Patent Description:
Hydrofoil crafts have foils which move through the water during flight, that is, during foil-borne operation of the craft, and develop lift in much the same manner as an airplane wing. The foils are carried on struts which are attached to the hull of the craft and hold the hull clear of the water during flight. The struts are usually mounted on the hull in a manner which permits the struts to be retracted so that the hull can float on the water and the craft can be operated in a hull-borne mode as a normal ship.

Many vessels are known in the art that adopt some sort of foil system for improving stability and/or performance of the vessel. Generally, such hydrofoils are utilized in both multi-hull designs and in mono-hull designs.

A hydrofoil, or more simply, a foil is a streamline body designed to give lift and is similar to aircraft wings. The foil generally has a different curvature or camber at opposed surfaces thereof. The angle of attack (AoA) of a foil is the angle between the chord, defined as the straight line connecting the leading and trailing edge of the foil, and the direction of movement of the boat.

Hydrofoil craft generally include one, two or more struts that extend downwardly from the hull. At the lower end of the struts, hydrofoils are fixed to extend substantially orthogonal to the struts. When the craft reaches an appropriate speed, the hydrodynamic properties of the hydrofoil cause the hull to be lifted out of the water, leaving the craft "flying" on its hydrofoils.

Foils have typically been used on boats to reduce drag and to maintain trim in planing vessels. Foils are generally not used for steering nor for yaw.

In <CIT> a hydrofoil craft is disclosed having a retractable hydrofoil-strut, which strut is attached to the bottom of the craft and which strut can be rotated forward and rearward by a hydraulic cylinder. The hydrofoil is fixedly attached to the strut, a fixed foil, therefore the craft is trimmed by adjusting the angle of the propulsion and/or the flap at the stern of the craft. The fixed foil has the disadvantage, that during a sudden retraction movement of the strut, e.g. caused by hitting a floating object in the water while traveling at high speed, the foil remains in fixed position and orientation with respect to the strut, resulting in an extremely rapid downward deceleration, thereby abruptly dragging the craft downwards into the water, usually leading to damaging of the foil-support system and of the hull structure itself. This abrupt deceleration between the time of impact and the time of becoming hull-borne as the craft settles on the water can be quite dangerous to passengers, crew members and cargo on board the craft. The structural damage may be quite extensive and may involve sufficient damage to the hull to result in a hazardous situation as well as requiring extensive repairs to the craft.

A foil design has been shown for mono-hull keel boats as represented by United States patent <CIT>, which discloses a hydrofoil craft that uses fully submerged foils wherein the lift is varied by changing the AoA of the foil relative to the water flow.

When foil-borne operation is desired, the struts are moved to their extended position and locked in place. <CIT> is also directed towards finding a solution for the problem of rapid deceleration of the craft after a collision of a strut with a floating or submerged object in the water in unexpected or unpredictable places.

In an alternative design of the means to trim or control the fly-height of the craft as it moves through the water, the foil is attached to the strut in a pivotable manner. Usually, movement or rotation of the foil with respect to the strut is controlled by an actuator that is located within the hull. A control rod is connected between the actuator and the foil for communicating the forces generated by the actuator to the foil.

The bow strut in <CIT> is retracted forward and upward for operation in hullborn mode and the bow strut is allowed to yield and to move aftward and upward upon meeting an obstruction. After impact, the hydrofoil is released from the control rod passing down the strut, thereby allowing the foil to retract to a downward and aftward pivoted position to allow the obstruction to pass clear.

The aftward and downward rotation of the foil, however, alters the lifting pressure causing a downward and aftward pressure to be applied to the strut resulting in a rapid and forceful aftward retraction of the strut leading to damage to the foil-support system and to the hull structure, and also to a dangerous and unsafe situation for the passengers, crew members and cargo on board the craft. Furthermore, the control rod is disconnected from the foil after the impact, so that the strut must be repaired first before being able to resume foil-borne mode. Furthermore, the retraction mechanism is complicated and expensive, because the bow strut must be able to pivot forward and upward during normal operation and also be able to pivot aftward and upward when meeting an obstruction.

