View Full Version : Bag Skirt attached to Chamber - How ?

17-Apr-11, 03:19 PM

So assuming I want to go with a bag design.
1- Would you suggest using a plenum chamber to evenly distribute the lift air to all of the bag? (Why or Why not)
2- If I were to use such a chamber, how can this be done? I mean, how can the bag skirt be attached to the chamber and the bottom of the craft and allowing at the same time air to exit from it?

Please help me as I do not seem to find any good information concerning bag skirts :(

Your help is greatly appreciated :)

17-Apr-11, 05:04 PM
The hassles I have had with my surveyor bag skirt I would go for segments, but thats just my opinion. There are a lot of people that like the bag skirts better than segments, but I cant think why, except maybe cost.

Keith Oakley
22-Apr-11, 10:06 AM
Keith Smallwood is the expert on bag skirts but he's usually offline in deepest France this time of year so hasn't come in. You can contact him at sales 'at' vortexservices.net - see ad under manufacturers. Its 30 years since I designed and built a bag skirt so I'm a bit rusty but here goes:

When designing how the lift air gets from the fan to the cushion under the craft there are two objectives:
1 Having ducting with a big enough cross section to minimise pressure buildup and thus loss
2 Minimising the effects of wave pumping

Taking 2 first, wave pumping occurs when a wave (or similar land object like a river bank) passes through the air cushion from front to back pushing the air out. The centre of lift will move back compared to the centre of gravity of the craft thus causing the craft to drop at the front (or plough in!) Small racing craft can counter this by the pilot moving his weight around but it gets more difficult with a larger craft. You need lift air to be injected in at the front behind the wave to restore lift.

A craft with a separate lift engine can have this mounted on the bow with the air going straight down into the cushion via large holes in front planing surface. Such a craft doesn't need a plenum chamber to distribute air around the sides of the craft if it uses a bag skirt. (It does if it uses a finger skirt because these ideally need a small feed directly into each finger) The 'no flow' bag here can be a sealed 'tube' fed by a small letterbox scoop under the fan feeding say 10% of lift air directly to inflate the bag. The advantage of this design is that the sidebodies are sealed and don't flood with water when afloat. The craft therefore has more buoyancy and being a wider 'boat' its more stable and less likely to rollover when afloat. The downside is the bag skirt will flood with water when afloat which will need purging when the craft starts up again. Small drain holes in the bottom rear of the bag are a solution but many craft used electric pumps.

Alternatively the bag can be used as a duct to distribute the air, either on its own or with air also passing through the plenum sidebodies of the hull. The choice is down to issue1 - having a big enough cross section to minimise back pressure and thus airflow loss. Bag only gives the benefit of better buoyancy and stability afloat but too big a bag cross section runs the risk of either too much height, leading to instability when hovering; or the skirt flattening at the bottom leading to a large area in contact with the ground which will cause excess drag. Solutions to the latter include a vertical wall strip on the bottom of the skirt (as used by Vortex) and/or internal cross webs within the bag to 'squeeze' the bag minimising the ground contact area.

Integrated craft with the lift fan at the back of the craft have the challenge of getting air around to the front. Most use both the bag and the hull sidebodies to do this, to maximise cross section. All these 'flow-bags' use round holes on the inside of the bag, usually only at the front, to transfer air to the cushion. The size and number of these holes adjust the pressure difference between the bag and the cushion. Most folk use a bag pressure 1.2 -1.5 times the cushion pressure to optimise stability.

Keith Oakley