Air injection for planing hulls

[CDK]

From my airforce days, long ago, I remember a unique feature of the Lockheed F-104 Starfighter plane.
Because the aircraft had unusually small wings, large flaps were needed to obtain enough lift at and below 200 knots. In addition, the flap angle was an extreme 35 degrees, causing air turbulence on the upper surface.
To resolve the problem, the designers used an air injection system. The rear wing edge had a row of small holes, connected to an air pump that was powered as soon as the flap angle exceeded a critical angle.

The above has nothing to do with boating, but is just a background for the following:

The friction between a solid surface and air is only a fraction of the hull-to-water friction. If a planing hull would have a row of holes at the beginning of the planing area, through which air was injected, there would be an enormous reduction of drag, resulting in a much higher speed and/or a lower fuel consumption. It might even me possible to reach planing speed with a much smaller engine that normally could never get the boat over the hump. The power requirements for the pump would be very modest, just a single V-belt and a vane type pump similar to the air injection pumps that were used in the automobile industry to meet CO requirements before the introduction of the catalytic converter.

Does anyone know of such an experiment being carried out? No skirts like on a hovercraft, just a row of holes, an air pump and of course a non-return valve to prevent water entering the boat when moored.

[rwatson]

The Lockheed method reminds me of the technique used by jacque cousteau for his 'solid' sails on his experimental wind assisted yacht.
The concept of injecting air bubbles in the water might have merit, but it would reduce bouyancy for the hull of course.
Should be easy to set up as a model experiment.

[mmd]

Yes. In 1995-96, I collaborated with a fellow designer to construct an air-lubricated planing catamaran hull for a Barbados client. The vessel, named "Ocean Mist", was 18.25m x 8.0 x 1.00m, arranged as an eight-passenger, three-crew charter boat. It was powered by twin 400-hp Cummins through conventional shafts & props, and achieved 31.25 knots during trials.



Her owner sadly was diagnosed with cancer a few years after delivery and, after a valiant struggle, passed away two years ago. The boat was sold prior to his death and I have lost track of it.

[GuestR01312011]

I know someone did a final year Bachelor Nav Arch thesis on this air injection into the boundry layer but it was on an Open 60 sailing yacht. I know that on hydrofoils (surface piercing) to alter the lift coefficient they use injection of air bubbles near the trailing edge of the upper surface of the foil to initiate early seperation. Also there is a method of shooting a fast stream/jet of water at the trailing edge to alter the stream speed over the foil.
I know alot of sailing rules (Open 60/Volvo 70/ AC90's/ ACC ver.5) stipulate that any device altering the boundry layer flow is forbidden. They mention devices to release films of varying viscocity over the hull and similar devices...
Another way to greatly reduce the viscous drag would be to resonate the hull at the frequency of the atomic forces that attract the fluid to hull...

[tom28571]

You need to differentiate between a hull that is partially supported by air pressure or an air/water mixture. Many suggest that injecting air into the water under the hull will lower skin friction. I don't know enough hydrodynamics to say but have read that introduced air bubbles increases total drag rather than decreasing it. As Watson says, it makes sense that adding air that does not support the hull by pressure will make the boat sink lower in the water to retain equilibrium displacement. Does this increase wave making and add to total drag? Some have run engine exhausts to the underside for the same purpose.

I know that many successful boats have used skirts at the chines to trap air and some even add blower fans for greater effect. These do work but are not widely used, maybe because the advantages are limited in practice.

curridronan's comments refer to rulings after some experimenters tried injecting long chain polymers into the boundary layer to reduce friction and retain laminar flow longer. This does work and so was outlawed in sailboat racing in the late 1960's I think. This method is used in industry to reduce friction in liquid in pipes, such as in steam powerplants.

[GuestR01312011]

Tom, from what Im studuing about Marine Hydrodynamics (MSc. Yacht & Small Craft) a turbulent boundry layer has less skin friction than a laminar one that will almost always trip to turbulent which is difficult to calculate and therefore design for. The air injection I was talking about above is purely for a planing/semi yacht hull form, it does not affect buoyant forces as hull is planing anyway. And it is not an air cushion vehicle. Also riblets/especially roughened surfaces like swimmers suits take advantage of the turbulent boundry layer theory...

[eponodyne]

Golly, I'd think you'd need a pump capable of moving like a thousand cubic feet of air a minute to see any real improvement. I might be mistaken but wouldn't you need like eight times the air when your speed doubled, following the ccube function rule of thumb for horsepower? Or is that only in air and in a denser medium you'd need correspondingly more? Dunno.

[Guillermo]

Air injection has been treated in these forums in several ocasions, as per example in:
The use of air

More radical use of air injection:
300knt torpedo

Cheers.

