GB2091646A - Hydrofoil Craft - Google Patents

Abstract

An inflatable craft substantially consisting of a U-shaped tubular buoyancy casing (10, 11, 12) exhibits under its hull structure, two V-shaped hydrofoils arranged transversally, the one in the forward and the other in the astern dinghy section. The hydrofoils have a cross-sectional outline with a shape-cornered leading edge and its maximum thickness in correspondence with the trailing edge. At least the hydrofoil applied to the forward section has the angle of setting increasing from the centre towards the two ends. Further expedients can be provided for decreasing the product of the lift coefficient by the immersed surface of the hydrofoils as the speed of the dinghy is increased, such as decreasing the foil chord of the hydrofoils from the ends towards the centre, the application of rigid trailing edge flaps having a width which is decreased from the centre towards the ends, or the adoption of flexible trailing edge flaps. <IMAGE>

Description

SPECIFICATION Rubber Dinghy This invention relates to a rubber dinghy, that is a small boat having an inflatabie structure consisting of a flexible casing which takes up its final shape and stiffness as it is inflated by pressurized air.

Rubber dinghies have gained an evergrowing acceptance. They are essentially composed of a tubular casing bent in the shape of a "U-letter" and having a comparatively large diameter, the branches of the U being connected together by a keel piece, the bottom gratings and the transom: such a structure affords the possibility of taking people on board and of applying an outboard motor.

While no doubt is left about the advantages tendered by such a boat structure, such as light weight, foldability for transportation purposes, small draught and others, it is likewise known that such rubber dinghies have very poor hydrodynamic properties which give rise both to a low cruising speed and a high fuel consumption.

With a view to do away at least partialiy, with these shortcomings, a few manufacturers have adopted several types of stiff hulls of fibreglassreinforced plastics or like materials but, by so doing, they have actually given up the advantage of an easy disassemblage and reassemblage of the component parts of the dinghy.

An object of the present invention is thus in the first place, to provide a rubber dinghy of the kind briefly referred to above but having a reduced hydrodynamic resistance and thus an improved hydrodynamic efficiency.

In order that such an object may be achieved, it has been envisaged to equip the rubber dinghy in question with hydrofoils installed below the water line of the dinghy, similarly to what has been adopted for the hydrofoil speedboats.

On taking, however, into account, the circumstance that the rubber dinghies to which this invention particularly relates are vessels having a comparatively reduced length, which, under any respect, cannot be compared with that of a hydrofoil speedboat, a few particular expedients have been adopted, of necessity, in order that the required longitudinal stability of the dinghy might have been achieved together with the possibility of a quick and short take-off, that is, an easy and convenient transition from the conventional navigation mode to the hydrofoil mode, and vice versa.

In substance, according to the present invention, the problems posed hereinbefore are solved by a rubber dinghy which is characterized in that it exhibits, under its water line, two Vshaped hydrofoils placed transversally of the longitudinal centre line of the dinghy and applied, respectively, the one ahead and astern, each of said two hydrofoils being composed of a foil having hydrodynamic lift, the cross-sectional outline having a substantially sharp-cornered leading edge and its maximum thickness in correspondence with the trailing edge, the Vshaped foil applied ahead having its settling angle gradually increasing from the centre towards the two ends.

In practice, it has been ascertained that the setting angle of the V-shaped foil can virtually be zero at the centre and can be increased towards the two ends to attain, in the vicinity thereof, a value of about 100 or more.

Indicatively, it can be said that the variation of the setting angle should be greater, the shorter is the dinghy and the shorter is the distance between the ahead V-shaped foil and the astern V-shaped foil.

Advantageously enough, not only the ahead Vshaped foil, but also the astern foil exhibits such a variation of the setting angle, even if, to the ends of the results aimed at, it is more important that such a feature be experienced in the ahead Vshaped foil.

It is known that the term "setting angle" as applied to a foil is the rigging angle of incidence as formed by the chord of a foil section and a reference horizontai axis, the latter, in the case in point, being the water line of the dinghy, It is outstandingly important to note that it is imperative to adopt outlines of the kind in which the maximum thickness is in correspondence with the trailing edge, the leading edge having a virtually nil radius of curvature, that which is equivalent to a sharp corner.

