These pages are are posted with friendly permission of 4wings.com Hovercraft design
THE BAG AND FINGER SKIRT
The integrated bag and finger skirt compromises between the flexibility of the finger skirt and the economy of the bag skirt. It is
the most complex and sophisticated Hovercraft skirt, it is a combination of a pressurized bag skirt and a finger skirt. This
combination uses the positive aspects of both skirt designs while only implementing marginal disadvantages of the finger and bag
skirt. You will see
this skirt design in nearly all commercial craft and some noncommercial. The advantages are a relative smooth ride over most
uneven surfaces with the finger section quickly adjusting to the surface contours. Low friction from the small cross section of the
finger compared to the wide and relatively inflexible bag skirt. As shown in several tests it is seldom that you damage several
finger at the same time and it is easier to repair in the field once damage occurs than a bag skirt. Even with the partial loss of up
to 3 finger your craft will fly safely. Most of the time you can get your craft at a save location if not even back to your home base or
trailer. It is easier and more inexpensive to replace some finger than a whole bag skirt of a craft. All patches and repair on a bag
skirt will only be quickly chaffed off if they are close to the ground contact line. While a replaced finger has the same ground
contact shape as all the other fingers it will not be worn off quicker than the surrounding finger.
Reduced "plow in" compared to a racing finger skirt provided by the upper bag section of this skirt. The bag section is not only a
flexible attachment for the finger but it is also a pressurized flexible cushion to the craft. The bag section provides higher stability
for the craft while absorbing great forces to the bow and side section just before "plow in" occurs. In the integrated bag and finger
skirt ( short B/F-skirt ) the bag section can be easily drained once water found its way in. Several small drain holes in the bags aft
section will allow the water to drain out of your skirt and hold craft weight low.
Where there is a lot of sunshine is always some shadow. The biggest disadvantages of the B/F-skirt are the high weight, large
amounts of material used and lots of labor required to build it. Because of these downsides this skirt design is not at all useful for
racing craft. A craft mostly used over ice or smooth water would also not benefit from the B/F-skirt. For all cruising craft with a
mixed use of land, water, ice and uneven terrain the skirt would provide the greatest benefit. If you use your craft over partially
frozen bodies of water you may also reduce damage from floating ice.
As you see the skirt selection is chosen from the final application of your craft.
Building sequence:
You can break down the building sequence into five major steps :
1.) Dimension of bag section and finger.
2.) Cut and mark the fabric for final glue joints
3.) Glue the finger to the bag sections and bag sections to each other
4.) Apply to craft and adjust bounce web.
5.) Set your craft on temporary feet, start lift and mark ground contact line on finger, cut in shape and enjoy flying.
Sequence 1.)
Before you can get your bag and finger cross section you have to determine your actual hover height which should be about 10%
of your craft width.
Once you know your hover height the bag section should provide 10 - 20 % and the finger section the remaining 80 - 90% of your
final hover height.
Hold in mind that your bag section has to provide a section which is parallel to the surface to attach your finger.
Once you have the shape you can either make a temporary mock up to get the dimensions or follow bag skirt and finger skirt
pages for the dimensions.
Bag section molds
Sequence 2.)
Take the dimension (length ) from the lower skirt attachment line and top skirt attachment line of your craft.
Mark and cut the fabric as well as mark the final glue joints. Start with the side sections of the craft , and mark the bag skirt section
first, remaining fabric can be used for finger. On the outer surface of the bag skirt panel mark the location of your finger - it is
useful to start from the CL of the bag and work yourself to the aft and front corner. Leave the last 1 ½ ' ( or 50 cm ) open for the
final corner finger - which can be easiest done if your bag skirt panels ( side and front / aft ) are temporary glued together. Mark
the corner glue joints and don't forget to allow at least ½" ( or 1.2 cm ) for overlap and glue joint.
Turn the bag skirt fabric around and mark location of the bounce web.
Once you have all lines on the fabric, make a template of a single finger and mark to the left over material. Satisfied with the layout
start cutting all the finger and bag skirt sections.
Sequence 3.)
Lay the bag skirt sections (sides of craft) on a flat surface and make sure that there are no wrinkles in the fabric. Glue every
second finger ( reference point A ) to your bag skirt section. Allow enough time for the glue to set before bending them over and
glue one side section ( reference point B ) to the bag section. Do not attempt to glue both at the same time since this would loosen
the wet glue joint. After the glue has adequate set apply the last finger section ( reference C) to your bag section.
Top= Finger / X-X-X- Line = folding line top = a, left = B , right = C
Bottom template for finger
Once you are done with the first set apply the remaining finger ( every second one) on glue joint (reference A) to the bag section.
