Hovercraft Skirt Design and Construction Tasks 

    A hovercraft skirt is an air-filled membrane that inflates and conforms to contours of the terrain that is mounted on the bottom of a hovercraft. The skirt confines the pressurized air that supports the weight of the craft.  The pressurized air, supplied by the lift engine, enters the skirt through ports in the side of the hull and exits via a series of holes in underside of the skirt under the craft.  The air then passes out from under the vehicle in a gap between the ground and the skirt. This page contains design documents that outline the decision to make steps used to design and construct the skirt for the hovercraft. 

Task Execution

Task one: Acquire technical data. This task had the following requirements. First we had to then get dimension of current hovercraft platform and then get material properties of the skirt.

The information that was left from the previous senior class includes detailed 3 dimensional models of the hover craft, their senior paper, which contain the formulas they used to determine lift and skirt pressure. Three two dimensional drawings have been created that show the exterior of the hovercraft.   The side of the hovercraft where found to be symmetrical with small deviations of about 1/16 of an inch.  Three 2 dimensional drawings where created.  There original purpose was for planning of the skirt design and documenting where the existing exhaust ports are.  The original three dimensional model from the previous year was modified so it reflected the actual condition of the model.  This entailed changing the angle and configuration of the inner ribs.

Task two: Skirt joinery; determining the best method for joining skirt material. For this task we need to research the joining of the fabric, de-construct the existing skirt, research using a compound to joining the seem, test the fabric joint which would include establish joint testing protocol, establish the strength different types of joints, build fixture for test and test old skirt joints. Then test proposed new skirts joining and from there, decide the best joining method.  


Seem Strength Testing 

        The purpose of this test was to determine if the force on the seems of the skirt could withstand the force created by pressure inside the skirt.  The experiment had the following setup.  The material used in the skirt-hull attachment experiment was cut down the middle.  Each end was folded over approximately 1” and then overlapped.  Three sewing lines were used.  Two on the very edge of each seem, and one seem down the middle.  A standard Singer sewing machine was used with 100% Polyester thread.  The top beam was suspended between two doors and a load was hung from the bottom beam.  The load amounted to 337 pounds, which created a pressure of 28.1 lbs/in (Eq. 8).  The hovercraft skirt will have an estimated 1.87 psi (Eq. 7, r =8in.).  Equations for the calculations can be seen in the Hull-Skirt Calculations Section.  The seem is approximately 15 times stronger than needed, and was still not tested to failure.  In the final design the skirt seem will be sewn using Nylon thread because it is stronger, which is not a factor, and it is more resistant to the affects of the environment, rot, and wear and tear due to friction and rough terrain.  


This is the seam used to join the fabric together.






This is a picture of the design used to test the seam.  The fabric was sandwiched between two pieces of wood on both sides then stretched between two doors with an upside down stool to hold the weights, 337lbs.






This is the design up close.






[1] S. Timoshenko. Strength of Materials. 3rd edition. Huntington, NY, Robert E. Krieger Publishing Company, 1976. Pg. 31

 Task 3: Determine the best method for attaching skirt material to hull. First we need to look at last years approach and then determine the failure strength of the skirt using a chosen test to determine failure strength.

Last year the class of 2007, determined the best skirt material for their current hovercraft. Their document is shown in this document:

Class of 2007 Final skirt write up.


Hull Attachment Tests

        The previously designed skirt to hull attachment was tested to determine if it is acceptable.  The design is to sandwich two pieces of wood around the skirt and using one screw over every two inches to fasten it to the hull.  See Fig. 2 for a design sketch.

            The test was done by making a U shaped basket with the skirt sandwiched between two 1”x2” pieces of wood with six screws over one foot.  The loaded weight weighed 267 lbs.  The test was loaded with all the available weight without failure.  See Fig. 3 for a sketch of the test design.  See Fig 4 through 8 for experiment pictures.

            The derivation of the force acting on the joint between the fabric and the hull is based on the derivation of hoop stress given by S. Timoshenko[1]. The force is found by integrating the internal pressure over the cylindrical surface of the inflated skirt. This is shown in equation1 above. Where  is the length, is the radius of the cylindrical of the inflated skirt, and  is the force per unit length acting along the joint. 

The calculated pressure under the hull is calculated to 0.11psi.  This pressure is calculated assuming the craft weights 450 Lbs and covers 28 ft2. Perozzo[2] states that the typical pressure in the skirt is 20% greater then the pressure under the craft.  The estimate of the skirt pressure is 0.132 psi.  Converting to a force using equation 2 from above.

