WO1996035607A1

WO1996035607A1 - A flap for an air duct outlet for a watercraft

Info

Publication number: WO1996035607A1
Application number: PCT/AU1996/000276
Authority: WO
Inventors: Kenneth Ronald Gale; Leanne Gaye Gale
Original assignee: Kenneth Ronald Gale; Leanne Gaye Gale
Priority date: 1995-05-09
Filing date: 1996-05-09
Publication date: 1996-11-14
Also published as: AUPN289195A0

Abstract

A flap (1) in combination with a watercraft having at least one duct (10) for the passage of a gas to a surface of the watercraft, located below the waterline, wherein the flap (1) is pivotally mounted at the duct outlet (2) and is movable between an open position and a closed position. In use, the flap (1) is in the closed position when the watercraft is travelling forward at low speed so as to substantially prevent water impinging on a rear peripheral region (9) of the duct outlet (2), and moves to an open position when the watercraft's speed increases.

Description

A FLAP FOR AN AIR DUCT OUTLET FOR A WATERCRAFT

FIELD OF THE INVENTION.

The present invention relates to a flap for minimising drag acting on a craft caused by turbulence resulting from the flow of water over a duct outlet located below the waterline of the craft, as well as a method for realising the minimisation of the drag. More specifically, the invention provides a flap for a duct outlet wherein the flap is in a closed position when the craft is travelling at low speed but which moves to an open position when the speed of the craft increases. The invention has application in fields such as boat and surfboard design. BACKGROUND OF THE INVENTION.

The provision of a duct in surfboards for the passage of air from the atmosphere to an underside of the surfboard is known. The duct consists of a passageway extending at an angle from the deck of the surfboard to the opposite underside surface of the surfboard. As the surfboard is propelled along a wave during a ride air from the atmosphere is drawn into the inlet of the duct by a venturi effect produced by the flow of water along the underside of the craft. The air subsequently passes out of the outlet of the duct located in the underside surface, and flows toward the rear of the surfboard. The presence of the air underneath the surfboard causes an increase in the speed of the craft. It is believed that the increase in speed is due to a reduction in the overall surface area of the underside surface of the surfboard in contact with the water. The provision of similar ducts in craft such as boats is also known and it is thought that the ducts act in a similar manner as for surfboards described above.

However, the water flowing around the craft impinges on the rear peripheral region of the duct outlet when the craft is travelling at low speed. This produces drag which inhibits the acceleration of the craft and is particularly undesirable in craft such as power driven boats as fuel is used in order to overcome the effect of the drag in order to bring the craft up to cruising speed.

STJMMI-JIY OF THE INVENTION. It is an aim of the present invention to ameliorate at least one of the problems of the prior art.

In a first aspect of the present invention there is provided a flap in combination with a craft having at least one duct for the passage of a gas to a surface of the craft located below the waterline of the craft, wherein the flap is movable between an open position and a closed position and is arranged so as to substantially prevent water impinging on a rear peripheral region of an outlet of the duct located in the surface of the craft when the craft is travelling forward through the water.

In a second aspect of the present invention there is provided a method f,or minimising drag acting on a craft having at least one duct for the passage of a gas to a surface of the craft located below the waterline of the craft, wherein the method comprises the step of attaching a flap to the surface of the craft to substantially prevent water impinging on a rear peripheral region of an outlet of the duct located in the surface of the craft when the craft is travelling forward through the water, and wherein the flap is attached to the surface of the craft so as to be oveable between an open and a closed position.

The term "craft" wherever used in this specification is to be taken to mean powered or unpowered water craft such as surfboards, sailboards, boats (including yachts) and ships. Moreover, the term duct wherever used in this specification is to be taken to encompass a passageway or conduit.

In use, the flap is in a closed position when the craft is travelling at low speed but moves to an open position when the craft's speed increases. In preferred embodiments of the invention the flap is drawn open by the flow of water over the flap in combination with the flow of air out of the duct outlet. The air is drawn into the duct from the atmosphere by a venturi effect produced by the flow of the water relative to the duct outlet as the craft travels forward through the water. It will be appreciated, however, that the flap may be connected to mechanical means arranged so as to cause the flap to be opened when desired by an operator on the craft. Preferably, the flap is pivotally connected to the craft through pivot means. However, the flap may be pivotally connected directly to the craft by a suitable fixative such as marine glue.

The flap may be formed from a variety of materials such as mylar or PVC. While it is preferable that the material from which the flap is formed is flexible, substantially rigid materials may also be used.

