Field of the Invention

The invention relates to hovercraft skirts and in particular to hovercraft skirts of the combined bag and finger type.

Background

Hovercraft (known also as air-cushion vehicles) are craft that can travel over water and land on a cushion of air. Known hovercraft include a hull and a flexible skirt that extends around and downwards from the periphery of the hull. A lift fan is used to create a pressurised cushion of air between the hull and the surface over which the craft is travelling, contained by the skirt. The craft is propelled forwards by one or more propulsors mounted on top of the hull. The hull typically also carries a cabin for the pilot/crew/passengers and, depending on the size of craft and intended use, may include further structures for holding equipment and/or cargo for example. There are three main types of hovercraft skirt: bag skirts, finger skirts (sometime referred to as segmented skirts) and combined bag and finger skirts.

A bag skirt can be thought of as being formed from a continuous tube (e.g. doughnut shape) of skirt material that is situated underneath the hull of the craft around its perimeter. The continuous tube is inflated with pressurised air to provide a compliant seal between the craft hull and the operating surface. The air cushion which is bounded by the bag is typically supplied with air from the pressurised air inside the continuous tube through holes in its inner wall. In this way the bag has a secondary function as a conduit which distributes air evenly from a small number of lift fan outlets to the main air cushion. A bag skirt can be designed to fit a variety of hull shapes. A finger skirt is formed from a series of fingers of skirt material projecting down from the hull to form a skirt underneath the hull and around its perimeter. Each finger is open to the air cushion on the inner side and is inflated out against its neighbouring fingers by cushion pressure. A pure finger skirt requires specific hull shaping to suit the required skirt attachments and to provide air distribution within the structure. A combined bag and finger skirt is a combination of the bag skirt design and the finger skirt design. This type of skirt has a bag skirt immediately underneath the vehicle body and then a finger skirt attached to the underside of the bag skirt. Finger skirts offer less resistance during travel over a surface and are more able to conform to rough surface shapes and obstacles being traversed but offer lower levels of craft motion damping than the combined skirt and are less tolerant of wave impacts. The segmented nature of finger skirts also means that when a portion of the skirt comprising one or more fingers is damaged, only the damaged fingers need be replaced, making the repair quicker and simpler than with a bag skirt, where the complete skirt may have to be removed for repair.

Combined bag and finger skirts offer the benefits of the low resistance, conformability and ease of repair of the finger skirt with the versatility of hull design, air distribution, impact tolerance and the craft motion damping of the bag skirt.

Combined bag and finger skirts typically comprise an "inner bag" portion underneath the craft at the interface between the bag portion and the finger portions. This inner bag portion typically has holes punched in it to allow air supplied by the lift fan to the bag region of the skirt to pass from the bag region to the finger region and finally to the main cushion. The size and number of these holes principally dictate the pressure difference between the bag and the main cushion.

Whilst combined bag and finger skirts offer advantages over both bag skirts and finger skirts, the location of the fingers under the bag region of the skirt, and the presence of the inner bag portion, make it difficult to access the tops of the fingers when they are to be removed for repair of replacement. In addition, if the top edges of the fingers are not fixed to the underside of the inner bag, relative movement of the fingers not only causes wear to the underside of the inner bag but the fingers are prone to snagging obstacles and scooping water which intrudes between the fingers and the underside of the inner bag.

Summary of Invention

A general aim of the present invention is to provide a combined bag and finger skirt in which it is more straightforward to remove the fingers, in which the top edges of the fingers can be easily attached to the underside of the inner bag to prevent wear and snagging and in which the inner bag can be easily replaced as separate sections. In addition, the invention is intended to improve the flexibility of the skirt bag.

Accordingly, in a first aspect the invention provides a skirt for a hovercraft, the skirt comprising: a bag section for attachment to the hull of the hovercraft; and a plurality of fingers extending downwardly from the bag section; wherein the bag section includes an inner bag portion forming an interface between the bag section and the fingers; the inner bag portion being formed of a plurality of panels that are arranged side-by- side along the length of the skirt, the panels extending over the top of the fingers.

Whereas a conventional inner bag is a continuous web making it difficult to remove the fingers, the multi-panel 'tiled' arrangement of the inner bag proposed now makes it possible to more easily access and release the attachments between the finger and the inner bag to remove the finger or, alternatively, to remove a finger along with the bag panel or panels to which it is attached. Particularly on larger craft, large inner bag panels tend to exceed the width of commonly available flexible materials. The use of relatively small tiled inner bag panels avoids the need for costly joining of material widths.

The fingers are preferably attached to the panels. Preferably the panels overlap one another. Most preferably they overlap such that the aft edge of each panel lies below the forward edge of the next panel. This helps to prevent uncontrolled leakage from the bag to the cushion that might lead to an undesirable loss of pressure differential between the bag and cushion. Overlapping of adjacent panels also offers the benefit of a stronger multi-layer structure without having to laminate layers together (as is often the case in conventional inner bags, which increases the cost and stiffness). Preferably the inner bag panels overlap one another by at least 5% of their plan area, more preferably by 40% or more of their area. The panels may overlap one another uniformly across their full depth or may overlap more towards one edge than the other. For example, the panels may overlap more towards an inner (hull attachment) edge than they do towards and outboard edge (e.g. up to 100% at the inner edge and no overlap at the outboard edge).

In some embodiments each panel of the inner bag has a single skirt finger attached to it. Advantageously, this can mean that the finger can be removed with its own section of inner bag panel still attached. In most bag and finger skirts, the fingers are attached at only two points, these being a) an attachment at the bottom edge of the outer bag portion (known as the finger head attachment) and b) to the lower surface of the inner bag in the vicinity of the lower hull attachment (known as the finger tail attachment). In some existing bag and finger configurations, the inner bag panel has paired flanges fixed to its underside using multiple fasteners. The flange pairs coincide with the pitch of the fingers. The finger upper edges are attached to these flanges to prevent damage caused by snatching and water scooping. This double-flange upper edge attachment is particularly difficult to use in practice.