As a matter of fact, the conventional hydrofoils having their maximum thickness between one third and one half of the foil chord and having a rounded leading edge would not be suited to the practical application envisaged according to this invention. Such outlines would be affected by the drawback of an extremely frequent aeration during motion, that is the detachment of the fluid fillets from the top surface of the foil, the result being a lift loss This is due to the circumstance that, in the case in point, the foils must work, as an average, at a distance of a few centimeters from the free air-water interface.

Moreover, the rounded leading edge would inevitably be conductive to the accumulation, on the edge, of filaments such as weed, algae and the like, which would bring about within a short time to a reduction of the hydrodynamic efficiency. On the contrary, by adopting the kind of outline selected according to the invention, there are no phenomena of aeration of the foils, nor of detachment of the fillets and, on account of the shape of the leading edge, the weed filaments and algae would be severed, thus ensuring the self-cleaning of the foils.The foil family as used according to this invention, moreover, has the features, which is favourable to the intended use, of having a ratio of the lift coefficient to the angle of incidence which is somewhat lower than that of conventional foils having a rounded leading edge, the result being a definite advantage as far as the longitudinal stability of the dinghy is concerned.

The lift of a hydrofoil moved in water is expressed, as is well known, by the formula: VZ P=c.p.S.

2g wherein Pisthelift cp is the lift coefficient p is the density of water S is the immersed surface of the foil, and V is the translational speed g is the gravity puli.

To impart the dinghy the necessary longitudinal stability together with the ability to a short take-off and thus a smooth transition from the conventional navigation mode to the hydrofoil navigation mode as the speed is increased, it is essential to have a maximum lift at the Idwer speeds and to decrease, as the speed is increased, the product of the lift coefficient by the immersed surface of the foils.

The V-shaped outline of the foils involves a decrease of the immersed surface which is proportional to the boat lift over the water.

However, in the case of short vessels, such as those considered in the case of the present invention, the decrease of the immersed foil surface as the dinghy is lifted has proven to be inadequate to ensure the longitudinal stability of the dinghy and the ease of its take-off. More particularly, it has been ascertained that, in the case of a short vessel, it was essential, in order to achieve the indispensable longitudinal stability of the vessel, to act also upon the lift coefficient of the foils.

In this connection, it has been possible to ascertain that the best and the most reliable way of influencing the lift coefficient is to vary the setting angle of the sections of the V-shaped foil, in the sense of increasing said angle as one proceeds from the foil centre to the foil ends.

As a matter of fact, indeed, it is possible rapidly to attain the minimum speed at which the dinghy draught is decreased and thus its hydrodynamic resistance is lessened, thus starting the hydrofoil navigation mode, even at full load, without thereby impairing the longitudinal stability of the dinghy.

According to the invention, in order further to improve the c.p.S. product as the speed is increased, other advantageous measures can be adopted, which are of an optional nature.

One of these supplementary expedients may be to shape the V-foils in such a way that their chord length is decreased gradually starting from the foil ends to the foil centre, the latter coinciding with the longitudinal centre line of the dinghy. By so doing, when the V-foils are totally immersed, a considerably wide lifting surface becomes available, which encourages a low speed take-off, whereas, as the dinghy is gradually lifted over the water, the wider portion of the foils emerge and do not offer any hydrodynamic resistance, so that a high speed can be attained.

A further additional expedient to be adopted would be that of-applying a flap to the trailing edge of the V-foil. Such a flap can be stiff and have a width which is increased from the centre towards the ends (if so, an effect akin to that of the first described additional expedient can be obtained); as an alternative, the flap can be of the flexible type so as gradually and progressively to decrease its angle of incidence as the speed is increased, by exploiting the effect of the pressure increase on the top surface of the foil and the pressure decrease under the bottom surface of the foil. This latter type of flap can be applied also only to the outer portions towards the end of the V-foils.The elasticity of the flap can be obtained by different constuctional approaches; for example, a flap can be adopted of a sheet metal which is flexible of itself and is rigidly connected to the trailing edge of a V-foil, or a flap can be adopted which is hingedly connected to the edge and has its yieldability adjustable by springs, for example coiled springs, plated at the ends.