Moreover, the flap may be arranged so as to lie substantially flushly on the surface of the craft in which the duct outlet is located. Alternatively, the flap may be arranged within the duct outlet itself so that an outer surface of the flap is level with the surrounding surface of the craft in which the duct outlet is located. The flap may also be distanced above the duct outlet by spacer means when the surface of the craft in which the duct outlet is located is substantially flat such as the underside surface of a surfboard.

Preferably, the flap is substantially planar in form and typically, is rectangular in shape. However, the flap may be one of a variety of other shapes.

It is an advantage that embodiments of the present invention may reduce drag acting on a craft at low speed and so allow increased acceleration of the craft. It is a further advantage that by reducing the drag acting on powered craft savings in fuel costs may be achieved.

The invention will hereinafter be described with reference to preferred, non-limiting embodiments of the present invention illustrated in the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS.

Figure 1 is a bottom view of a flap arranged over the outlet of a duct extending through a surfboard.

Figure 2 is a cross-sectional view of the embodiment illustrated in Figure 1 taken through X-X.

Figure 3 is a cross-sectional view of the embodiment of figure 1 with the flap in an open position. Figure 4 is a perspective view of a flap located over a plurality of duct outlets on the rudder of a boat.

Figures 5a and 5b illustrate the flow of water around a conventional rudder at different speeds.

Figure 5c illustrates the flow of water around the conventional rudder shown in Figures 5a and 5b when partially rotated during a turning manoeuvre.

Figures 6a and 6b show the flow of water around the rudder of Figure 4 at different speeds.

Figure 6c illustrates the flow of water around the rudder of Figure 4 when partially rotated during a turning manoeuvre.

Figure 7 is a side view of a displacement hull incorporating a plurality of ducts and having flaps over the respective duct outlets. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION.

Figure 1 illustrates a flap 1 arranged over a duct outlet 2 on the bottom surface of a surf oard 3. Flap 1 is connected to spacing means 4 in the form of an arrow- shaped member through pivot means 5. In the embodiment shown, pivot means 5 is a rod about which flap 1 pivots between an open and closed position while spacing means 4 is tapered so that the water flows smoothly over its surface 6. Spacing means 4 is also provided with rearwardly extending wings 7 to inhibit the flow of water under sides 8 of flap 1.

The bottom surface of surfboard 3 is substantially flat and spacing means 4 serves to space flap 1 from surface 3 to enable flap 1 to be drawn open by water flowing over the flap when the surfboard reaches sufficient speed during a ride. As illustrated in Figure 2, the rear end 11 of flap 1 is in contact with the underside surface of surfboard 3 when in the closed position thereby substantially preventing water impinging on a peripheral region 9 of duct outlet 2 and so minimising drag acting" on the surfboard as it moves through the water during a ride.

However, in the open position, flap 1 is substantially parallel with the underside surface of surfboard 3 as indicated in Figure 3. When flap 1 is in the open. position air flows through duct 10 and out underneath rear end 11 of flap 1. The air then continues along the underside surface of surfboard 3 and exits the tail region of the craft.

.Another embodiment of the present invention is illustrated in Figure 4 which shows a rudder 12 of a boat having a plurality of duct outlets 2 which are covered by flaps 1 arranged on each side 13 of rudder 12. Flaps 1 lie flushly on sides 13 and are fixed to rudder 12 at leading end regions 14 of the flaps. Curved portions 15 of rudder 12 direct the flow of air along opposite sides 13 of rudder 12 when flap 1 is in the open position. In use, air flows into duct 10 from conduit means (not shown) which is in communication with the atmosphere.

While this aspect of the invention is described with reference to a rudder it will be appreciated that flaps l may be located over duct outlets 2 on the keel of a yacht or, for example, the centre board of a sailboard. Moreover, each flap 1 may be replaced by a plurality of flaps, one flap being provided for each respective duct outlet 2. The working of flap 1 on rudder 12 will now be described with reference to a conventional rudder as illustrated in Figures 5a to 5c. Figure 5a shows water travelling along sides 13 of rudder 1 in a laminar flow. However, when the speed of the boat increases, turbulence, indicated by the numeral 16, is produced at the rear of rudder 12 as shown in Figure 5b. The turbulence is caused by a combination of the speed of the boat and the shape of the rudder. That is, after water flows over the widest portion of rudder 12 (indicated by the line P-P) it flows inwardly towards sides 13 to fill the space left by the passage of rudder 12. Accordingly, the laminar flow of the water over the rear section of rudder 12 is broken by the inflow of water which produces drag that acts on the boat by inhibiting acceleration of the craft.