Each of the inner bag panels in a multi-panel tiled arrangement in accordance with embodiments of the present invention may have a drooping trailing edge. This provides a ready-made flange to allow a simple attachment of the finger top edge to the underside of the panel. Each panel may also be attached, as with conventional arrangements, to the bottom edge of the outer bag portion and to the hull at its inboard edge.

In a second aspect the invention provides a hovercraft including a hull and a skirt extending around and downwardly from the hull to form a plenum for an air cushion, wherein the skirt is a combined bag and finger skirt in accordance with the first aspect above.

Brief Description of the Figures

An embodiment of the invention is described below by way of example with reference to the accompanying figures, in which:

Fig. 1 is a schematic side view of a hovercraft according to an embodiment of the present invention; Fig. 2 shows a section from the skirt of the hovercraft of fig. 1

Fig. 3 is an isometric view of the hovercraft side skirt showing the individual fingers and inner bag tiled panels. The figure includes a view of an inner bag panel shown laid out flat and a section through the attachment at the finger top edge; and

Fig. 4 is an underside view of the skirt system along one side of the craft (with fingers omitted for clarity).

Description of Embodiment

Fig. 1 shows a hovercraft having a central hull that carries a passenger cabin 4. A skirt 10 surrounds and is attached to the hull 2. Between them the hull 2 and skirt 10 form a plenum for a cushion of air on which the hovercraft rides. Air is supplied to the plenum by a lift fan (not shown). Propulsion for the hovercraft is provided by one or more motor driven fans or propellers 12 on the hull 2, usually mounted towards the stern.

As best seen in fig. 2 and 3, the skirt 10 is of the combined bag and finger type, including a bag part 20 and, below the bag part 20, a plurality of fingers 22. Lift air supplied by the lift fan inflates the bag part 20 and fingers 22. More specifically, air from the lift fan is blown into the bag part 20 through openings 24 in the side of the hull 2, from where it passes into the main cushion via the fingers 22.

The bag part 20 includes an outer bag portion 26 and an inner bag portion 28. The outer bag portion 26 has a generally semi-circular cross-section and is connected at its top edge to the hull 2 at the line known as the upper skirt attachment 30. The outer edge 32 of the inner bag portion 28 is connected to the bottom edge of the outer bag portion 26 and is connected at its inner edge 34 to the hull at a line known as the lower skirt attachment. In this example, as is commonly the case, the shape of the hull 2 means that the connection of the inner edge 34 of the inner bag portion 28 is inboard of the connection of the outer bag portion 26 to the hull 2.

In accordance with the present invention, the inner bag portion 28 is 'tiled', as best seen in figs. 3 and 4. That is, it is made up from a plurality of overlapping panels 40, each of which has one end attached to the bottom edge of the outer bag 26 and the other end attached to the hull 2 (at the lower skirt attachment line). The forward end 42 of each panel overlaps on top of the aft end 44 of the adjacent panel by about 100% at the inner hull attachment reducing to little or no overlap at the attachment to the lower edge of the outer bag 20.

The inner bag 28 extends across the top of the skirt fingers 22, which are detachable, and there is one panel 40 per finger 22. By suitable removal of finger and inner bag panel attachments, the finger 22 and attached panel 40 can be removed from the hull 2 and outer bag portion 26 together.

Each panel 40 has a plurality (in this example three) of air feed holes 46 through which air passes from the bag portion 20 of the skirt to the underlying fingers 22. The size of these holes 46 can be chosen, as in conventional bag and finger skirts, to control the pressure difference between the bag 20 and the main cushion space. If suitably placed, they also allow hand access to finger attachment fasteners below the panel 40.

The shape of the panels 40, the position of the air feed holes 46, and the degree of overlap between panels 40 are selected to ensure that the intended air feeds are not blocked by an overlapping panel. This is best seen in fig. 4, where the broken lines show the aft edge of each panel 40, which it can be seen stops short of the air feed holes 46 of the next panel 40. To achieve a good average overlap whilst avoiding covering the holes, the panels are wider at their hull attachment end than they are at their outer end. In this example, each panel is about twice as wide at the hull attachment end as it is at its outer end. In most bag and finger skirts, the fingers are attached at only two points, the bottom edge of the outer bag portion (finger head attachment) and to the lower surface of the inner bag in the vicinity of the lower skirt attachment (finger tail attachment). The same or similar attachment points may be used for embodiments of the present invention, although as seen in figs. 2 and 3 the illustrated example two finger top edge attachment points 48 to the trailing edge 50 of the associated inner bag panel 40 are additionally provided (in other examples there may only be one of these top edge attachments, more than two, or none at all). These additional attachments 48 help to better prevent damage to the fingers 22 from snatching and water scooping when operating in rough conditions.

By using the tiled inner bag 28 removal and replacement of the fingers 22 becomes much easier than with a conventional bag and finger skirt because the finger attachments 48 can be accessed from above by lifting the forward edge of an adjacent panel 40 to access the attachments on the drooping aft edge 50 of the panel to which the finger in question is attached. Hand access is also facilitated through the air feed holes 46. This allows maintenance personnel to enter the skirt system via hull access points and unfasten finger attachments without having the weight of the skirt bearing down on top of them. This makes it relatively easy to release the attachments and remove the finger 22. Similarly, the attachment of a replacement finger 22 can be achieved relatively easily. If an inner panel 40 is damaged or worn, it can be replaced individually without the need to remove and replace a large unitary panel. Various variations and modifications to the specifically described example are possible within the scope of the invention.