Another foil design and configuration are disclosed in United States patent application <CIT>, which is summarized as a load-bearing wing (foil) of the immersed type, of an upside-down "T" shape. This foil is connected to the strut through a pivot connection, the axis of which extends parallel to the keel of the craft, so that the foil can fold from a horizontal to a vertical orientation parallel to the strut. The foil is not connected to a control rod, so that the AoA is fixed. The axis of the pivot connection is not centred but offset in relation to the centre of hydrodynamic pressure of the foil. This asymmetry in construction enables the foil to fold back automatically when the strut is retracted aftward and upward caused by reversing of the lift of the fixed foil. The resulting reverse force acting on the outside surface of the foil causes the foil to fold into a position substantially parallel to the strut.

The aftward and upward rotation of the strut having a fixed foil, however, alters the lifting pressure causing a downward and aftward pressure to be applied to the strut resulting in a rapid and forceful aftward retraction of the strut leading to damage to the foil-support system and to the hull structure. Furthermore, a foil that can be pivoted around two orthogonal axes at the lower end of the struts is a complex and costly construction.

Documents <CIT> and <CIT> disclose mechanical fuse devices comprising means for restraining the bow strut in its foil-borne position by a mechanical support which is designed to rupture upon striking a large, sizable object to permit the strut and the foil to pivot aftward and upward around the retraction pivot axis to a position near the hull or within a recess in the hull. For hull-borne operation of the craft, the bow strut moves pivotally in the forward direction in a slot or recess in the bow of the craft to the retracted position. The retraction mechanism must be able to both pivot the bow strut aftward when meeting an obstruction and to pivot the bow strut forward and upward from the extended position to the retracted position during normal operation. The mechanical linkage of the retraction mechanism is complicated and therefore expensive and susceptible to malfunction.

Document <CIT> discloses a hydrofoil flap control rod system for transferring actuator induced forces to a hydrofoil flap. To control the attitude of the craft, a rectangular-shaped rod with wear pads within a rectangular conduit is connected between the actuator and the flap for communicating the push and pull forces generated by the actuator to the flap. Continuously exerting two-directional push and pull forces to the flap or the foil by the control rod requires a fixed connection between the actuator and the flap, so that the flap will decelerate the craft when hitting an floating object, thereby abruptly dragging the craft downward into the water. Furthermore, the bow strut is fixedly mounted to the hull and cannot be aftward retracted to the hull for hull-borne operation of the craft.

Document <CIT>discloses a strut with a hydrofoil, wherein the orientation of the hydrofoil is controlled by a mechanical mechanism comprising a surface sensor and a control rod extending through the strut. The mechanism is arranged such that an upward motion of the surface sensor causes a downwards trust in the control rod such that the angle of attack of the hydrofoil is increased, and that a downwards motion of the surface sensor causes an upward trust in the control rod such that the angle of attack of the hydrofoil is decreased. The control rod must be able to exert pulling and pushing forces on the hydrofoil, requiring a complicated design, guidance and control, and a thick, heavy weight control rod. Retraction of the strut and the surface sensor mechanism is done by slackening and tensioning cables, no measures are taken for preventing unsafe situations when hitting a floating object or when electrical systems fail.

The object of the invention is therefore to wholly or partly remedy the disadvantages of the prior art. It is an object of the present invention to provide a safety strut assembly for a hydrofoil craft, more particular for an electrically powered hydrofoil speed boat, which safety strut assembly is intrinsically safe, e.g. in case of fault or breakdown of the electric systems. It is further an object of the invention to provide a strut assembly, which safely retracts when hitting an object floating in the water, without abrupt deceleration of the craft leading to dangerous and unsafe situations for the passengers, crew members and cargo on board the craft. It is an object of the invention to provide a safety strut assembly, which has a relatively simple construction incorporating a mechanical fuse device, retraction means and control means for the hydrofoil. In particular, it is an object of the invention to provide a safety strut assembly, which can be retracted and can be repositioned in the extended position easily. It is a further object of the invention to provide a control assembly in the safety strut assembly for controlling the AoA of the foil at the bottom of the safety strut, the control assembly allowing retraction and repositioning of the safety strut. It is a further object of the invention to provide a control rod and actuator assembly which enables a maximum power efficiency. It is a further object of the invention to provide a craft and hydrofoil construction which is cost efficient and relatively cheap and easy to manufacture and which has a lowest possible environmental impact. It is yet a further object to provide a hydrofoil craft which is safe and reliable.

