4wings.com Hovercraft development 47751 Michigan ave., Port Isabel - 78578 - Texas, US Tel.: (956)943 5150


Hovercraft development
47751 Michigan ave. , Port Isabel, TX, 78578
Tel:(956) 943-5150

The finger skirt is comprised of a large number of separate segments which are able to slide and bellow individually to conform with the shape of the water surface or terrain over which the craft is traversing.

The fingers should not be sewn or connected together in any way or else the design of the skirt which is to form a very flexible air seal between the hovercraft and terrain, will be defeated.
To achieve this, the fingers should only be attached to the craft by a rigid strip along the upper edge and by some from of corner tie at the two innermost apexes.
The straight finger serves well as a bow skirt due to the advantageous receding angle of attached which helps in riding over waves and ground undulations and at keeping frontal spray and dust to a minimum.  The straight finger however, is very unforgiving to being dragged backwards over obstacles.  The extended finger is best used at the side and rear of the hovercraft as it has a better obstacle clearing characteristic and also its design helps to maximise the effective cushion area because the ground contact point is much further out towards the hull perimeter than for that of the straight finger.
The inner fact of the finger skirt is completely open which means that apart from a very marginal increase in pressure due to the peripheral jetstream, is a very soft and flexible with only actual cushion pressure holding it firm.  This is  vastly different to the bag skirt which has a peripheral pressure much greater than cushion pressure.  The finger skirts at the rear of the hovercraft must be equipped with either a single antiscoop flap or for improved flexibility, individual strips may be sewn to the inner edges of each finger which extend out belong the ground contact point.  This reduces their efficiency  in retraining cushion air from escaping, but unfortunately is quite essential if the hovercraft is to be used anywhere than over smooth terrain.

To get a basic idea about the flexibility and difference of the finger skirt in relation to a bag and finger skirt as described at : "Bag and Finger skirt" take a look at the following clip out of 007 and pay close attention to the smaller crafts with finger skirt in relation to the bigger craft with a bag and finger skirt ...lol and not so much to the action ..lol

The straight finger skirt is easy to design and construct and repair work is also easy because of the small size of each segment.  The extended finger however is a little more difficult to design .
The finger skirt, straight or extended, gives a very smooth ride and has a low friction characteristic and so long as the cushion height is adequate,  is ideal for travelling at high speed over waves and rough ground.  Apart from the rear fingers which should have an anti-scoop flap, a hovercraft with a finger skirt does not trap water like a bag skirt at lift off.  An extended finger uses a great deal more material than a straight finger or bag skirt and for this reason, it has become common practise for the expensive low friction neoprene coated fabric to be used only on the knuckle which constantly rubs across the on-coming waves and for less expensive non-coated fabric to be used on the bellow and sides of the finger.
To construct a Straight Finger Skirt.
Straight Fingers are best used at the bow of a hovercraft.
To design a straight finger you must draw the cross-section of the skirt under the bow and make a paper template to this shape.  Draw a rectangle of width equal to half the hoverheight and of height equal to the skirt length from the nose of the hull to the ground contact point.  Duplicate the cross section pattern and lay these alongside the rectangle.
Allow for a seam at the top of the rectangle and then using this completed shape, make a
full template out of plywood.
To make the skirt, simple mark around the template and cut out as many fingers as are
necessary to evenly fit across the bow of your hovercraft.

Extended fingers are best used at the side and rear of the craft although it is common practise to use them as bow fingers also, in lieu of the straight finger which has a set back ground contact point.

To draw a development, you must draw the cross  section of the skirt under the bow and make a paper template to this shape.  Next draw a rectangle of height equal to the straight fore edge of the cross section and of width equal to half the hoverheight.  Transpose the cross-section shape along both sides of this rectangle  and then draw in two smooth curves from the centre of the top of the rectangle out to the outer hull contact points of the two adjoining skirt cross-sections.

The extended skirt knuckle is developed by extending two curved lines upward from the centre of the top of the rectangle to intersect with a horizontal line equal in width to half the hoverheight and at such a height that the distance along the outward radiating curves is exactly equal to the upward radiating curves.  The lengths of these curves must be equal as they represent the common stitch lines on the finished finger and the hull contact point on the cross-section shape and the top of the knuckle must come together at the same point.  Allow for a seam at the top of the knuckle and then using this completed shape, make a full template out of plywood.  The knuckle template will naturally be separate from the lower part, but when making the skirt, it may be an advantage to leave a web of material between the two curve lines on each side to facilitate stitching.

If you use straight fingers on the bow, you will need to make approximately three fingers on each side with progressively more knuckle bellow to accommodate the transition between the straight fingers and the extended fingers.

Crossection of a partially removed / installed finger skirt

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