Yet another expedient of an ancilliary nature, which can be provided according to the invention, is that of connecting the ahead V-foil not rigidly to the dinghy, but rather, to afford the possibility of varying its position, more particularly of having it oscillably to move about a transversal axis which is perpendicular to the middle longitudinal plane of the dinghy. By so doing, it becomes possible to modify the angle of setting of the V-foil and, more exactly, to have such angle wider at the start for narrowing it gradually afterwards consistently with the speed decrease.

This variation of the position of the V-foil (ahead) can manually be controlled by the dinghy driver.

Furthermore, it should be noted that the astern V-foil can be reinforced, with advantage, by a central vertical strut having a streamline outline of symmetrical type with a sharp-cornered leading edge. Such a strut, in addition to reinforcing the structure, by bearing in mind the heavier weight astern of the dinghy, has the task of improving the directional stability of the dinghy during navigation in the hydrofoil sailing mode.

It should be remembered, moreover, that the dinghy according to this invention does not require any special particular modifications, with the only exception of the appropriate fittings, preferably of the releasable type so as to be quickly released and reassembled, for connecting the V-foils to the rubber dinghy hull structure proper. The necessity of having an outboard motor with a sufficiently long driveshaft, or a level-adjustable configuration should also be taken into account in order that a thorough immersion of the propeller may be ensured during sailing in the hydrofoil mode of navigation may be obtained.

The features and the advantages achieved by this invention will become more clearly apparent from the ensuing description of a few exemplary embodiments of the rubber dinghy in question, the description being aided by the accompanying drawings, wherein: Figure 1 is a side elevational view of a rubber dinghy, embodying the features of this invention, Figures 2 and 3 are cross-sectional views, taken along the lines Il-Il and Ill-Ill, respectively, of Figure 1, the showing of Figure 3 being on an enlarged scale, Figure 4 shows the dinghy of Figure 1 in plan view, Figure 5 is a fragmentary cross-sectional view taken along the line V-V of Figure 3, Figure 6 is a fragmentary plan view of a modification of a hydrofoil constructed according to this invention.

Figure 7 is a cross-sectional view of the hydrofoil of Figure 6, taken along the line VIl-VIl of Fig. 6, Figure 8 is a partial plan view of a detail of another alternative embodiment of this invention.

Figure 9 is a cross-sectional view taken along the line IX-IX of Figure 8, Figure 10 is a partial plan view of a detail of another modification of the invention, and Figure 11 is a cross-sectional view taken along the line Xl-Xl of Figure 10.

Figures from 1 to 5 are illustrative of a rubber dinghy, essentially comprised of a flexible tubular casing inflated by air under pressure, and generally indicated at 10, having a large diameter and a U-shaped outline in plan view, the two side prongs of the U, 11 and 12, being connected by an arcuate portion 13 ahead, the stern ends being of a pointed shape.

The prongs 11, 12 of the tubular casing 10 are connected by a semiflexible structure, which consists, in its essence, of a keel piece 14, a bottom grating 15, a rubberized canvas bottom 16 and the transom 17: such a structure makes it possible to accommodate people aboard and to apply an outboard motor 18. The stalk 19 of the motor is long enough as to ensure a complete immersion of the propeller 20 at any instant of time during sailing.

According to the invention, beneath the water line of the rubber dinghy, there are applied two hydrofoils 21 and 22, respectively, which are Vshaped and are arranged transversally with respect to the longitudinal axis of the dinghy: the hydrofoil 21 is mounted near the bow and the hydrofoil 22 is mounted near the stern.

Each of the V-shaped foils 21 and 22, is a hydrofoil having a cross-sectional outline with a sharp-cornered leading edge and with its maximum thickness in correspondence with the trailing edge: this is clearly shown in Figure 5 for the foreward foil 21.

The hydrofoils 21 and 22 are made of an adequately resistant material such as steel, aluminium alloys, fibreglass-reinforced plastics and the like.

The "astern" foil 22 is rigidly connected, by means of two struts 23, 24, to two longitudinals 25, 26: these are secured in any appropriate way to the bottom portion of the prongs 11 and 12 of the tubular casing 10 (best seen in Figure 2) whereas a central upright 27 having a streamlined shape, symmetrical, and with a sharp-cornered leading edge, not only acts as reinforcing member, but also improves the directional stability during the hydrofoil sailing mode.