These and further objects are realized by the present invention, wherein he first linkage means comprises a first drive ring mounted around the foil axle, wherein the first drive ring on the radial inner surface is provided with at least one first ring cam element, and wherein the foil axle on its radial outer surface is provided with at least one foil axle cam element,
wherein the at least one foil axle cam element and the at least one first ring cam element are arranged to mutually engage and disengage.

In order to prevent deceleration of the craft by a negative AoA during retraction of the strut in de aftward direction, the foil is able to remain parallelly oriented in the direction of the movement of the craft and of the flow of the water by means of the first drive ring. When the strut pivots aftward with respect to the strut axle, the first drive ring also pivots with respect to the craft. Because the at least one first ring cam element disengages from the foil axle cam element of the foil axle, the foil axle and the attached foil are not forced to follow the pivoting movement of the strut and of the first drive ring, thereby allowing the foil axle and the foil to maintain their parallel position with respect to the flow of the water and to the hull of the craft.

In a further aspect of the present invention, the at least one first ring cam element is arranged to engage the at least one foil axle cam element and pivot the foil axle when a pulling force is exerted on the first linkage means by the control rod.

In another aspect of the present invention, the foil axle is able to freely pivot away from the engaging position of the at least one foil axle cam element with the at least one first ring cam element.

When the at least one first ring cam element engages the at least one foil axle cam element, the foil axle is pivoted and the AoA of the foil is adjusted. By continuously adjusting the AoA of the foil, the position and the orientation of foil-born craft above the waterline is maintained. By disengaging the at least one first ring cam element and at least one foil axle cam element, the foil axle and the attached foil may freely pivot within the first drive ring thereby preventing deceleration of the craft when retracting the strut aftward.

In another embodiment of the invention, the foil axle and the first drive ring each are provided with three cam elements, wherein these first cam elements are equally spaced apart, so that the foil axle and the foil are able to pivot freely over at least <NUM>° degrees. When the foil is able to pivot over at least <NUM>° degrees, the strut may be easily retracted for hull-borne operation, and may also be easily retracted when the strut hits an object, without the foil decelerating the craft. Furthermore, the force distribution on the foil axle is optimized over its circumference, allowing a smaller and more robust construction of the first linkage means, thereby decreasing drag and energy dissipation of this submerged portion of the strut.

A further embodiment of the invention provides for the second linkage means comprising a transverse oriented actuator axle, the actuator axle being coaxial with the strut axle, the second linkage means further comprising: a second drive ring mounted around the actuator axle, wherein the second drive ring on the radial inner surface is provided with at least one second ring cam element, and.

With these measures, the linear actuator assembly is not required to follow the pivoting movement of the strut. The at least one second ring cam element disengages from the actuator axle cam element of the actuator axle, thereby allowing the linear actuator assembly to maintain their position within the hull of the craft during aftward pivoting of the strut.

In another aspect of the present invention, the at least one actuator axle cam element is arranged to engage the at least one second ring cam element and pivot the second drive ring when a pushing force is exerted on the second linkage means by the linear actuator assembly.

In another aspect of the present invention, the second drive ring is able to freely pivot away from the engaging position of the at least one second ring cam element with the at least one actuator axle cam element.

When the at least one actuator axle cam element engages the at least one second ring cam element, the second drive ring is pivoted by a pushing action of the linear actuator assembly and the control rod is pulled upward, and the AoA of the foil is adjusted by means of the first linkage means.