Allow at least 24 hours for your glue to set before applying the side sections of these finger in the same building sequence as
described above for the first set. Before gluing the finger to the front and aft sections of your skirt verify your CoG on the craft and
be sure that the fingers ground contact line will have an equal distance ( CoG to front ground contact line as well as CoG to aft
ground contact line.) to the CoG. Only if this is the case start gluing the finger in the same sequence as on the side panels of the
skirt.
side front and aft finger attached, ready for corner layout. Once all fingers are attached to your bag skirt section, decide on a nice
clean layout for the corner finger. This is best done when you glue the corners of the bag skirt along the lower corner together
while the fabric is still laying flat on a surface. Even if it is very time consuming to apply the corner finger, if you rush now you will
increase friction of the craft. Once you have all finger in place you can turn your skirt around and it should stand on all the finger
45 degree pointing upwards. Apply the bounce web to the inner side of the skirt and if you are 100% confident that the
dimensions are correct , glue as well to the top craft attachment line (not recommended). Now is the best time to glue all corner
joints on your bag section. Now it is more or less done deal.
Sequence 4.)
Once your skirt is all glued and well dried, you can start to attach it to your craft, start with the aft lower craft attachment line and
work yourself forward. Once the skirt is attached to the lower hull set your craft on a leveled surface. Now the bag and finger skirt
should lay quite flat to the floor and it is the best possibility to attach the bounce web ( temporary with SS staples) before you fold
the bag section over and attach to the top hull attachment line. When the skirt is all sealed it's time to start your lift unit and apply
pressure to the skirt, the bag section should inflate while the finger should slowly expand and hold the air under your craft. Most of
them will be tucked under the craft and show wrinkles in the ground contact line. If you have no major wrinkles in the bag section
and the appearance is more or less smooth it's time to get to the final adjustment of the finger.
Sequence 5.)
Final adjustment. Try to support your craft on a leveled surface to actual hover height. The easiest way is to let the lift unit lift your
craft and slide precut temporary feet under the craft. Once you have supported your craft even, reduce lift pressure to just over
idle. This should provide enough pressure to hold the bag section inflated and apply a small amount of pressure to the finger.
Mark all finger along the ground contact line and shut lift unit down. Once the pressure escaped from your skirt it is relatively easy
to cut your finger in shape. Just for safety reasons leave at least ½" (1.2 cm ) from your marked ground contact line to the actual
cut.
Once all finger are even the required air volume should be less and friction between surface and finger nearly nothing. Start your
lift unit and your craft should fly gently off the temporary feet, ready for your first test flight.
Various methods can be used to attach a skirt to the hull. For finger skirts the inner attachment is either a regular tie-wrap, a synthetic
fiber rope or simply a piece of wire. On the outside the attachment method is identical to the one use on both sides of a bag skirt.
The most common bag skirt attachment is by simply holding the skirt between the hull and a piece of flat aluminum. Every 4-6" a
screw tightens and holds the assembly together and fixes it to the hull. This method is simple, parts are available everywhere but it's a
pain to remove and attach the skirt as a normal 14'x7' craft will require the builder to use around 150-200 screws! Imagine you want to
replace the skirt and you have to remove all of them.... After time the screw holes get bigger and bigger and if regular screws are
used they start to rust. If mounted into wood care has to be taken that no water can intrude into the holes and allow the wood to rot.
Some typical Universal Hovercraft and Sevtec attachments:
Our fellow club member H. Harold Carter detected a nice material which makes the skirt attachment easy and durable and easy to get
it off again!
A material called Poly-Fastener, made by Curry Industries Ltd. 19 Burnett Avenue Winnipeg, Manitoba, Canada R2G 1C2, is the
solution to an old hovercraft problem. Their web site is: http://www.escape.ca/~curryind/ .
Poly-Fastener was designed to attach Polyethylene or other tarps to greenhouses and other more or less temporary buildings. Based
on the tough holding requirements of buildings under windy conditions one can imagine that the holding forces of the attachment
strips have to be very high.
The material looks like a very strong PVC and abrasion and "holding" tests were passed with glory!
The C-channel (2) can be attached to the hovercraft hull with 3M 5200 sealant and stainless screws. This makes it not only watertight
but also stick to the hull extremely well. The small "lip" on one side of the C goes either upwards (outer skirt attachment) or to the
inside (inner skirt attachment). Make an alignment line on your skirt, lay it over the channel and push the cover (1) into one side of the
channel (2) (left side on the picture above). Make sure your alignment line is still where it should be, i.e. at the "corner" of the C. Now
push the cover (1) into the other side of the channel. This can be either done with your fingers or even better with the tool (4) which is
also supplied by Curry Industries. This tool is very important to have if you want to take the skirt off again! The job can be done with a
screw driver and your fingers but it's dangerous for both, yourself and the skirt........ You want your skirt to hold, well, it doesn't release
it that easy.......
