View Full Version : Skirt Hole Mod

peterd51 - n/a
12-Jun-07, 07:04 AM

I've read the earlier thread about varying the size of the skirt holes and wondered about altering mine...

it's the pocket type of skirt and has 59 holes all the same size, about 3 inches.

As I'm changing the deck panels I could glue some wood or plastic over the inside of the existing holes to reduce the diameter. Possibly all the holes across the back, the diagonals and, say, three along each side? Allowing the holes to increase slightly along the diagonals and then more so along each side?

Would anyone consider this to be a good idea please?

If so, should it be a large square of material, say, 3.5 x 3.5 inches with a round hole or could I get away with a simple strip, say 1" x 3" that would leave the hole round at the bottom with a straight bit across the top?

I can't see where it would cause any problems with airflow even if it caused some turbulence.

Final questions on this topic...what sort of pressuse could I expect at each hole and how could I measure it please?

I have engine compression testers but I guess these measure too high?



Sean Atterbury - n/a
12-Jun-07, 08:09 AM
I too have had the same thaught, and am going to do the same with just chucking in a strip of wood to block off about holf of the hole.

HOWEVER...... http://hovercraft.org.uk/images/icons/smiley_icons/icon_twisted.gif HERE WE GO AGAIN.....

What if we put a flap that covers the hole almost completely and hinged, then polace a spring onto that so that when pressure under the craft drops, like when plough in is immenent (?), then the lack of pressure will alow the holes to close and let more air go towards the fromt or whereever it is needed most.... http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

Will this work or am I thinking too hard again. I know .. I know... What about the weight????? bla bla bla... http://hovercraft.org.uk/images/icons/smiley_icons/icon_sad.gif

Just a thaught.... http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

tonybroad - n/a
12-Jun-07, 08:29 AM
i don't think there are any definite answers to your question as skirt pressure is often dictated by the weight of you and your craft and the contact area of the skirt - 3" holes all round is not a bad start

skirt pressure is a lot lower than people expect and certainly the compression tester is out! 550-600 Pascals(0.08psi) is a regular amount of pressure for a skirt and can be measured with a simple manometer(U tube)which you can make yourself and is pretty accurate

get a length of clear tubing and form a U bend with 3-4" height in the parallel section, fix it to a piece of wood with a base so you can stand it up - blank off the end that is going to be probed into the skirt but drill a few small holes through the side of the tube near the blanked off end, fill the tube with coloured water until it fills the U and mark off a scale in inches

with someone in the craft, stick the manometer either in the plenum or skirt or underneath the craft and when it is hovering look at the difference in height of water in the U tube - every 1" height of water equals 500 Pacals pressure

perhaps do a pressure check before you start any modifications

hope that helps


Paul Fitz - n/a
12-Jun-07, 07:42 PM
Perhaps I can modify Tonys' suggestion a little.

First, 1 inch water is equal to approx 250 Pascals. As Tony suggests the typical cushion pressure of a single seat is usually between 350-750 pascals dependent on total load (Mass), ie craft weight plus driver weight.

Assuming a cushion pressure of about 500 Pascals the depression in the U tube will be 2" which is actually quite difficult to graduate. If one leg of the U is modified to slope at 45 degrees This effectively doubles the movement to 4". It will be important to ensure that the u tube is kept upright when measuring. You will require a tube leg length of approx 9-12 inches to get practical values.

To measure static pressure it is important that the end of the tube is at 90 degrees to the direction of any flow. Tilt it slightly so the tube 'scoops' any air and the reading will be wrong. For this reason it is better to connect the flexible tube to a piece of rigid e.g 1/4" od Stainless or CuNi with the end cut square. I put a 90 degree bend in this tube as it makes it easier to judge the position and angle of the unseen end of the pipe when using it. Velocity pressure can be measured by facing the right angled tube into the airflow. The actual velocity can then be read off of a table.

The next meeting I can get to is Whittlesey. I will bring a meter with me should anybody want to play. http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)

peterd51 - n/a
12-Jun-07, 08:02 PM

thanks for the desciptions.

It's been a few years since I used a manometer but it sounds easy enough to make one.

I guess I'll be placing the 'probe' just outside the skirt holes with the air blowing past the end of the tube?

I also guess that I can do some initial testing with the skirt off and take readings at each hole. That way calibration wouldn't matter too much as it's simply comparing one reading with another?

I'll be a few weeks away from this point as I've not finished re-building the engine yet and I'd need to get the deck fully fitted to do these tests.

But if the pressure is that low I assume I could add the masking bits outside the holes? I'd thought they'd need to be inside as the pressure would be much higher and it'd blow them off.

It's given me some points to think about...thanks!