In another embodiment of the invention, the actuator axle and the second drive ring each are provided with three cam elements, wherein these second cam elements are equally spaced apart, so that the second drive ring of the second linkage means are able to pivot freely over at least <NUM>° degrees. When the actuator axle with the connected linear actuator is able to pivot over at least <NUM>° degrees, the strut may be easily retracted for hull-borne operation, and may also be easily retracted when the strut hits an object, without the need for displacing the linear actuating assembly together with the strut. Furthermore, the force distribution on the actuator axle is optimized over its circumference, allowing a smaller and more robust construction of the first linkage means, thereby decreasing drag and energy dissipation of this submerged portion of the strut.

In a special embodiment of the invention, a foil spring is provided between the strut and the foil axle, which foil spring is tensioned by retracting the strut into the horizontal hull-borne position within the recess of the hull, so that the tensioned foil spring will rotate the foil axle and the attached foil into a safe vertical transport position.

In yet another aspect of the invention, the linear actuator assembly is arranged for exerting pushing forces to the second linkage means and the control rod. When the linear actuator only has to exert a pushing force without requiring the option of a pulling force, the linear actuator can have a less complicated design and control, saving weight. Furthermore, this feature enables disengaging of the control rod when retracting the strut upward and aftward to the hull, preventing the need to displace the actuator assembly and preventing deceleration of the craft by the hydrofoil.

In yet another aspect, the second linkage means comprises a spring element, biasing/tensioning the second linkage means in the pushing direction of the linear actuator assembly.

Preferably, the spring element is a compression spring.

Due to the position of the centre of rotation before the centre of pressure, the hydrofoil will be forced to a negative AoA by the moving water during foil-borne operation. As a result, the control rod will be pushed downward, thereby pushing the second linkage means in the direction of the linear actuator and compressing/tensioning the spring element. This spring element substantially decreases the force the AoA actuator needs to generate, thus reducing the energy consumption of the control assembly of the foil.

In a special embodiment, the assembly further comprises a retraction assembly comprising a retraction actuator and retraction linkage means connected to the strut, the retraction assembly being adapted to pivot the strut aftward and forward in the keel direction about the strut axle.

In a further aspect of the present invention, the foil axle of the safety strut assembly, being the centre of rotation of the hydrofoil, is not coinciding with the centre of pressure of the hydrofoil in the keel direction of the craft, thereby enabling varying the angular orientation of the hydrofoil by a single direction displacement of the control rod and the first linkage means in the height direction of the craft.

In order to ensure that hydrofoil craft during high speed traveling in de foil-born mode will safely change/switch from foil-borne mode to hull-borne mode, e.g. in case of fault or breakdown of the electric systems, the centre of pressure of the force exerted by the water flow on the hydrofoil will orient the hydrofoil to an intrinsically safe position without requiring actuation and a force from the control rod.

In another aspect of the invention, the present subject matter is directed to the centre of rotation of the hydrofoil being located before the centre of pressure of the hydrofoil in the keel direction of the craft. The positioning of the centre of rotation of the hydrofoil before the centre of pressure creates a negative AoA of the hydrofoil and a force on the control rod during movement of the foil-borne craft. As a result, the negative AoA will force the craft to intrinsically safely switch to the hull-borne mode, e.g. in case of fault or breakdown of the electric systems. Furthermore, the intrinsically safe negative AoA without actuation ensures that switching from hull-borne mode to foil-borne mode is only possible with electrical control systems working properly.

In particular, the first linkage means is located before the centre of rotation of the hydrofoil in the keel direction of the craft, thereby enabling varying the angular orientation of the hydrofoil by a pulling force and displacement of the control rod in the height direction of the craft. By exerting a single direction pulling force on the first linkage means by the control rod, the hydrofoil will leave the intrinsically safe negative AoA orientation and hull-borne mode, enabling switching to foil-borne operation.

As a result, the AoA can only be adjusted from negative to positive by exerting a pulling force by the control rod originating from (a properly working) electrical control system such as the actuator assembly.

The invention and the following detailed description of certain embodiment thereof may be understood by reference to the following figures.

