located within the cavity. The hydrofoil has near its pivoted end a fairing plate (25) which is adapted to substantially close the opening to the cavity when the hydrofoil is in the first position. <IMAGE>

Description

RETRACTABLE HYDROFOIL This invention relates to marine craft fitted with hydrofoils.

In the construction of marine craft, and particularly of sailing craft but also in certain other vessels such as air cushion vehicles, it is known to fit a hydrofoil to the hull such that it projects either vertically downwards or at some downward angle to the vertical into the water; the hydrofoil being intended to improve windward sailing ability or directional control of the craft. Such a hydrofoil is preferably moveable to be retracted into the hull of the craft when the hydrofoil is not needed, in order to reduce hydrodynamic drag and to reduce the depth of water needed for navigation.

The hydrofoil may be arranged to be operable to slide in a substantially vertical direction, into and out of a close-fitting cavity or trunk extending well above the waterline of the craft. In this arrangement, the hydrofoil is commonly called a daggerboard.

In another known arrangement, the hydrofoil may be pivotally mounted so as to be retractable sternwards into a cavity or trunk. The cavity may be substantially as narrow in width as the daggerboard trunk, but may be proportionally longer in the direction of the length of the craft so as to accommodate the full length or span of the hydrofoil when it has been pivoted through approximately 90 degrees from its projecting position. In this arrangement the hydrofoil is commonly called a centreboard.

A "high aspect ratio" design of centreboard comprises a centreboard which has a span dimension which is substantially greater than its chord dimension, and which when pivotally lowered fully emerges from its trunk or cavity.

A problem that arises with such a centreboard is that when it has been rotated through approximately 90'degrees to move from its fully retracted position to its fully lowered position, there is then, astern of the lowered centreboard, a substantial length of unoccupied cavity which leaves an open slot. This slot, if left unmodified in any way, will receive high energy, turbulent water flow from the centreboard, which will pass into the interior of the cavity and may result in unacceptable noise, increased hydrodynamic drag and even structural damage to the vessel.

Some preferred designs of centreboard have a rather thick hydrodynamic section, such designs both enhance efficiency and make it easier to incorporate adequate structural strength into the centreboard, and are therefore desirable.

However, in designs with thick hydrodynamic section the problems of noise and drag are exacerbated because the slot must be wider in order to accommodate the thicker section of the centreboard.

An attempt to alleviate the problem of the open slot has been made by providing flexible rubber strips on each side of the slot that touch or overlap at the centreline of the slot. These flexible strips are pushed out of the way by the centreboard as it is pivoted up or down, but return to their original position over the open slot. Although reducing the ingress of high velocity water flows and vortices, such slot closing devices are commonly rather prone to damage, and rarely provide really good hydrodynamic efficiency.

The present invention is intended to reduce or eliminate noise and turbulence, and to provide the minimum possible hydrodynamic drag in vessels using such centreboards.

According to the present invention there is provided a marine craft comprising a hull and a high aspect ratio hydrofoil extendable from a cavity in the hull via a slot, the arrangement being such that the hydrofoil is pivotable from a retracted position in which the slot is substantially occupied by the hydrofoil to an extended position in which the hydrofoil projects from the hull characterised in that a member is provided that pivots with the hyrofoil to occupy the portion of the slot vacated by the hydrofoil when it is extended.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1A shows for clarification a longitudinal cross-section of a craft with a daggerboard; Figure lB shows an end view of the daggerboard according to Figure lA; Figure 2A shows for reference a low aspect ratio centreboard; Figure 2B shows an end view of the centreboard according to Figure 2A; Figure 3A shows a longitudinal cross-section of a craft with a rotatable high aspect ratio centreboard; Figure 3B shows an end view of the centreboard according to Figure 3A; Figures 4A and 4B show a section through a centreboard slot according to the prior art; Figure 4C shows an end view of the centreboard according to Figure 4B;; Figures 5A and 5B show an arrangement of a centreboard according to an embodiment of the present invention; and Figure 6 shows detail of a section through the centreboard slot of the embodiment shown in Figures 5A and 5B.

In Figure 1A a craft having a hull 1 is fitted with a daggerboard 2 which slides substantially vertically in a cavity 3 between a retracted position indicated by the dashed outline and a projecting position indicated by the solid outline. An end view of the cavity and the daggerboard such as that shown in Figure 1B indicates that the cavity in this arrangement need only be slightly larger than the hydrofoil, thus excessive turbulence in the cavity does not occur.

For reference, Figure 2A illustrates a low aspect ratio centreboard 7 which is rotatably retractable into a cavity 8 with a low aspect ratio centreboard, the cavity also remains substantially occupied by a portion of the centreboard along its open edge. The end view of the low aspect ratio centreboard illustrated in Figure 2B shows that the cavity in this arrangement is only slightly larger than the hydrofoil and excessive turbulence in the cavity does not occur.

Figure 3A shows a hull 1 fitted with a hinged or rotatable high aspect ratio centreboard 4 which rotates between a retracted position indicated by the dashed outline and a projecting position indicated by the solid outline wherein the centreboard is stored in a cavity 5 in the hull. When the centreboard is in the projecting position, a large gap results at the opening 6 of the cavity as can be clearly seen in the end view shown in Figure 3B. It is towards reducing the adverse effects of such a cavity that the present invention is directed.