Conversely, the ahead foil 21, in the embodiment shown and described herein, is connected to the branches 11 and 12 of the tubular casing 10 so as to be able to effect a limited rotation about a transversal axis which is perpendicular to the vertical longitudinal middle plane of the dinghy. More particularly, as best seen in Figures 3 and 5, the bottom sections of the branches 11, 12 of the tubular casing 10 are secured to the longitudinals 25, 26 each of which carries a pivotal pin 30 and 31, respectively, and, about such pivotal pins there are, mounted for rotation, arms 32 and 33, respectively, which are integral with the ahead foil 21.In order to control the limited rotation of the foil 21 about the axes of the pins 30 and 31, either end of the foil in question is secured to a connecting rod 34 the distal end whereof is connected to a clevis 35, the length of which can be varied by a handle 36 (best seen in Figures 1 and 4). By rotating the hydrofoil 21 as described above, its angle of setting relative to the dinghy can be varied.

It should be noted, also, that the hydrofoils 21 and 22 may carry, at their ends, floats, such as those indicated at 37 in the drawings.

An important feature of the present invention is that at least the ahead hydrofoil 21 , V-shaped, has an angle of setting which becomes wider and wider proceeding from the hydrofoil centre, which latter coincides with the longitudinal middle plane of the dinghy, towards both the ends of the hydrofoil (best seen in Figure 5). The angle of setting may be virtually nil at the centre, whereas, in correspondence with the ends, said angle of setting may take values as wide as 1 0-1 5.

By so doing, the result is that, as the hydrofoils are totally immersed, not only their supporting surface is at a maximum, but also their lift coefficient (which is a function of the setting angle) is at a maximum, whereas, as the dinghy is being gradually lifted and is raised over the water as the speed is increased, both the supporting surface and the lift coefficient are gradually decreased. This circumstance facilitates the takeoff at a low speed but without decreasing the maximum speed which could be attained with a certain installed power and improves the longitudinal stability of the dinghy. As a matter of fact, while the V-shaped outline of the hydrofoils provides a wide supporting surface when the hydrofoils are totally immersed, while the supporting surface in question is decreased as the dinghy is raised over the water level, thus making a short take-off possible, this effect would not suffice, alone, to provide also the necessary longitudinal stability. Thus, according to the invention, the possibility is afforded of varying the angle of setting, and thereby also the lift coefficient of the hydrofoils, in the sense that such angle is decreased as the dinghy is raised over the water level and as the hydrofoils correspondingly emerge, so that this effect is summed to the effect of the concurrent decrease of the supporting surface of such hydrofoils.

According to a few alternative embodiments of the V-shaped hydrofoils, the effects which can thus be achieved can further be exalted.

One of these embodiments, best shown in Figures 6 and 7, provides a V-shaped hydrofoil, generally indicated at 40, in which not only the angle of setting becomes wider and wider proceeding from the centre towards the ends but also the foil chord is gradually lengthened from an appropriate value at the centre towards the ends.

By so doing, the supporting surface of the hydrofoil is decreased as the dinghy is raised over the water level and the hydrofoil itself emerges, not only by virtue of the V-shaped outline, but also by virtue of the gradual shortening of the foil chord.

A similar effect can be achieved with a Vshaped hydrofoil having a constant chord-length and a variable angle of setting (such as 21 and 22 in Figures from 1 to 5 inclusive), which additionally carries, applied to its trailing edge a rigid flap, the width of which is increased from the centre towards the ends. This alternative embodiment is shown in Figures 8 and 9, in which the V-shaped hydrofoil is generally indicated at 41 and the variable-width flap is indicated at 42.

Still another embodiment, shown in Figures 10 and 11, provides the application to the trailing edge of a V-shaped constant-chord and variableangle-of-setting hydrofoil, generally indicated at 43, of a resilient flap 44 for a certain setting angle. As the speed is increased, this angle of setting of the resilient flap 44 is gradually narrowed (as depicted in Figure 11 in dash-anddot lines) due to the direct effect of the pressures over the top surface of the foil and of the negative pressures under the bottom surface thereof. This kind of flap can be applied over the entire extension of the hydrofoils (as shown in the drawing), or, also, only on the portions near the hydrofoil ends.