The invention is now described by the following aspects and embodiments, with reference to the figures.

For convenience of interpretation of the figures, the following terms are used. The terms vertical, horizontal and straight are to be understood as substantially vertical, horizontal respectively straight, whereby horizontal meaning: in the transverse direction of the width of the craft parallel to the waterline, whereby vertical meaning: in de height direction, perpendicular to the water surface, whereby the keel direction meaning: perpendicular to the transverse direction parallel to the water surface, from the stern to the bow.

<FIG> show an overview of a hydrofoil craft for the safety strut assembly according to the invention.

Arrow V indicates the vertical direction, directing upwards from the water; arrow H indicates the transverse direction, directing from starboard to port side of the craft; arrow K indicates the keel direction, directing from the stern to the bow.

<FIG> shows a side view of a typical hydrofoil craft <NUM> which is here illustrated in foil-borne operation, traveling at high speed, indicated by arrow <NUM>, above the water surface <NUM>. Safety strut assembly <NUM> at the bow of the craft comprises a strut <NUM>, which is supported on the hull <NUM> of the craft in a manner to permit pivotal movement in the aft direction of arrow <NUM> but is normally held against such movement during foil-borne operation by substantially rigid restraining means. The strut is retracted in the aft direction indicated by arrow <NUM> in a recess in the hull when hull-borne operation of the craft is desired. For foil-borne operation of the flight, the strut is repositioned in the extended position. If an impact force, or a force in excess of the normal load, is applied to the strut, it is adapted to yield and permit pivotal movement in the aft direction indicated by arrow <NUM>.

Furthermore, a floating (semi)submerged object <NUM> is shown that could cause a collision, indicated by arrow <NUM>.

<FIG> shows a perspective view of the typical hydrofoil craft <NUM> of <FIG>, which craft is illustrated here as including a pair of hydrofoils, there being a bow foil <NUM> mounted on the bow strut <NUM> beneath the bow of the craft and a second hydrofoil <NUM> mounted on two vertical struts <NUM> beneath the stern of the hull <NUM> of the craft. The stern struts <NUM> are provided with propellers <NUM> for propulsion of the craft. The type of propulsion for the craft is not essential for the invention. A mechanical propulsion may be provided, but also alternative propulsions may be provided, like a hybrid diesel electrical propulsion, a waterjet or an electrical propulsion including batteries for storage of electrical energy. Nevertheless, it has to be understood that other means of propulsion may be employed without departing of the gist of the invention.

<FIG> shows a side view of the bow strut retraction mechanism <NUM>, here illustrating the movement indicated by arrow <NUM> of the bow strut <NUM> with the bow foil <NUM> when being retracted into the boat's hull.

The bow strut retraction mechanism <NUM> comprises a retraction actuator <NUM> and retraction linkage means <NUM>. When hull-borne mode operation is required, the bow strut <NUM> can be retracted aftward and upward about a bow strut axle <NUM> into a recess <NUM> provided in the hull <NUM>, by means of the retraction linkage means <NUM>. When foil-born mode operation is desired, the bow strut <NUM> is moved into the upright, extended position by the bow strut retraction mechanism <NUM>.

A safety release system (also indicated by mechanical fuse device) is provided to the bow strut retraction mechanism, which release system ensures that the bow strut retracts and moves aftward and upward upon meeting an obstruction such as submerged object <NUM> (see <FIG>). The safety release system may be of any suitable type, it may be designed simply to rupture and release the strut, permitting it to swing in the aft direction in response to the impact force. The safety release system thus functions as a mechanical fuse device by rupturing or failing in a predetermined manner. The mechanical failure is thus confined to an easily replaceable element or device and any other structural damage is prevented or minimized. In the embodiment shown, a breaking pin <NUM> is mounted between the retraction actuator <NUM> and the retraction linkage means <NUM>. The breaking pin <NUM> ruptures upon striking a sizable submerged object <NUM> by the bow strut <NUM> permitting the bow strut and the bow foil <NUM> to yield and to move aftward and upward into recess <NUM> according to arrow <NUM>.