Referring now to Figures 4A to 4C, which illustrate a high aspect ratio centreboard 4 according to the prior art, the opening 6 of cavity 5 is fitted with flexible strips 9.

When the centreboard is retracted as shown in Figure 4A, the flexible strips cover the opening of the cavity. Figure 4B shows the situation where the centreboard is rotated into the projecting position and the flexible strips are deflected to accommodate the centreboard. The end view illustrated in Figure 4C shows how the flexible strips deform around the centreboard but substantially cover the opening to reduce the ingress of turbulent water.

Unfortunately such strips are prone to damage, to some extent by virtue of their own flexibility.

The present invention provides an alternative method of closing the slot that is left unoccupied as a high aspect centreboard is pivoted into the extended position. This is achieved by provision of an additional member or attachment that pivots with the centreboard to occupy the portion of the slot that is vacated. As the member is only positioned in the slot when the centreboard is extended it is not subject to wear and tear, in the same way as permanently fixed strips, and neither does it have to be pushed out of the way. In a preferred embodiment the member is attached to or formed with the centreboard and provides structural advantages to the centreboard in addition to closing the slot.

A preferred embodiment of the invention is shown in Figures 5A and 5B, in which a centreboard 20 has a leading edge 18 and a trailing edge 19, and is hinged at a pivot 21 so as to be raised and lowered by a means 22, which may be hydraulic or any other suitable means. It is installed in a trunk or cavity 23, which extends into the hull 24 above the waterline of the craft.

The member for closing the slot is constituted by a permanent fairing plate 25 is rigidly fixed to the centreboard 20 and supported by a gusset 26, which extends rearwardly from the top of the trailing edge of the centreboard. As a result of the integrated structure, a fillet 27 can now be incorporated for the structural support of plate 25 and in order to reduce hydrodynamic drag and improve hydrodynamic efficiency. The height of the cavity, if it is closed at the top, must be adequate to accommodate the plate 25 when the centreboard is fully raised, as shown by the dashed lines in Figure 5A.

When the centreboard is fully lowered into the projecting position shown by the solid outline in Figure 5A, the fairing plate 25 fits across the opening of the cavity 23 in order to substantially cover the opening and prevent the ingress of turbulent water into the cavity. This is clearly shown in Figure 5B which illustrates an end view of the centreboard and fairing plate within the cavity when the centreboard is in the projecting position.

In Figures 5A and 5B, the opening in the cavity 23 is shown to be larger than the plate 25, for clarity of explanation.

However, the gap will, for optimum efficiency in such an arrangement, be kept as small as is practicable.

However the plate 25 may also be made wider than the opening in cavity 23, as shown in Figure 6 which illustrates detail of a section through a centreboard cavity according this alternative embodiment. Edges 28 and 29 of the opening of the cavity 23 are preferably only a little wider apart than the maximum thickness of the centreboard 20 to permit it to be pivotably raised and lowered. The fairing plate 25 is, however, wider than this opening, so that it overlaps edges 28 and 29 on each side of the opening.

In most situations it would not be necessary to attempt to make an actual seal between plate 25 and edges 28 and 29, a small gap being adequate to damp down any undesired water flow into the cavity and to reduce hydrodynamic drag.

However, if the best possible performance is desired, then foam rubber or other sealing means may be fitted in gaps 30 and 31 between the fairing plate and the edges of the opening; the sealing means may be fixed either to edges 28 and 29 or to the plate 25.

As shown in Figure 6, the lower surface of plate 25 may be profiled so that the exterior 33 of the plate aligns with the exterior of the hull in order to bring the hull shape abaft the centreboard to as efficient a hydrodynamic profile as possible. The simpler profile indicated by the dashed line 32 may instead be used where cost is a greater consideration than hydrodynamic efficiency.

This invention has been described in the foregoing embodiments with reference to a single centreline hydrofoil.

However, the invention is equally applicable to hinged and angled side hydrofoils such as are commonly fitted in the side hulls of multi-hull craft. Whereas centreline centreboards acting in broadly the central plane of symmetry of the craft are usually of symmetrical section, such "side boards" are commonly installed in pairs, one on each side of the craft, and may have asymmetrical "lifting" hydrofoil sections. The principle however remains the same, and a fairing plate may be provided on the hydrofoil in accordance with this invention.

Claims (7)

1. A marine craft comprising a hull and a high aspect ratio hydrofoil extendable from a cavity in the hull via a slot, the arrangement being such that the hydrofoil is pivotable from a retracted position in which the slot is substantially occupied by the hydrofoil to an extended position in which the hydrofoil projects from the hull characterised in that a member is provided that pivots with the hydrofoil to occupy the portion of the slot vacated by the hydrofoil when it is extended.

2. A marine craft according to claim 1 further characterised in that the member is connected to the hydrofoil.

3. A marine craft according to claim 1 in which the member is formed integrally with the hydrofoil.

4. A marine craft according to any preceding claim wherein the hydrofoil has a leading and a trailing edge and the member is supported by extensions of the hydrofoil from the trailing edge.

5. A marine craft according to any preceding claim wherein the member is adapted to overlap interior edges of the cavity opening.

6. A marine craft according to any preceding claim in which the member comprises a fairing plate hydrodynamically profiled to the hull.

7. A marine craft according to any preceding claim wherein said hydrofoil is pivotable between its first and second positions by hydraulic means.