Marine

ASV hulls would dramatically improve ship efficiency by riding on a cushion of air

ASV hulls would dramatically i...
The Air Supported Vessel uses a cushion of air to increase speed and improve efficiency (Image: Effect Ships International AS)
The Air Supported Vessel uses a cushion of air to increase speed and improve efficiency (Image: Effect Ships International AS)
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From the surface, the unusual hull configuration of the Air Supported Vessel is hidden (Image: Effect Ships International AS)
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From the surface, the unusual hull configuration of the Air Supported Vessel is hidden (Image: Effect Ships International AS)
Bottom view of an Air Supported Vessel hull undergoing tank testing (Photo: Effect Ships International AS)
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Bottom view of an Air Supported Vessel hull undergoing tank testing (Photo: Effect Ships International AS)
Air Supported Vessel hull undergoing tank testing in Sweden (Photo: Effect Ships International AS)
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Air Supported Vessel hull undergoing tank testing in Sweden (Photo: Effect Ships International AS)
The Air Supported Vessel uses a cushion of air to increase speed and improve efficiency (Image: Effect Ships International AS)
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The Air Supported Vessel uses a cushion of air to increase speed and improve efficiency (Image: Effect Ships International AS)
The Air Supported Vessel uses a cushion of air to increase speed and improve efficiency (Image: Effect Ships International AS)
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The Air Supported Vessel uses a cushion of air to increase speed and improve efficiency (Image: Effect Ships International AS)
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A European project is developing new Air Support Vessel (ASV) hull designs that allow watercraft to ride on a cushion of air to greatly reduce friction between the hull and the water, resulting in more hull speed for less power than conventional designs. The project is part of a EUR10,000,000 (approx. US$13,225,000) project funded in part by the European Union, the Norwegian Research Council and Innovation Norway, and Norwegian company Effect Ships International AS has recently completed tank-testing in Sweden of two ASV hull models.

Getting a ship to move through the water is more difficult than it looks. Though it's very easy to move a floating object with the push of a hand that would be impossible to shift on land without a bulldozer, water itself has friction that even the smallest craft must work against. We think of water as being slippery, which anyone who has gone into a skid on a wet road can attest to, but as far as ships are concerned, it is actually quite viscous. It tends to stick to things like ship hulls and form a boundary layer that is dragged along with the ship like invisible seaweed. You can't see it, but it drags at the surrounding water and slows the ship down. This effect isn't noticeable on small craft going a low speeds, such as row boats, but in high-performance craft where every factor counts or in large ships were the effect is very large, it is very noticeable and very expensive in terms of speed and fuel consumption.

Also, when a conventional hull moves, it pushes the water ahead of it out of the way. We like to talk about a ship "slicing" its way through the water like a knife, but what is really happening is that the ship is pushing the water ahead of it. When the ship is going slowly, the water can easily fall off to either side, but as the ship goes faster, the water starts to pile up on itself until forms a wave. This wave swells and falls away constantly to form transverse waves along the hull that slow the craft down. Eventually, this effect becomes so large that the ship simply can't go any faster. Putting more power behind it just makes the waves bigger without any increase in speed. The limit is calculated by the formula Hull Speed = 1.34 * (LWL)1/2 where LWL is the length of the hull at the waterline in feet.

Put simply this means that the only way to make a conventional hull go faster is to make it longer, so that, all things being equal, a 50-foot boat will outrun a 30-footer. Of course, the friction also increases with size, so the two tend to balance out. These two problems, among others, puts a real limit on the design of maritime vessels and the only way around it is to abandon conventional hulls in favor of ones that either reduce the friction or the bow wave.

Boundary layers and hovercrafts

Reducing friction on a hull means getting rid of or reducing the boundary layer. Golf balls do this in the air with their dimpled surface that sets up currents that break up the layer, allowing them to fly straighter, while dolphins do the same in the water by altering their skin to form similar depressions when they swim at speed. This isn't practical for a steel-hulled ship (though some companies have tried using ridged rubber mats for the same effect), so Mitsubishi has developed a system for breaking up the boundary layer by blowing air bubbles under a ship's hull. This is a sort of air lubricant that makes the hull more slippery in water, but that's different from what Effects Ships International (ESI) is doing. Mitsubishi is trying to mitigate the problems of traveling through water. ESI is trying to avoid them altogether.It's basically a variation on how speed boats work. Speed boats are able to blast across the waves far above their hull speeds because that is exactly what they are doing - blasting across the waves instead of through them. They use a planing hull that acts like an airplane wing that generates lift and raises the boat out of the water. With very little of the hull immersed, there's little or no bow wave and very little resistance, so the boat can go like stink.

Another way of solving this is by taking the get-out-of-the-water strategy to it's logical conclusion with the hovercraft, which lifts the vessel out of the water entirely. That's why hovercrafts have pilots instead of captains. Since their invention in the 1950s, hovercraft that travel on a cushion of air have become a common sight. They're used for passenger services, search and rescue, military operations, moving heavy equipment and just plain fun.

Hovercraft operate by blowing air under high pressure beneath a vehicle where it is contained momentarily by a rubber skirt. This allows the vehicle to float a short distance off the ground and makes it virtually frictionless. This allows the hovercraft to make remarkable bursts of speed as well as traverse water, marshes and dry land, but that frictionlessness comes at a price. They are notoriously difficult to steer and riding across a choppy bay in a high wind can be an alarming experience as the craft travels as much sideways as it does forward.