This safety release system limits or reduces the possible structural damage by providing a predetermined failure path. However, when during the retraction movement in the direction of arrow <NUM> the bow foil <NUM> remains in fixed position and orientation with respect to the bow strut, a large negative AoA will occur which will decelerate the craft and will cause a downward deceleration dragging the craft downwards into the water. The linkage system according to the invention permits the bow strut <NUM>, in response to the impact force of arrow <NUM> of a floating or submerged object <NUM>, to make a pivotal movement in the direction of arrow <NUM> in the aft direction, while keeping the foil in the horizontal position <NUM> in the water flow <NUM>. As a result, abrupt deceleration is prevented, allowing the craft to slow down and to settle onto the water <NUM> at a safe rate of deceleration.

<FIG>, <FIG> show a first embodiment of the safety strut assembly for the hydrofoil craft <NUM> according to the invention.

<FIG> shows a side view of the foil Angle of Attack (AOA) control mechanism of the bow strut, here illustrating the position of the main components, including the foil <NUM>, second linkage means <NUM>, actuator axle <NUM>, spring element <NUM> and linear actuator assembly <NUM>.

<FIG> shows a perspective view of the foil <NUM> and the strut <NUM>, with a fragmentary sectional view illustrating: the mechanism to control the AOA of the foil <NUM>, control rod <NUM>, first linkage means <NUM>, foil axle <NUM>, the Center of Pressure (COP) line <NUM>, the Center of Rotation (COR) line <NUM>, and foil spring <NUM>.

<FIG> shows a bottom view of the foil <NUM>, illustrating that the position of the Center of Rotation (COR) line <NUM> is placed in front of the Center of Pressure (COP) line <NUM>.

<FIG> shows the strut <NUM> of the retractable safety strut assembly according to the invention, with second linkage means <NUM> and first linkage means <NUM> connected by control rod <NUM>.

<FIG> and <FIG> show in more detail the hydrofoil control mechanism in the strut <NUM> for maintaining the optimal orientation of the craft during foil-borne mode traveling of the craft. Linear actuator assembly <NUM> is connected with second linkage means <NUM> to control rod <NUM>. Control rod <NUM> extends downward through the bow strut <NUM> to first linkage means <NUM>. First linkage means <NUM> connects the control rod <NUM> to the hydrofoil <NUM>. The linear actuator assembly <NUM> is now able to adjust the AoA of the hydrofoil <NUM> and maintain the optimal position of the craft during foil-born travel.

In <FIG> is shown, that according to the invention the Center of Rotation (COR) line <NUM> of the hydrofoil <NUM> is not coinciding with line of the Center of Pressure (COP) <NUM> of the hydrofoil <NUM>. Thus, during travel/flight or movement through the water of the hydrofoil, the pressure of the water, exerted on the hydrofoil, will force the hydrofoil into an (safe) orientation determined essentially by the mutual distance and position of the COR and the COP.

The non-coinciding placement of the COR <NUM> and the COP <NUM>, compared to a coinciding COR <NUM> and COP <NUM>, has the advantage that a single direction displacement of the control rod <NUM> is sufficient in controlling the orientation of the foil <NUM>. If the COR <NUM> and COP <NUM> were coinciding, e.g. were located in the same position, this would result in requiring both push and pull forces for controlling the orientation and the AoA of the foil <NUM>, so that the control rod <NUM> must be a fixed connection between the linear actuator assembly <NUM> and the foil <NUM>. Having non-coinciding COR and COP, and a single direction displacement of the control rod <NUM>, allows for an intrinsically safe foil <NUM>, which has the freedom to return to its safe orientation by the pressure exerted by the flowing water.

In a more advanced embodiment as shown in <FIG>, the Center of Rotation (COR) line <NUM> is positioned in front of the line of the Center of Pressure (COP) <NUM> of the bow foil <NUM>. Thus, during travel/flight, the trailing portion <NUM> of the bow foil is moved upward and the leading portion <NUM> is moved downward into a negative AoA creating a downward acceleration of the craft.