Poly-Fastener is available from several sources. If you want to buy online a search in google with the keywords "poly fastener" will
come up with several suppliers (mainly for white material but black is available too). Cost of the fastener is reasonable too, it ranges
from $0.65 to $1.00 per foot and it's available up to 100 ft length in several widths. The most usable width for our applications should
be 1.5-2".
35 lbs on only four inches of Poly-Fastener!
Glued to the surface with 3M 5200 Marine Sealer.
© 2002, K. Juergen Schoepf
web-site © 2000-2002 by THCC
This page shows how skirt corner can be glued. The calculation of the shape is difficult due to the pressure differences but if the
shape is known here's the trick how to get them together........If you know the required shape you can find the math on the Skirt
Sections page.
Label text:
1. Do not cut tabs within 1/8", 3mm, of outline. Example:.... This will eliminate possibility of tiny holes at base of tabs.
2. Draw pattern outline on this piece and then draw - freehand - the cut line one inch, 25mm, away.
3. Note: cut wedges from tabs to prevent overlap of tabs when folded over. Outside curve only.
Label text:
4. Dry fold tabs over to establish glue line on top piece. Apply adhesive per directions. Carefully pull tabs over including the 1/8" and
press together. Use laminate "J" roller if available to get best bond possible.
5. HH66 glue for Vinyl coated fabric
6. Adhesive 628 for Neoprene
7. All tabs not glued down for clarity.
The left side of this picture shows some samples of the Poly-Fastener discussed in the Skirt Attachement page.
© 2002 H. Harold Carter
web-site © 2000-2002 by THCC
These pages are are posted with friendly permission of 4wings.com Hovercraft design
THE FINGER SKIRT
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 form 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 better obstacle clearing characteristics 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 other than over smooth terrain.
CHARACTERISTICS OF THE FINGER SKIRT
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 practice 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, simply mark around the template and cut out as many fingers as are necessary to evenly fit across
the bow of your hovercraft.
TO CONSTRUCT AN EXTENDED FINGER SKIRT.
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 center 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.
(c) 2000, Michael Dolezal, 4wings.com
These pages are are posted with friendly permission of 4wings.com Hovercraft design
HOVERCRAFT SKIRTS
Why have a skirt?
All modern hovercraft, large and small, use a skirt of one sort or another for their suspension system so that the power required to
lift the craft can be minimised.
A hovercraft skirt is required to fulfil the following functions:






Contain the cushion of air beneath the craft at the required hoverheight.
Have the ability to conform or contour efficiently over obstacles so as to keep to a minimum, the loss of cushion air.
Return to its original shape after having been deformed.
Give adequate stability to the craft.
Offer little resistance to the passage of obstacles beneath it.
Have the ability to absorb a large proportion of the energy which is produced on impacts or collisions with obstacles greater
than hoverheight or cushion depth
A hovercraft skirt should have the following features:





Be easily maintained on site without the need to lift or jack-up the craft.
Have a long operating life.
Be relatively simple to construct and fit.
Have a low maintenance cost. The initial cost of making the skirt may not be very low but it is important that once made
and fitted, the skirt be inexpensively maintained.
Be tailored so that it is even in height above the ground all the way around the craft. One part of the skirt should not drag
whilst another edge is higher and not contacting the ground.
SKIRT DESIGNS
There are several major designs of hovercraft skirts but only the three main types will be
presented in the following pages. Each has its own applications.
HOVERCRAFT SKIRTS
The stability of a hovercraft is dependent upon the pitch (fore and aft) and roll (side to side) stiffness of the air cushion. This
stiffness is derived from two main sources:


The bag skirt employs an inflated bag surrounding the air cushion and it is the pressure of the bag which provides the
stability.
All three designs use the movement of the centre of pressure on the collapse of the skirt to provide stability, ie., when the
skirt crumples as the edge of the craft drops, the effective contact point where the skirt touches the ground moves.
Therefore extra cushion area and thus more lift is provided at that side, moving the centre of lift pressure over and tending
to raise the craft to restore it to a level position.
DESIGN SEQUENCE
The first action when designing a skirt is to decide upon the main use to which you will put the hovercraft and the type of terrain
over which you intent to operate it. For a cruising craft that will be operated over water you will probably opt for the smooth but
stable ride of the bag and finger skirt. If you fancy yourself as the driver of a racing hovercraft you may choose the full finger skirt
which has very low drag characteristics or maybe a straight bag skirt if racing on terrain where hull damage would otherwise occur
if a soft skirt were used.