Air Supported Vessels

ESI's Air Supported Vessel is sort of a wedding of the hovercraft and the planing hull. Sitting at the pier, it doesn't look much different from your garden variety cabin cruiser. It may be a bit boxy in the bow, but that's about it. But if you put on your scuba gear and take a dive underneath, there's where the difference shows. Instead of a smooth V-shape, the bottom has a pair of sharp, blade-like projections in the bow that funnel water into a trough down the middle. On either side of this are a another pair of cavities and at the stern are a pair of pods containing the screws to propel the craft. As the boat moves through the water, high-pressure air is pumped into the port and starboard cavities and this forms a kind of bubble under the hull that lifts as much as 85 percent of the hull's displacement out of the water. There are no rubber skirts needed as in the hovercraft and the cavities are integral with the hull.

Bottom view of an Air Supported Vessel hull undergoing tank testing (Photo: Effect Ships International AS)
Bottom view of an Air Supported Vessel hull undergoing tank testing (Photo: Effect Ships International AS)

This arrangement greatly reduces the wetted area of the hull and reduces hull friction by about 50 percent with increased efficiency of 40 percent once the energy cost of running the air compression system are deducted. It also produces very little wake, so it is ideal for sheltered waterways, such as harbors and canals.

The aim of this hull is for use in calm, protected waters to create a new line of fast, efficient and environmentally-friendly commuter ferries. Such ferries would be about 67 feet long (20 m) and 20 feet abeam (6 m) and carry 100 passengers at speeds of up to 35 knots. It should also be a pretty smooth ride for those prone to seasickness because ESI says that the hull has excellent motion damping characteristics.

ESI plans to make its ferries out of carbon fiber sandwich composites to make them as lightweight as possible. If sufficient weight savings can be made, the ferries will be equipped with electric motors powered by new lithium-ion nanotech batteries under development for the project. However, the hulls can employ an alternative diesel propulsion, such as the Volvo Penta IPS.

ESI is currently seeking shipyards and partners with an eye toward developing commercial vessels, crew boats, offshore and windmill support vessels, patrol crafts, paramilitary and fast navy vessels and pleasure boats ranging from 50 feet (15 m) to 330 feet (100 m). It also believes that ASV principles may be applicable to large commercial ships.

Source: SES Europe

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17 comments
piolenc
How is this different from the "captive air bubble" ships of fifty years ago?
Pat O'Leary
Or John Thorneycroft's patents in 1877 for trapped bubble craft?
Nacho Lotitto
100 year old news...
Dawar Saify
Why not a wing on water. There will be a small buoyant areas within the water on the sides with propellers or jets. The wing will be covered by solar panels. This is not a ground effect vehicle, the propellers remain under water.
Common.Sense.Lost
A special thanks to you all for making a point...... STOP REINVENTING THE WHEEL!
warren52nz
I've been thinking that a boat with underwater wings that lift the hull completely out of the water would be the trick. Like a hydroplane but under water. The angle of attack of the wings could be computer controlled so the boat stays at a constant height while the waves wash away under the hull and above the wings so they have essentially no effect. The ride would be smooth. Anyone see any problems with this concept?
jerryd
Sounds like a more complicated version of the Hickman Sea Sled. Next this makes the same wake, bow waves as any other boat does. A far less expensive way to go fast economically is cat and tri hulls, By having an 8-1 beam/length ratio hulls they don't make enough of a bow wave to matter thus keep going faster at far less fuel than any of the mentioned craft. On my bucket list is a wing in ground effect boat, like a seaplane but only flys just above the water at high eff is how to really go fast.
Lindsey Roke
Hydrofoils do exactly that - and they began doing it well before computer technology moved out of main-frames. See - for example - www.youtube.com/watch?v=759A774qf68
Mythobeast
Almost everyone here seems to be missing the point. With a 40% improvement in fuel efficiency, don't you think they would have been implementing this if someone had already come up with it? The difference between this and a hovercraft or other "air hull" is that the rear skirt is designed to perpetually exhaust air, which is replaced by air being pumped in from the front. This means that the air underneath it is moving at roughly the same speed as the water, resulting in almost no friction between the air/water layer. The wake is in the same form as, but substantially smaller than with an equivalent sized vehicle. It's maybe displacing a third of the volume of water, so there is less of an issue with the water rushing back in creating waves. @warren52nz, in your scenario, you would wind up with water friction on the upper side of your blade. While this would, in fact, result in less overall friction than the hull, it would be more friction than if the blade were lifted half out of the water, as with hydrofoils. HOWEVER, that doesn't mean that what you suggest is a bad idea. The result would be an amazingly smooth, wave-resistant ride on water surfaces like lakes that don't have tall wave crests. The pleasure cruise industry considered a design similar to this, but with pontoons under water. That allowed it to ignore waves even when standing still. I believe they had issues with the stability of the design during storm-level waves, but that wouldn't effect what you have in mind. The real issue is that it would have some serious material strength problems for really big vehicles, and those are the ones that have the biggest fuel efficiency issues.
Expanded Viewpoint
On a PWC up to a medium sized boat, how about running a turbocharger off the exhaust gases, and then dump all of it into a manifold that is a part of the hull near the bow? Large volumes of gases at low pressure should work just as well as small volumes of air at high pressure. With this system, you'd not be wasting money on expensive air compressors and burning more fuel. It would be win, win, win, win when you consider weight/cost/fuel economy/mechanical complexity considerations. Now watch someone make a gazzillian bucks off this idea. Randy