Positioning the COR <NUM> of the hydrofoil before the COP <NUM> creates an intrinsically safe negative AoA of the hydrofoil during travel/flight of the craft and a force on the control rod during movement of the foil-borne craft through the water. As a result, the negative AoA will force the craft to switch to the intrinsically safe hull-borne mode, e.g. in case of fault or breakdown of the electric systems. Furthermore, the intrinsically safe orientation of the foil <NUM> is advantageous during the start of the travel of the craft, when the craft is hull-borne. The intrinsically safe negative AoA of the foil <NUM>, when not actuated, ensures that switching from hull-borne mode to foil-borne mode is only possible with electrical control systems working properly.

<FIG> and <FIG> show in more detail the first linkage means <NUM> according the invention for maintaining the foil <NUM> in a safe horizontal position, parallel to the water surface <NUM> and in the direction of movement <NUM> of the craft, during retraction of the bow strut <NUM>.

<FIG> & <FIG> show the bottom part of the strut <NUM> and the foil <NUM> at the bow of the craft. The control rod <NUM> passes down through the strut <NUM> (see <FIG>) from the linear actuator assembly <NUM> (see <FIG>). The foil <NUM> is pivotally mounted on the strut <NUM> in a manner that it is controlled by the linear actuator assembly <NUM> by means of control rod <NUM> and generates lift when the strut is in the extended position and locked in place. In case the strut <NUM> makes a pivotal movement <NUM> in the aft direction, the foil <NUM> can move freely with the flow of the water <NUM> (see <FIG>) thus not generating lift or decelerating the craft. Control rod <NUM> is connected through first linkage means <NUM> with foil axle <NUM>. The first linkage means <NUM> comprise a first drive ring <NUM> mounted around the foil axle <NUM>. The first drive ring <NUM> on the radial inner surface is provided with at least one first ring cam element <NUM> and the foil axle <NUM> on the radial outer surface is provided with at least one foil axle cam element <NUM>. The first ring cam element <NUM> and the foil axle cam element <NUM> are arranged to mutually engage and pivot the foil axle <NUM>, when the control rod <NUM> exerts a pulling force on the first linkage means <NUM> in the direction of arrow <NUM>. A pulling force of the control rod <NUM> pivots the foil axle clockwise in <FIG> and <FIG>, thereby moving the leading portion <NUM> of the foil <NUM> upward and the trailing portion <NUM> downward creating an upward acceleration of the craft. The foil axle <NUM> is permitted to pivot freely counter-clockwise when the pulling force of the control rod <NUM> is removed or the strut <NUM> is retracted and moves aftward and upward into recess <NUM> of the hull <NUM> according to arrow <NUM> (see <FIG>). As a result, the horizontal, parallel orientation of the foil <NUM> with respect to the water surface and the direction of the water flow <NUM> is maintained, so that abrupt deceleration of the craft is prevented and the craft can slow down and settle onto the water <NUM> at a safe rate of deceleration. In the embodiment of the invention shown in <FIG>, <FIG> and <FIG> three sets of engaging cam elements are provided, equally spaced apart, thereby permitting the foil axle <NUM> and the connected foil <NUM> to pivot freely over at least <NUM> degrees.

In <FIG> (circle X of <FIG>) a foil spring <NUM> is mounted between the strut <NUM> and the foil axle <NUM>. The foil spring <NUM> is tensioned when the strut is retracted and rotating aftward and upward in the direction of the recess <NUM> in the hull (see <FIG>). When the strut <NUM> has reached position <NUM> after intermediate position <NUM>, the foil spring <NUM> will rotate the foil axle <NUM> and the attached foil <NUM> counter clockwise into a safe vertical transport position.

<FIG>, <FIG> and <FIG> show a third embodiment of the hydrofoil craft <NUM> according to the invention.

