Refining the hovercraft concept, Norwegians hope to take a quantum leap in fuel efficiency
Plain is probably an appropriate term for the M65, with its angular deckhouse, small windows and conspicuous air intake grilles on each side of the bow, as the white 65-footer sits at a dock at Antalya on Turkey's southern coast.
But looks don't matter to an air-supported vessel, which blows bubbles out the stern as it rises on a cushion of air, reducing the friction between hull and water and allowing the boat to go faster and farther on less fuel and horsepower. Such qualities get attention these days because they save money and are easier on the environment. Traditional planing hulls need plenty of horsepower because they push lots of water out of the way as they get on plane and up to speed.
An air-supported vessel works like a hovercraft, which travels on a layer of air, except that the ASV doesn't need the funky skirt and air-drive propellers. In fact, the ASV concept is suitable for monohull designs, preferred for recreational boats, or catamarans for commercial or military vessels that need to carry big payloads at high speeds.
"We have scientifically documented fuel savings of 50 percent, compared with the leading European and U.S. brands in the 50- to 70-foot range," says Ulf Tudem, the general manager of Effect Ships International AS in Sandefjord, Norway.
ESI is a small company that has spent more than $10 million to develop this idea, including about $3.7 million in European Union support. The M65 - a monohull - won the innovation award in the 2011 European Powerboat of the Year contest.
The ins and outs of an ASV A Q&A with Ulf Tudem, general manager of Effect Ships International Q: What is ESI? A: Effect Ships International AS was established in 2002 with a goal to develop and document new hull forms and solutions with air cushion support. The technology was patented or has patents pending in almost 70 countries. Q: Which hull form and which kind of propulsion system work best with the concept? A: The ASV concept works for mono- and multihulls in a wide range of configurations. ESI has designs for most propulsion systems, like water jets, pod propulsion (Volvo Penta IPS, Cummins Zeus, ZF, Rolls-Royce Pod Prop), Servogear, fixed shafts. Surface-piercing propellers may also be used. For smaller craft, Volvo Penta and MerCruiser outdrives will be popular. For pleasure boats, pod propulsion seems a perfect match. Q: Does an ASV need lightweight hull construction? A: No. With the ASV system, designers will enter a positive design spiral due to the fact that everything will be much smaller and considerably lighter. Comparing conventional boats and ASVs, the ASV will be several tons lighter while delivering the same performance. The truth remains: Less weight improves performance, fuel consumption and operational efficiency. Q: Are there any weight, displacement or size limitations? A: The ASV concept has proven very scalable. However, each application will have to be evaluated. It’s not one-size-fits-all. Q: Does an ASV hull change maneuverability? A: All who have tested the ASV with pod propulsion were amazed about the ease of handling in port, easily moving sideways with joystick control and at sea. Q: What are the effects on the interior and accommodations? A: The interior of ASV monohulls actually is more spacious. That’s because the fan is in the bow and the air-feeding channel leads under the hull, while the propulsion system is near the transom and is much smaller in size than a conventional fixed-shaft installation. ASV hulls also are a bit wider and almost vertical, which contributes to larger volume and easier arrangements of the interior. Q: Doesn’t the fan use a lot of fuel? A: The powering of the air fan system depends on how well the air is entrapped within the air cushion chamber. Calm waters require less ventilation than rougher seas. To maintain a 65-footer on the cushion in 30 knots of speed requires 10 to 12 liters of diesel per hour, corresponding to less than 10 percent of total consumption. Q: What’s the premium for an ASV? A: The extra cost will be marginal. The builder will pay ESI royalties based on the value of the vessel. Construction cost will be very similar to a conventional hull of the same size. The propulsion and all other systems will be smaller and less expensive. The fan and air cushion enclosure system will add some cost, but that will be less than the price difference for the smaller engines and systems. Q: Did you have to license any part of the technology? A. No, the ASV technology is a sole ESI-owned technology. Q: Have you licensed the technology to commercial builders? A: At this time we are negotiating with several builders, but I can’t disclose names — yet.
ESI calls the ASV the "world's greenest hull concept," an oblique reference to the carbon savings that are being achieved through efficiency. "It's even slightly more [than 50 percent savings] in some cases," Tudem adds. "The massive fuel-saving advantage is applicable throughout most of the speed range from 12 to 15 knots upward." For fast passenger commercial ferries in the 130- to 410-foot range, ESI also tested catamaran models.
The cushion of compressed air under the hull lifts the boat in the water and reduces performance-robbing drag. One collateral benefit of an efficient hull is weight reduction. Weight also detracts from performance. ESI says tests show hull resistance could be reduced by 50 percent or more at design speed, which means the engine could be as much as 50 percent smaller. That, in turn, begets more space and weight savings through smaller tanks that carry less fuel.
The concept requires a special underwater hull shape with a large cavity to trap the air, which is blown in by a fan mounted in the bow. The cavity is closed off at the stern with a special flap. On the prototype, the hydraulically driven fan is powered by a Volvo Penta D3 diesel, but future versions might use electric power from a genset or from a hybrid main propulsion system. The hull also needs pronounced "rails" on each side that prevent the air from escaping and lend stability under way.
When the main propulsion system is engaged - Tudem says it can be any type, including pod drives, jet drives or outdrives, fixed-shaft and even surface-piercing props - the M65 accelerates and gets on plane much faster than conventional boats because it doesn't need to get over the proverbial hump.
Although the ASV concept is new, the underlying ideas are not. Lifting the hull out of the water to reduce drag without hydrofoils also is part of the M-hull concept that M Ship Co. in San Diego developed for recreational, commercial and military applications. Instead of using air, the M-hull captures its own bow wake to generate lift by channeling the water between the main hull and the outside "skirts."
Another sworn enemy of drag is Oakland, Calif., designer Tom Wylie, who is known for sleek and fast sailing vessels. A few years ago, Wylie tinkered with the hull shape of a center console skiff that uses sponsons to boost stability and reduce resistance - and the need for extra horsepower. With a 40-hp 4-stroke, the Wylie Skiff has enough power for duties as a crash boat at dinghy regattas or as a tender to a Wylie-designed research vessel that tags great white sharks near the Farallon Islands in the Pacific, 25 miles west of San Francisco.
What impressed testers about the Norwegian ASV M65 was its nimble acceleration once it was "pumped up" and riding on the air cushion. "After 5 seconds we had reached 15 knots, after 13 seconds we passed 25, and we hit the top speed of 33.1 knots," Hans Wirscher writes in the German magazine Meer & Yachten. "These numbers are quite common in the 65-foot class, but there was no mountain to be conquered with a lot of power and gasoline consumption to get to planing speed."
ESI says it did add weight with containerized sandbags to emulate "real-world" conditions, which means boats that are heavily built. In one test, the company added nearly 5 tons and still achieved a 32-knot top speed, Tudem says.
So what's the recorded fuel consumption of the ASV M65 prototype? ESI says the test vessel achieved a verified fuel consumption of 4.9 liters (about 1.3 gallons) per nautical mile when cruising at 28 knots once the boat was optimized and in proper balance.
"The trim remains almost neutral throughout the whole speed range," Tudem says. "The acceleration is much faster from zero to full speed, compared with similar-sized conventional vessels with twice the installed power, due to much less resistance."
The plain appearance of the M65 prototype notwithstanding, the sea trials in Turkey confirmed the workability of the ASV idea and gave Tudem and his business partner Tor Kolbjorn Livgard, who handles engineering, confidence to market the concept. Considering the volatility of fuel prices and boat owners' desire to reduce the pain at the pump, they might have good cards. Now they have to play them.
This article originally appeared in the May 2011 issue.