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.
four 90° angles
two 90° angles at the rear
four 135° angles at the front
two 90° angles at the rear
six angles at the front (3 different values)
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.
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.