# Tutorial : custom hovercraft bag skirt

### This tutorial focuses on "vented bag skirts", which are a good compromise between ease of design, ease of assembly and efficient operation.

First, if you don't know much about hovercrafts, I strongly recommend you to read this page that contains a bit of theory about skirts.

Special thanks to Ian Smith, who created parts of the following tutorial, for giving me the authorization to complete it and publish it.

The least we can say is that designing a hovercraft skirt from scratch can be confusing. Fortunately, to make things easier, the skirt can be divided into several parts, as shown on the picture above : one piece for each side of the hull. Joined together, these weird parts will give the hovercraft skirt that characteristic cylindrical shape. Although the assembly of a skirt for a bigger hovercraft will obviously be different, the principle is the same :

First of all, choose the material for your skirt : I personally use trash bag pieces, which can be melted and joined together using a soldering gun (you can check out the end of the BR6 DIY guide to understand how it's done), or K-way pieces which can be sewn together. You can use other materials or other fabrics as long as they are resistant enough, airproof and waterproof, while being flexible enough to inflate.

Then, you need to choose what your hovercraft hull will look like. Here are a few examples :

four 90° angles

4 sides

two 90° angles at the rear

four 135° angles at the front

6 sides

two 90° angles at the rear

six angles at the front (3 different values)

8 sides

Logically, the more complex your shell is, the more work and calculations you will have to do : the red hull, above left, will require only one angle template (only 90° angles), and 4 pieces of skirt, as it has 4 sides. The black one, as it has four different angle values, will require four templates and four times more calculations. And as it has 8 sides, it will require 8 pieces of skirt to cut and join together.

Note that the following tutorial also works if you need to design a "vented bag skirt" for a real hovercraft.

**For better understanding, it is recommended to**** enlarge the following pictures by right-clicking on them and select "open in a new tab" or "display image".**

The skirt will be fixed to the hull in two places: under the edges of the hull, and on two strips - polypropylene strips in my case - under the hovercraft. Therefore, you need to set these strips first.

Take the hovercraft hull upside down. With a pencil, draw two lines parallel to the sides of the hull. They must be separated from the edges of the hull by 30% of the hull width.

With a protractor, draw the bisectors of all the angles of the hull. Like shown on the picture, use these bisectors to reproduce the shape of the hull with a smaller ratio.

You can now cut the two strips and fold them in the right places to match the lines drew previously. For small models (up to 40 cm hull width), the height of these strips can be 10% of the hull width. For larger models, I suggest limiting this size to a maximum of 4 cm.

You will notice that the strip is not perfectly straight, and that there should be two strips facing each other - this was a quick draft for me, you have to do better :-)

Note that the front and back of the "vented bag skirt" must be kept open to allow air to escape (see this page for more explanations). This is why we don't need skirt attachments for the front nor for the rear.

You now need to draw the shape of your skirt using drawing software like Geogebra or graph paper. Draw it at 1 : 1 scale to avoid mistakes, you have to be as precise as possible to get a good result.

Draw a horizontal line that represents your hovercraft hull. Draw a vertical segment that represents one of your attaching strips (3 cm height in my case, because my hull is 30 cm large).

Draw a semi-circle, the center of which is exactly below the edge of the hull, and facing the end of the attachment strip.

Join the end of the semi-circle to the end of the attaching strip with an arc.

Sorry to say that I have no specific instructions to position the compass nor to set its spacing, I usually try several times until I get a correct arc. In this case, the compass was positioned at the top of the drawing, right above the center of the semi-circle.

Finally, you obtain a vertical cut of your skirt : this is what your skirt will look like when inflated.

From this single shape, you can design all the corners templates you need, so be sure to save it or copy it if you need. Remember that you will need one template for each angle value on your hull.

Erase the hull but keep the attachment strip.

Draw two horizontal and vertical axes which pass through the center of the semi-circle drawn previously.

Divide the semi-circle into 8 equal circular sectors, each one being 22.5°.

With a compass, measure the length of the arc marked in red. Use this arc length to divide the rest of the skirt. If necessary, add a last point "manually" at the very end of the skirt, like I did.

On a real hovercraft, where the semi-circle must be around 40 cm in diameter, you can go up to 12 equal circular sectors to get more precision - in that case you will get 15° sectors. Of course, this level of precision is not required for models.

In all cases, it is important to divide the semicircle with an even number, to obtain a correct positioning of your points.

I will rotate the skirt 90° to the right to continue the process, but you're not forced to do the same. Draw three horizontal lines like shown and a vertical axis.

In this example, let's say I'm making a template for the rear of the hovercraft, where I have right angles (90°). Whatever the value of the angle on your hull, divide it by two and use this result to draw the diagonal as shown.

Note that the diagonal musn't come too close to your semi-circle, draw your vertical axis accordingly.

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We will continue the tutorial with older drawings but the whole process remains unchanged. Draw a corner above like shown.

We begin by drawing the first mark : draw a horizontal line joining the beginning of the skirt to the diagonal drawn previously, and from there draw a right angle to join the horizontal line of the corner.

Draw a horizontal line above the last point that was drawn and repeat the process to get your second mark.

The space between two lines, noted X on the drawing, is calculated from the following formula : θ × (π/180) × r.

In this example, θ = 22.5 because we divided the semi-circle in 8 equal sectors, and r = 3 because the semi-circle radius is 3 cm.

Therefore, I get : 22.5 x (π/180) × 3 = 1,2 cm.

Once your mark is drawn, erase the two lines that you used to place it, to make your work easier.

Repeat this process for every mark.

Pay attention to the last point that you may have added manually during the previous steps - in that case, the two last lines, at the top of the drawing, will not have the same "X" gap as the others.

Use your compass to measure the distance between the penultimate point and the last point on the profile of the skirt, at the bottom left, and use this length as your last "X" gap. The result may contain a very small margin of error, which will have no bad consequence.

Join the dots, and voilà, you have your template, ready to be reproduced on a piece of cardboard.

As said before, as you need to draw one template for each angle value on the hull, repeat this operation until you have all the templates you need.

You can see here the final result of two 45° corners facing each other. When joined together by soldering or sewing, this part of the skirt will have an angle of 90°.

This is what you get when you have cut out the different parts of your skirt. You can check the BR6 DIY guide pictures for better understanding.

Note that you will need to add a margin to the right and left of each piece for sewing or soldering the pieces together : personally I add a 1mm margin to the right and left of each piece - on the other hand, I didn't ever have the use of margins at the top and bottom of each piece because I use duct tape to set the skirt edge-to-edge under the hull and at the bottom of the skirt attachments.

To go further, the following drawings and picture can give you ideas to get smoother finishes : instead of having only two 45° pieces, you can get a right angle with four 22.5° pieces, by adding an extra piece of skirt in the middle. Keep in mind that you may need a seam / joining allowance on all pieces. See the full tutorial written by Ian Smith for more details (please note that the first part of this guide is outdated).

You now have all the knowledge to design your own vented bag skirt. This tutorial may seem difficult to understand at first but it cannot be simplified more at this point. I personnally learned by doing it a few times and making mistakes at first. Good luck !

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If you have any questions, send me an email at diyhovercraft01@gmail.com.