The turbulence caused by the passage of the rudder through the water at higher speeds increases when the rudder is partially rotated during a turning manoeuvre as indicated in Figure, 5c. While there is a substantially laminar flow of water along one side 13a of rudder 11 severe turbulence is present along substantially the entire opposite side 13b of the rudder. The drag resulting from the turbulence effectively causes the rudder to act as a brake and indeed, steering may be lost if the angle of the rudder is partially rotated by an angle of about 11° or greater. The rudder shown in Figure 6a is provided with flaps

1 arranged over duct outlets 2 which are in communication with the atmosphere through duct 10. At low speed, the water moves in a laminar flow around sides 13 of rudder 12 as with the conventional rudder illustrated in Figure 5a. Flaps 1, however, inhibit the formation of turbulence at rear peripheral regions 9 of duct outlets 2 and so avoid an increase in drag acting on the rudder caused by the presence of the duct outlets.

When the speed of the boat increases flaps 1 are drawn open by the movement of the water over the flaps in combination with the flow of air out through duct outlets

2 from duct 10. The air fills the space left by the passage of rudder 12 through the water thereby decreasing turbulence to the rear of the rudder as indicated in Figure 5b. The decrease in turbulence translates to a reduction in drag acting on the boat and increased acceleration of the craft.

Figure 6c shows the position of the respective flaps 1 when the rudder is partially rotated during a turning manoeuvre. More specifically, flap 1 on the side 13a of rudder 12 is in a closed position while the opposite flap 1 on side 13b of rudder 11 is in an open position. The water travels along side 13a of rudder 12 in a laminar flow as for the conventional rudder illustrated in Figure 5c. However, the turbulence is substantially reduced on side 13b of the rudder as the air flows out from duct outlet 2. The open flap 1 assists in reducing the turbulence by directing the air along side 13b of rudder 12 and by guiding the water around the widest portion of the rudder.

A further embodiment of the present invention will now be described with reference to a displacement hull. The contour of a displacement hull reaches its deepest point in approximately the middle region of the hull and then gently rises- at the rear of the hull toward waterline 18. At low speed water flows smoothly around the hull but at higher speeds the water is drawn inwardly towards

hull to fill the space left by the passage of the craft causing turbulence and increasing the drag acting on the hull. Figure 7 shows a displacement hull 17 having a plurality of duct outlets 2 located in a rear region of the hull slightly forward of where the turbulence occurs with a conventional hull. Each duct outlet 2 is covered by a flap 1 to avoid drag caused by water contacting rear peripheral regions 9 of the duct outlets when the craft is travelling at low speed. As with the embodiments shown in Figures 1 to 4 and 6a to 6c, flaps 1 are in a closed position when the craft is travelling at low speed. However, they are drawn open by the flow of the water and by the flow of air from duct outlets 2 when the speed of the craft increases. The air acts to reduce the turbulence produced at the rear of the hull by the passage of the craft as discussed above.

It will be appreciated that the deeper duct outlets 2 are located on hull 17 the greater the speed of the craft required to cause flaps 1 to be opened due to the pressure of the water acting on the flaps. Typically, duct outlets 2 are located within a depth of 1 metre from the water line. However, embodiments may be provided wherein duct outlets 2 are located at greater depths. Moreover, flaps 1 may be operated by mechanical means controlled by an operator on the deck of the boat.

While duct outlets 2 in the accompanying drawings are shown as being covered by a single flap 1, embodiments of the invention may be provided wherein a duct outlet 2 is provided with a plurality of flaps 1 which co-operate with each other to substantially prevent water impinging on the rear peripheral region 9 of duct outlets 2 as the craft travels through the water.

Although the present invention has been described hereinbefore with reference to several preferred embodiments, numerous variations and modifications are possible without departing from the scope of the invention which is defined in the following claims.

Claims

The Claims Defining The Invention Are As Follows:

1. A flap in combination with a craft having at least one duct for the passage of a gas to a surface of the craft located below the waterline of the craft, wherein the flap is movable between an open position and a closed position and is arranged so as to substantially prevent water impinging on a rear peripheral region of an outlet of the duct located in the surface of the craft when the craft is travelling forward through the water.

2. A method for minimising drag acting on a craft having at least one duct for the passage of a gas to a surface of the craft located below the waterline of the craft, wherein the method comprises the step of attaching a flap to the surface of the craft to substantially prevent water impinging on a rear peripheral region of an outlet of the duct;.' located in the surface of the craft when the craft is travelling forward through the water, and wherein the flap is attached to the surface of the craft so as to be moveable between an open and a closed position.

3. A flap in combination with a duct outlet substantially as hereinbefore described with reference to any one' of Figures 1 to 4 or 6 to 7 of the accompanying drawings.

4. A method for minimising drag acting on a craft which comprises the step of attaching a flap to the craft so as to be over a duct outlet located below the waterline of the craft, substantially as hereinbefore described with reference to any one of Figures 1 to 4, or 6 to 7 of the accompanying drawings.