ASV hulls would dramatically improve ship efficiency by riding on a cushion of air
By David Szondy
March 6, 2012
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 hovercraftsReducing 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 VesselsESI'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.
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