[Chris Ostlind]

A few years ago, I visited a website where a Danish designer had been studying the performance characteristics of classic Viking craft. His take was that they, quite often, had constructed channels in the hull from bow to stern in which an entrained air column moved while underway. This column functioned much like the air bubble trail you suggest, CDK.

Obviously, they had little in the way of pumping said air such as the bubble stream suggested. Well, unless you'd buy a Viking warrior sitting up at the bow running a blacksmith's manual bellows for the entire voyage. ;-)

Anyhow, I have lost connection with the Danish guy's website in the ensuing years since forst reading it. I do know that he had built several classic sailing boats, of the harbor and short coastal variety, using this entrained air channel technology and was reporting them to be faster than the non channeled version of same.

If anyone knows of this site, I'd love to get the URL and refresh my understanding of his studies.

Chris

[Guillermo]

More interesting info:
http://www.turbulence-control.gr.jp/...004/Kodama.pdf

Search internet for "microbubbles drag reduction" "microbubbles planning hull" or the like, to find out a wealth of info.

Cheers.

[tom28571]

This is all interesting for thought but I would sure like to see some objective testing and reporting.

It is difficult to believe that turbulent flow has less skin friction than laminar flow when so many engineers spend lifetimes trying to maintain laminar flow of wings and other foils. Of course that includes lift/drag ratios among other factors.

Anyway, whether a boat is planing or not, it still requires exactly the same lift force to hold it up. Buoyancy plus dynamic lift must equal the weight of the boat, unless Newton was wrong.

I'm reading the links above but it requires a lot of study to see it through.

[TollyWally]

Forgive my possible misunderstanding but isn't this very similar to what happens in an old fashioned step hydroplane? I was under the impression the the step created a sheet of bubbles that decreased the friction between the aft section of the hull and the water. Several modern go fast boats appear to have ducts that channel air to the aft areas of the hull.

National Fisherman published an article 10 years back or so about some experiments with a big airduct channeling air underneath an experimental power catamaran. They got quite a difference by opening or closing the top of the air duct. This was a fairly unsophisticated demonstration, very much home made and low tech.

In the same article, more sophisticated experiments regarding Russian freighters running on rivers were mentioned. The Russians were reportedly injecting the exhaust under the freighter hulls and achieving some increase in effiency. There were no details of the Russian experiments other than the report they had done it.

Once again I mention that I may have completey misunderstood the premise of this thread, and if so, look forward to being set straight by the collective wisdom of our assembled panel.

[mmd]

Tom, what type of more objective testing and reporting would you like? The boat in the photo above is a shallow-vee planing hull form terminating in a flat after section bounded by rails, into which an air stream is introduced to reduce skin friction in the after part of the hull. Top speed of the hull exceeded calculated expectations for a non-ventilated hull by approximately 20%. I thought it a rather successful real-world demonstration of the principle. Too bad we didn't have an additional half-million dollars to build a similar conventional hull as a trial horse, I guess.

[CDK]

Originally Posted by eponodyne

Golly, I'd think you'd need a pump capable of moving like a thousand cubic feet of air a minute to see any real improvement. I might be mistaken but wouldn't you need like eight times the air when your speed doubled, following the ccube function rule of thumb for horsepower? Or is that only in air and in a denser medium you'd need correspondingly more? Dunno.

No, all that is needed is a very thin air layer. Of course the output must increase with speed to obtain the same thickness but that need not be the objective. Too much air would cause the hull to loose its grip and behave like a hovercraft.
That's the way I think it is, but I have NOT yet tried it.

[tom28571]

Originally Posted by mmd

Tom, what type of more objective testing and reporting would you like? The boat in the photo above is a shallow-vee planing hull form terminating in a flat after section bounded by rails, into which an air stream is introduced to reduce skin friction in the after part of the hull. Top speed of the hull exceeded calculated expectations for a non-ventilated hull by approximately 20%. I thought it a rather successful real-world demonstration of the principle. Too bad we didn't have an additional half-million dollars to build a similar conventional hull as a trial horse, I guess.

Well Michael, some experts claim that air entrained water under the hull decreases drag and some say that it increases drag. I have read these claims but can't direct you to them. The boat in your example gave results with injected air but no performance data without the air. I don't think you would buy that without further data and I don't either. Your reply adds some information that would have helped (the 20%), but it's still calculation, not demonstrated. I don't have a dog in this fight, I'm just interested.

As I said, the material in the links above looks interesting and seems to support lower drag for injected air under a flat plate. I don't have the time, money or expertise to work on this myself and will just wait to see what falls out.