<FIG> shows in more detail (circle VII of <FIG>) the second linkage means <NUM>, which connects the linear actuator assembly <NUM> (see <FIG>) to the control rod <NUM>. The second linkage means <NUM> comprises a second drive ring <NUM> mounted around the actuator axle <NUM>. The radial inner surface of the second drive ring is provided with at least one second ring cam element <NUM>. The actuator axle <NUM> on its radial outer surface is provided with at least one actuator axle cam element <NUM>. The actuator axle cam element <NUM> and the one second ring cam element <NUM> are arranged to mutually engage and disengage. When a pushing force is exerted by the linear actuator on the second linkage means <NUM>, the actuator axle <NUM> will pivot the second drive ring <NUM> counter clockwise, and will pull the control rod <NUM> upward in the direction of arrow <NUM>. The second drive ring <NUM> is permitted to pivot freely further counter-clockwise when the strut <NUM> is retracted and moves aftward and upward into recess <NUM> of the hull <NUM> according to arrow <NUM> (see <FIG>). Accordingly, by allowing free pivoting of the second drive ring <NUM> of the strut <NUM>, the actuator axle <NUM> and the linear actuator assembly <NUM> are permitted to maintain their position and are not forced to follow the pivoting movement of the strut <NUM>. Because the second ring cam element <NUM> disengages from the actuator axle cam element <NUM> of the actuator axle <NUM>, the linear actuator assembly is allowed to maintain its position within the hull of the craft during the aftward pivoting of the strut <NUM>.

In the embodiment of the invention shown in <FIG> and <FIG> three sets of engaging cam elements are provided, equally spaced apart, thereby permitting the second drive ring <NUM> of the second linkage means <NUM> to pivot freely over at least <NUM> degrees.

The actuator axle <NUM> is coaxial with the strut axle <NUM>, e.g. an axle in axle construction extending on two sides of the strut in the transverse direction of the craft. The actuator axle <NUM> is coaxial with the strut axle <NUM> to be able to retract and rotate the strut with the control rod.

<FIG> shows the strut <NUM> of the bow of the craft, as seen from port side, provided with a control rod <NUM> connected to first linkage means <NUM> comprising a first drive ring <NUM>, and connected to second linkage means <NUM> provided with a second drive ring <NUM>. Advantageously, this construction for the safety strut assembly allows for a safe and swift retraction of the strut, while minimizing the chance on damage to the control mechanism for the bow hydrofoil.

<FIG> is a cross-sectional view over line VIII - VIII in <FIG>, which shows a rear view of the strut <NUM> with retraction assembly <NUM> and actuator assembly <NUM>. Seen from starboard, a cross-section of retraction assembly <NUM> over line III- III is shown in <FIG>. Seen from port side, a cross-section of actuator assembly <NUM> over line IV- IV is shown in <FIG>. <FIG> is a cross-sectional detailed view over line IX- IX in <FIG>, as viewed from port side of the craft.

Claim 1:
Safety strut assembly (<NUM>) for a hydrofoil craft (<NUM>) comprising a strut (<NUM>), which is attached to the hull (<NUM>) of the craft by means of a transverse oriented strut axle (<NUM>) for pivotal movement with respect to the hull, the assembly further comprising:
a control rod (<NUM>) passing down through the strut (<NUM>);
a linear actuator assembly (<NUM>);
a hydrofoil (<NUM>) pivotally mounted to the bottom portion of the strut (<NUM>) about a transverse oriented foil axle (<NUM>);
first linkage means (<NUM>) connecting the hydrofoil (<NUM>) to the control rod (<NUM>) to vary the angular orientation thereof;
second linkage means (<NUM>) connecting the linear actuator assembly (<NUM>) to the control rod (<NUM>);
characterized in that
the first linkage means (<NUM>) comprises a first drive ring (<NUM>) mounted around the foil axle (<NUM>), wherein the first drive ring on the radial inner surface is provided with at least one first ring cam element (<NUM>), and
wherein the foil axle (<NUM>) on its radial outer surface is provided with at least one foil axle cam element (<NUM>),
wherein the at least one foil axle cam element (<NUM>) and the at least one first ring cam element (<NUM>) are arranged to mutually engage and disengage.