The next step is to develop a section through the side skirt geometry, deciding where to put the hull structure to pick up the skirt
and remembering the hydro-dynamics of the hull form. The skirt and the hull shape must be designed together. Finger skirt hulls
generally have rounded corners whereas those fitted with bag skirts usually have square or angular corners. At this point in
designing a hovercraft a great deal of variation in the exact shape and size of the skirt and structure can be considered until the
happy medium is found. The bow shape can now be developed in a similar way.
The final stage is the development of the tailoring, splitting it into a sensible number of panels which, when sewn or glued together,
adopt the shape required. These panels should be sufficient in number to enable the skirt to look smooth and free from stress
when it is inflated. There should, however, be few enough panels to make construction of the skirt fairly simple and straightforward.
Finally, you must decide on the type of material to use for constructing the skirt.
(c) 2000, Michael Dolezal, 4wings.com
web-site © 2000-2002 by THCC
This page is provided with permission of Ronald van der Wijden , the original can be found on
http://www.geocities.com/ronaldb66/index.html.
Some remarks to the math: This approach to derive the skirt pattern does not incorporate the venturi effect in the gap nor the
pressure profile which will form from about 10% inside the skirt to 10% outside the skirt. These two effects will have impact on the
skirt shape in "real life", thus it will be required to tune the skirt. For full size crafts it may be a costly experiment to cut and glue
several skirts to achieve the best performance. For model hovercrafts however this approach should lead to reasonable results.
Skirt geometry
First, let's start with the cross section of a typical bag skirt. Although actual sizes vary, depending on the size of the craft and on
chosen design parameters, uniform elements can be recognized:
Skirt cross section
The curved line is the actual skirt;
A is the point where the skirt is fixed to the hull outside, B is a fixation point under the hull. Although not drawn, allowance should
be made for attachments, i.e. an overlap.
G is the ground contact point; in theory, this should lie slightly within the outer edges of the hull, it should not extend beyond the
outer edge of the hull for stability reasons. To simplify things, it is drawn on the outer edge in this example.
The cross section shape is made using two intersecting circle segments: R1 is a semicircle that defines the portion of the skirt that
"bulges" outside the hull when inflated; R2 is (in this case) a quarter circle that continues R1 up to the skirt attachment point B.
The radius R1 is determined by the desired ride height for the craft, typically this is between 1/8 and 1/10 of the craft's width; The
radius of R2, and thereby the width W, depends upon the ratio between the skirt pressure and the plenum pressure. The skirt
pages on this site give more information on determining these values.
In the example drawing, a ratio of 1:2 is used, which is an unrealistic value, but serves to simplify the example.
Finally, ride height H follows from 2 x R1.
Segment length
The total length of the combined curve can be calculated using
C = 2 x R x pi
where C is the circumference of a circle with radius R and pi is the constant 3.1415926...
For calculating the length of a circle segment, the angle is needed; when using degrees, divide the angle by 360 (full circle) and
multiply the obtained factor by the circumference of the full circle.
For the R1 segment, this simply is half the circumference of the full circle, since it is a semicircle; L1 = R1 x pi.
The R2 segment in the example is a quarter circle, so that would be L2 = 1/2 x R2 x pi, but for other ratios it's a bit more
complicated; let's look at a more generic procedure.
To determine how many degrees the R2 segment is, we use the following diagram:
Circle segment
Shown here is a circle segment with radius R and angle A.
S is the distance between the skirt attachment points on the hull of the craft (see the "skirt cross section" diagram), H is the ride
height.
Radius R is calculated earlier on, but we'll have to determine angle A. Shown in the diagram is distance C, for which applies:
C = R - H. C now is the cosine of angle A times there radius R:
C = cos(A) x R
or,
cos(A) = C / R
and therefore
A = arccos(C / R)
Arc cosine is the reverse of cosine: arccos(x) is the angle of which the cosine is x.
Now, the length (say, L) of this circle segment follows from
L = 2 x R x pi x (A / 360)
The total width of the skirt material needed follows out of adding the calculated lengths of the two circle segments.
The distance between the skirt attachment points, S is the sine of angle A times the radius R:
S = sin(A) x R
Laying it out
Shown in the following diagram is a section of skirt laid out flat:
Skirt layout
W is the total width of the section (not incorporating seams or other overlap!), which is composed out of L1 (the length of the outer
circle segment) and L2 (the length of the inner circle segment). Also indicated are the skirt attachment points A (outside) and B
(inside), the ground contact point G where the two circle segments meet, and the point M, which is halfway A and G; this is the
point where the skirt sticks out the furthest. The significance of this point will show later on.
