In the last few years there has been some impressive developments with air cavity hulls. Fuel savings around 35% are being achieved and that includes the fuel used in the blower.

I built a model to get a better understanding of the issues involved in getting it to work well on a slender displacement hull that could be used on a proa:
http://www.rickwill.bigpondhosting.com/100_2565.JPG
The hull has a clear bottom so I can see how the cavity pressurises and what wave pressure does in the cavity. The blower is much larger than needed for this small model.

The model is powered by a tiny air prop so the force (around 0.3N) is near constant irrespective of the speed variation due to drag changes on the hull.

Without air pressure the boat does 1.6kph (be prepared to be under awed):
http://www.rickwill.bigpondhosting.com/100_2562.MOV
I also tested the model with the cavity filled in with foam so it had a flat bottom. Speed lifted marginally to 1.7kph.

With the blower running and cavity under air pressure the speed increased to 2.8kph;
http://www.rickwill.bigpondhosting.com/100_2560.MOV
The excess air is making bubbles out of the starboard side as there is a slight list to port. Still not startling speed but the difference is impressive. The wave making is the best indicator of the comparative speed in the clips.

The drag v speed relationship is not quite a squared function due to influence of waves but close to it. So speed ratio of 2.8/1.7 indicates drag rises 2.7 times without the air. To get the unblown hull to 2.8kph would take 2.7 times more force than that required for the blown hull. The area of the cavity is 0.14m^2. Total wetted surface including the outrigger is 0.18m^2. So a reduction of around 4 could be expected if the cavity contributed no drag and allowing for the small degree of wave resistance.

Cavity hulls seem well suited to calm water performance improvements but could still be beneficial in waves. I played around with the air cavity concept on a planing hull first. It is a bit more spectacular:
https://www.dropbox.com/s/h4dt2qtm0icq98u/100_2536.MOV?dl=0
The air cavity helps it to get on the plane but then opens up. This actually works like a hovercraft on flat ground. I have used a ducted fan and bleed off a bit of air to lift it. Lifting does not require much air pressure or air volume as the cavity fills most of the bottom, area. Even on the displacement boat the big oversize blower is using about the same power as the tiny propulsion motor.

Rick

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