The resilient character of such a flap can be obtained, as outlined above, with different constructional approaches. For example, a blade flap can be adopted which is inherently endowed with a sufficient flexibility and is rigidly connected to the trailing edge of the hydrofoil (as suggested in Figures 10 and 17),our, as an alternative, a flap can be provided, which is hingedly connected to the trailing edge of the hydrofoil concerned, and the yieldability of such a flap can be made adjustable by means of springs, such as coiled springs, secured to the ends of the hinged connections.

The alternative embodiments of this invention as described above, which can also be variously combined together, are all intended to influence the product of the lift coefficient by the immersed surface of the hydrofoils applied to the rubber dinghy, in the sense of causing this product to be decreased as the speed is increased so as to ensure the longitudinal stability of the dinghy in the transition from the conventional sailing mode to the hydrofoil sailing mode, as well as during the hydrofoil sailing mode and also so as to make possible a quick and smooth transition from either sailing mode to the other.

As regards the way of connecting the hydrofoils to the dinghy structure, it should be remembered that one of these ways has been described herein merely to provide an indication, it being, however, understood that many other connection means can be adopted, preferably of the quick fit kind so as to make possible a convenient and reliable assembling of the hydrofoils and their overhauling as the dinghy must be prepared for transportation.

In addition, it should be noted that the outboard motor for the dinghy, in the case that it should be inconvenient to equip it with an exceedingly long driveshaft, could be easily mounted in a level-adjustable way so as to permit to depress it relative to the dinghy hull in the transition from the conventional sailing mode to the hydrofoil sailing mode to ensure under these conditions a thorough immersion of the propeller.

Claims (12)

Claims

1. A rubber dinghy comprising a tubular Ushaped casing inflatable by compressed air and a semiflexible structure connecting the two branches of said U-shaped tubular casing, characterized in that beneath the tubular Ushaped casing there are installed two V-shaped hydrofoils placed transversally with respect to the longitudinal centre line of the dinghy and applied, respectively, the one in the forwards section and the other in the astern section, each of said hydrofoils being composed of a hydrofoil having a cross-sectional outline with its leading edge substantially sharp-cornered and with its maximum thickness in correspondence with the trailing edge, at least the forward-mounted V shaped hydrofoil having its setting angle gradually widened from the centre to both ends of the foil.

2. Rubber dinghy according to Claim 1, characterized in that the angle of setting of said hydrofoil is varied from a magnitude which is virtually zero at the centre to a value of from 100 to 1 50 in correspondence with the hydrofoil ends.

3. Rubber dinghy according to Claim 1, characterized in that the hydrofoil which is applied in the foreward section of the dinghy is mounted for limited oscillation about an axis transversal of the longitudinal centre line of the dinghy, means being further provided for adjusting the position of said hydrofoil.

4. Rubber dinghy according to Claim 1, characterized in that the hydrofoils have a constant foil chord length.

5. Rubber dinghy according to Claim 1, characterized in that the foil chord of at least one of the hydrofoils is gradually shortened from the ends to the centre, the latter coinciding with the longitudinal centre line of the dinghy.

6. Rubber dinghy according to Claim 1, characterized in that to the trailing edge of at least one of the hydrofoils a flap is applied.

7. Rubber dinghy according to Claim 6, characterized in that said flap is a stiff flap.

8. Rubber dinghy according to Claim 7, characterized in that said stiff flap is fixedly connected to the trailing edge of said hydrofoil and has a width which is increased from the centre towards the ends.

9. Rubber dinghy according to Claim 7, characterized in that said stiff flap is hingedly connected to the trailing edge of the hydrofoil and is held in a predetermined setting angle by resilient means capable of yielding under the action of predetermined positive and negative pressures which are originated over the top surface, and under the bottom surface, respectively, of the flap.

10. Rubber dinghy according to Claim 9, characterized in that said flap is applied only to the end portions of the hydrofoil.

11. Rubber dinghy according to Claim 6, characterized in that said flap is flexible and is rigidly secured to the trailing edge of the hydrofoil concerned:

12. Rubber dinghy according to Claim 11, characterized in that said flexible flap is applied only to the end portions of the hydrofoil concerned.

1 3. A rubber dinghy substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.