If we bring this section into skirt shape, the top view will look something like this; for more clarity, on the left a side view is added:
Skirt shaped and cut through
In the diagram, the slanting line S supposes the skirt section cut through at 45 degrees. This cut runs through point P, where it
meets reference line R.
Now, imagine traveling along the skirt edge starting in point A, towards M. The distance we would be away from A in the horizontal
plane (although drawn vertically here) we call D1. D1 increases until we reach M; after this, going on to G, D1 decreases again.
Traveling on to B, D1 increases again, but in the other direction.
For every point on the skirt, a distance D1 can be determined and plot onto the reference line R.
Suppose the skirt was cut like line S, and we would make the same trip along the edge; for every value of D1, we'd be away from
line R at distance D2. Starting in A, D2 would be zero, increase as we reach M, decrease again to zero in G, and increase but in
the other direction as we move towards B.
It's that distance D2 that is the key to drawing out the desired cutting line for our skirt. To be able to relate it to a certain point on
our skirt material however, one more step needs to be taken.
Edge Profile
Determining where we are on the skirt edge uses trigonometry again. Look at the following skirt side view (which should be familiar
by now):
Skirt side view
Of any point P on the circle segment R1, the position can be determined exactly using angle A. The same applies to point Q on R2
using angle B.
To simplify things, we look at both segments separately.
First, let's tackle semicircle R1:
Circle segment R1
Again, the location of point can described by using angle A. What we are interested in are:


the distance D1 perpendicular to reference line R;
the length of the segment between starting point S and point P.
We need D1 to relate point P to the cut we want to make through the skirt later on, and distance S-P to figure out where on the
skirt material we are.
D1 can be calculated with
D1 = R1 x sin(A)
and the length of S-P with
SP = 2 x R1 x pi x (A / 360)
as described previously in "Segment Length;".
Analogously, we follow the same approach for segment R2:
Circle segment R2
Again, distance D1 follows from
D1 = R2 x sin(B)
and the length of the skirt segment from ground contact point G to point Q follows from
GQ = 2 x R2 x pi x (B / 360)
Cutting the skirt
Let's bring the skirt top view we saw earlier on back:
Skirt shaped and cut through
Now we can chose points on the skirt circumference, calculate where on the skirt they are and determine distance D1; what's
failing is the matching distance D2.
In the diagram, cutting line S is drawn at a 45 degree angle to line R, which is perpendicular to the skirt section. Such an angle
would make the skirt follow a 90 degree hull corner.
At 45 degrees, each distance D1 is paired to the exact same distance D2:
D2 = D1 x tan(45)
with tan(45) being the tangent of a 45 degree angle, which is 1.
Notice that to have the skirt follow a X degree corner, the adjoining skirt sections both have to be cut at X/2 degrees.
Naturally, any angle other then 45 can be used; for a 45 degree corner, the intersection angle would be 22.5 degrees, for a 30
degree corner it would be 15, etc.
To construct the cutting line, make a list of suitable points across the entire skirt edge; 15 degree intervals are a decent choice.
For each point (angle), calculate:



the length of the circle segment from the starting point;
the distance D1 as shown in the above diagram;
the paired distance D2.
With the list, the skirt cutting line can be drawn. An example:
Below, a skirt side view is shown with a number of points on the perimeter. For R1, one inch is chosen; R2 is two inches. On R1,
the points are at 45 degree intervals, on R2 they're at 30 degrees. For the skirt intersection, 45 degrees is assumed.
Of course, for an acceptable precision more points need to be determined, but this small number of points is enough to explain the
principle.
Points on the skirt perimeter
From this, the following list can be drawn up:
Point
Angle A
Angle B
D1
Length
D2
P0
0
-
0
0
0
P1
45
-
0.71
0.79
0.71
P2
90
-
1
1.57
1
P3
135
-
0.71
2.36
0.71
G
180
0
0
3.14
0
P4
-
30
1
4.19
1
P5
-
60
1.73
5.24
1.73
P6
-
90
2
6.28
2
Notice that the skirt segment length for point P4-P6 is the total, including the R1 segment length; the value 6.28 for point P6 is
therefore the total width of the skirt material to be used.
With these values, we can draw the cutting line for a skirt section, intersected at 45 degrees:
Skirt cut for 45 deg. intersection
In this diagram, the points are connected by straight lines; of course, for a more accurate cut a more flowing line should be drawn.
Distances D2 are measured from reference line R, skirt segment lengths are measured from starting edge S.
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