The force on the skirt is 1.76 lbs/in minimally.  The calculated force per unit length from our design is 11.125 lbs/in.  This experiment shows that the method of the skirt to the hull attachment is ten times stronger then needed.  See calculations attached for derivations of equations shown above.

< Figure 2

Fig. 2. The sketch shows the attachment of the hull to the skirt.  The skirt is sandwich between the two pieces of wood.

<figure 3

Fig. 3. This is the design set up to test the strength of the hull to skirt joining.  Once the force is found, it can then be converted the psi to determine if the design is acceptable.



Fig. 4.(above)  Experiment picture, top view.

Fig. 5. (above)  Experiment picture, side view.



Fig. 6.  (above) Experiment picture, top view.


Fig. 7.  (above) Experiment picture, front view.

 Fig. 8. (above)  Experiment picture, loaded with all available weights.


[1]         Timoshenko, S. and G.L. MacCullough, Elements of Strength of Materials. 1949, Toronto: D. Van Nostrand Company.

[2].         Perozzo, J., Hovercrafting as a Hobby. 1995, Bend, Oregon: Maverick Publishions.


 Task 4: Design new skirt. First for this task it is nesscesary to determine type of skirt, and the shape of the skirt. After the design is created in solid works, it is printed and used for a layout of the cut pattern of the skirt material. 

The hull of the craft is 7 feet long, 4 feet wide, and 8.5 inches thick.  The skirt is designed to inflate into a cylinder like shape around the edge of the hull. (See figure #9)  The skirt is made up of four separate pieces of ballistic nylon that will be sewn together at the corners of the platform in a 90 degree miter like joint.  When inflated, the design of the skirt wraps around the sides of the hull from the top edges to 8 inches in from the edge of the bottom. (See figure #12) The design was prepared in SolidWorks as a solid hollow cylinder. (See figure #11) Then it was converted to a sheet metal within SolidWorks so that the surface of the cylinder could be flatten out, and the dimensions of the skirt flattened out could be determined.  The long side pieces are 8 feet 8 inches by 3 feet 6 inches. (See figure #13) The short side pieces are 5 feet 8 inches by 3 feet 6 inches. (See figure #14) This design will require about 14448 in2 or 100.3 ft2 of fabric.  [(8ft-8in x 3ft-6in)x2 + (5ft-8in x 3ft-6in)x2]  Two inches of extra fabric is added around the edge of the each pieces for room to sew the pieces together and to allow attachment to the hull.  The dimension for the exhaust ports to be cut into the fabric will be determined at later date.  The drawing files have been archived and sent to the fabrications team.  Instructions have been included so that the fabrication team can produce full scale templates.


Figure 9

Hover Craft platform with skirt on three sides



Figure 10

Bottom view of Hover Craft platform with skirt on three sides



Figure 11

7 feet side skirt



Figure 12

Side Skirt connect to edge and bottom of platform



Image 13

Blue Print of 7 feet side skirt



Image 14

Blue Print of 4 feet side skirt

Task 5 : Purchasing materials. First, the types of materials, prices and delivery times were compi, get quotes and then submit purchase requests to Ms. Hicks. Then compile a spread sheet of the budget and item costs. Cost Sheet of Hovercraft (through November 29, 2007)


Task 6 Assembly of the Skirt. First we need to find a sewing machine, and then cut material and attach the layout of the cut and sew outline to the material. Then join the skirt together and mount it to the hull.

         The skirt is used for the hovercraft to hover. It is filled by air and creates a cushion for the craft to be supported on. The construction of the skirt is done by sowing the material at the corners given a design that is sufficient for the craft to hover. 

            We traced out the design from a printout and had to trace it on both sides because the curve needed to be the other direction for two of the corners. It was also needed because we can then use the pieces of paper interchangeably throughout the construction of the skirt. Once the paper was pinned precisely to the material, it took more people to guide it through the sowing machine for more accuracy. If there was only one person operating the sowing machine it proved to be difficult to hold the skirt and guide it through the sowing machine while rotating the material at the same time of making sure it is sowing on the line exactly. This process was repeated four times for the four corners and then took the skirt to the craft to observe the precision of the skirt construction compared to the craft. The skirt was bigger than originally calculated but proved to be a minor problem. It can be solved by taking the skirt and overlapping the excess material at the corners to tighten the skirt against the craft for attachment.

            The total time for the skirt to be constructed was a week. It took a maximum of four people at one time to help with the construction.

 Hole Calculations of the skirt