This invention relates to drag reduction and more particularly to a microbubble injection device for the purpose of reducing skin friction in a turbulent boundary layer by the injection of microbubbles of gas between a body of fluid and the surface, or skin, of an object immersed therein. Generally, the gas of interest is air, the liquid water and the object a waterborne vessel. BACKGROUND ART

As early as about 1800 a patent was granted for the engendering of a layer of air between a ship's hull and its water boundary layer, for the purpose of reducing skin friction. In 1973, some experimental work was reported by

McCormick and Bhattacharyya involving microbubbles of hydrogen, and more recently the effects of microbubbles upon a turbulent boundary layer have been studied in the Soviet

Union. Instead of using an electrolytic method for the generation of the hydrogen microbubbles, air was forced through a porous surface; simple flat plate geometry was employed and measurements were made downstream of the porous region. Results indicated that local skin friction might well be reduced by upwards of 50%; however, no way was ever postulated to actually produce engineering - particularly marine engineering - benefits from the discovery.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to overcome the above and other disadvantages by the provision of a microbubble injection device which comprises a member attachable to, or incorporatable in, the external surface, or skin, of a waterborne vessel; the said member having a longitudinal duct extending therethrough, said duct being adapted to be in communication with a body of water, in which said member is immersed via a foraminous region provided on that part of the said member which is

remote from said external surface, or skin, of the said vessel.

Ideally, the member should have an aerofoil-shaped transverse cross-section, while the pores of the foraminous region preferably have a diameter of less than about 100 microns. Throughout the drawings like integers are referred to by the same numeral. BRIEF DESCRIPTION OF DRAWINGS

In order that the reader may gain a better understanding of the present invention, hereinafter will be described some preferred embodiments thereof, by way of example only and with reference to the accompanying drawings in which:-

Figure 1 is a schematic drawing showing an -injection device according to the present inventions;

Figure 2 is a transverse cross-section through a typical device;

Figure 3 shows an alternative cross-sectional form; Figure 4 illustrates an inventive device particularly applicable to surboards and sailboards;

Figure 5 shows a further embodiment; and

-Figure 6 depicts yet' another embodiment. MODES OF CARRYING OUT THE INVENTION;-

Figure 1 shows schematically a typical microbubble injection device according to the present invention, the device comprising a member, generally referenced 1, in the form of a strap 2 of aerofoil-shaped cross-section and having a longitudinal duct 3 extending therethrough, best to be seen in Figures 2 and 3. Strap 2, as it is depicted in Figure 1, is affixed to the external surface or skin 4 of a waterborne vessel 5; strap 2 extends part-way round the vessel 5 from gunwhale to gunwhale and may be kept in contact with surface 4 by any suitable means such as by gluing, or by maintaining it under tension, or

other like methods.

Figure 2 is a transverse cross-section through the strap 2 showing a foraminous region 6 provided on the part of strap 2 which is remote from surface or skin 4. The pores of this foraminous wall region 6 ideally have a diameter of less than about 100 microns.

The inventive microbubble injection device operates in the following manner: when vessel 5 is in motion, the movement of water over the aerofoil shape hydro-dynamically generates a low pressure area adjacent the foraminous region 6. When sufficient speed has been attained, the pressure drop is such as to cause air to enter strap 2 through the open ends 7, located above the waterline', flow through duct 3 and out through the pores of foraminous region 6. The combination of small pore size - less than 100 microns - and the velocity of the water over the foraminous region results in microbubbles of air entering the turbulent boundary layer to thereby cause a reduction in skin friction drag on the hull surface aft of the device.

As this drag reduction effect persists only for a distance of about two metres downstream of the injection point, vessels of some considerable size may well be provided with a plurality of inventive straps 2, spaced at approximately two metre intervals along the hull. The location of the device or devices can be varied to conform with the position on the hull of different kinds of friction, such as skin friction and form friction. In the case of planing hulls, the or each device would be aft of the so-called "stagnation point".

Figure 3 shows an alternative transverse cross-sectional form in which duct 3 is bounded on both sides by walls of the strap 2.

While Figures 2 and 3 show straps having therein foraminous regions 6, it is also envisaged that the strap's entire outside wall could well be porous. In another

(- variation the device may be in communication with atmosphere via ducting within the vessel. There again, the member- 1 may be ^' incorporated into the structure of the vessel. r- "^Figure'4 illustrates a device in accordance with Jkhe invention which is particularly applicable to surfboards and sailboards. In this embodiment, the strap 2 continues

-'..abross the top of a board 8. Air enters strap 2 through its foraminous _. region 6 located on the top of the board and then travels through the duct 3 to exit through the foraminous region 6 located"beneath t\ e board. Because of the very fine pore size the strap is far more permeable to air than it is to water, thus, even when water washes over the top of the board, only a very small quantity will enter the strap 2. This water will eventually find its way out beneath the _board*.-and similarly any water . ntering the strap when board 8 is not- moving will also -be ejected once movement commences.

While it has been stated that an inventive strap is attachable to a vessel hull, or a board, in a number of ways such as by gluing, etc., in Figure 5 there is shown an embodiment suitable for fitment to, say, a sailboard having a keel, or a centre-board, retractable or otherwise.

In this embodiment a sailboard 9, having a centreboard 10-, is provided with _a divided porous strap 11, each half being held at its immersed end by being anchored in co-acting grooves 12 in the fabric of the board. The •ι?p.per end of each half-strap 11 being held in place by the tension exerted by fasteners 13 on the upper surface of the board; fasteners 13 may well be conventional, over-centre action.toggle s eners.

Figure 6 depicts yet another embodiment wherein a board, again referenced 8, has a length of inventive strap 2 affixed only to the underside of the board; ends 14 of this strap are- closed-off. In this embodiment, atmospheric air -is caused to enter the duct within the strap through a bore 15 in board 8. _. Furthermore, the mouth of bore 15 is

advantageously provided with a simple, foot-operated flap valve 16, preferably made of rubber. Such a bore-and-valv combination is operable to control microbubble injection an is usable by a rider to give controlled speed increase decrease as and when required or desired. INDUSTRIAL APPLICABILITY

The microbubble injection device according to th present invention is applicable to virtually all kinds o watercraft from large oceangoing vessels, all pleasur craft, trimarans, catamarans, planing-hull craft, hydrofoils, and the like down to sailboards and surfboards. It is envisaged that the greatest commercial advantage will be gained by the provision of the inventive, foraminous, aerofoil-shaped straps in a range of lengths and sizes fo attachment to, or incorporation in, the external surface, or skin, of a waterborne vessel.

From the abovegoing, it will be readily appreciated by those skilled in the art that numerous variations and modifications may be made to the invention without departing from the spirit and scope thereof as set out in the following claims.