Air cushion advances

Using an air chamber under the hull to lift a vessel and reduce the drag started with the hovercraft development back in the 1950s. Initially the air chamber was kept in place with flexible skirts but then came the sidewall hovercraft with flexible skirts just at bow and stern.

This was followed in the US by the Air Ride Craft concept where the air chamber was contained within rigid ‘skirts’ on a catamaran type hull. Now finally there appears to be a practical monohull solution where the air chamber is contained within the shape of a monohull, a concept that can combine the efficiency of the hovercraft with the seaworthiness of a monohull.

It was over 14 years ago that trials started in Norway on this latest version of hovercraft technology using updated design techniques and modern materials. Effect Ships International (ESI) was established in 2002 with the goal of developing new air assisted hull concepts offering improved efficiency, higher speed and other operational benefits. Their Air Supported Vessels (ASVs) initially focussed primarily on fast passenger only catamaran designs in the 30 -40 m size range and with a design speed as high as 50 to 70 knots. At that time the market for fast passenger ferries was booming and the EFI designs could offer better performance than other designs at a reasonable cost

Over the years, nearly 20 different ASV hull concepts have been evaluated for resistance and behaviour in both calm waters and in a wide variety of sea conditions to verify the concepts. Ulf Tudem, the General Manager of EFI comments, "We believe that tank testing our concepts for advanced hull designs creates a massive resource of information from which we can confidently predict the performance of new designs".

In 2005 ESI shifted the focus of their development to a new ASV monohull concept which was aimed at creating designs that outwardly would look like conventional craft but have the air cushion contained within the monohull design. During the last decade the company has carried out a massive amount of additional ASV related research and documentation on this concept.

In this work ESI has been supported by European Union, Norwegian Research Council, Innovation Norway, Vinnova in Sweden and leading maritime shipyards, academia and propulsion manufacturers such as Rolls Royce and Volvo Penta. All of this research into monohull ASVs led to the construction of the first full size ASV monohull test vessel being built in 2009. This 20 metre craft was designed to offer outstanding efficiency and to provide large onboard volumes and was extensively tested to validate the tank testing work.

This 20 metre ASV was fitted with Volvo Penta diesels coupled to IPS drives for propulsion. The fuel consumption was found to be reduced by up to 50% when operating at speeds of around 30 knots compared with conventional planing hulls. Equally important was the ability of this ASV to cope with rough sea conditions, the air cushion acting to moderate the movements of the hull in waves to provide a better ride that could be achieved with a conventional deep vee hull.

"It's been a long journey but now we are starting to see some positive results" said Tudem. "We have patented the concept in over 60 countries around the World and now we are entering into licensing arrangements with shipyards and designers. The BB Green electric ferry project opens up a new world of possibilities for efficient electrically propelled vessels".

BB GREEN is a €3.2 million European Union funded project to develop a zero emission commuter ferry for operating mainly on commuter routes. This 20 metre by 6 metre fast ferry has the hull and superstructure made from a carbon fibre/Divynicell sandwich using vacuum infusion techniques. The main electric motors supplied by Enchandia Marine are two 280 kW units driving Volvo Penta IPS drives to give a speed of just under 30 knots. The lift fan in the bow generates the air cushion with a power of 80 KW, will support approx 75% of the vessel's displacement.

The lithium titanate batteries can have a 400 kWh capacity but on the test ASV these are half-size, sufficient to operate at 25 to 30 knots for about 7 miles. Recharging can be done in about 20 minutes. This is adequate to allow for an hourly service carrying 90 passengers and some bicycles. The prototype BB Green is currently undergoing evaluation in Stockholm Harbour.

Tuco Marine has worked with ESI to develop a18 metre fast offshore support/crew transport vessel based on the ASV hull form. According to Tuco this prototype monohull design could form the basis of a wide range of workboat applications such as pilot boats, SAR vessels, patrol vessels, fire-fighting, coastal fast ferries, paramilitary crafts as well as high speed vessels for navy and security operations.

Powered by a pair of Volvo Penta 350 hp diesels coupled to IPS Drives this vessel can offer speeds up to 40 knots. A third Volvo Penta diesel of 130 hp powers the lift fan. Like the BB Green vessel this Tuco vessel is constructed in a carbon composite sandwich material to reduce weight but conventional GRP construction is possible. Not only is this 18 metre prototype vessel fast for its installed power but it combines this with excellent sea-keeping with the air-cushion help to smooth out the ride in waves. The wide beam also contributes to the reduced rolling motions and helps the good manoeuvrability from the widely spaced drive units.

ESI has also established collaboration with Atlantic Eagle Shipbuilding in Portugal and their first ASV will be a fast 16 metre coastal patrol vessel. This vessel will have a near vertical bow shape combined with a beam of 5.2 metres. The vessel has been designed to deliver speeds in excess of 40 knots using twin 450 kW Volvo Penta diesels coupled to IPS Drives and to have a range in excess of 600 miles. Seakeeping is an important requirement and the air cushion is expected to give good motion damping in waves.

EFI is confident that the benefits of the ASV concept can be applied to larger vessels. An initial design for larger fast crew transfer/offshore support ASV with a length of 40 m and a beam of close to 10 m has been prepared. The light ship weight and resistance will be very low leading to improved fuel efficiency and the generous beam of this ASV will contribute to a significantly larger deck area and more accommodation space than comparable conventional monohulls. These designs will also have a reduced rolling motion at low speeds which can be important for many offshore operations.

ESI also developed and tested several hull forms that would be suitable as the basis for larger RORO and ROPAX vessels and car ferries. These have included advanced designs for a vessel that was 125 metres long with a beam of 45 metres and with a design speed of 60 knots. EFI also looked at the military market and developed an ASV landing craft design for navy use. All of these advanced concepts were based on catamaran hulls.

With two, quite different, full size ASV demonstrators in the water, and other vessels in the pipeline, Effect Ships International AS (ESI) and it’s collaborators intend to show the market why “riding on a cushion of pressurized air” makes a lot of sense. The “skirt-less” and patented ASV concept enables up to 80% of the vessel’s weight to be supported by pressurised air contained in a hull integrated cavity. Large areas of the underwater hull are separated from direct water contact thus reducing the wetted surface area which is the main resistance component of fast hulls. For several ASV designs this reduction can be more than 40%, contributing to an almost similar fuel saving.

“By removing the requirement for flexible skirts to help seal the air chamber as used on conventional surface effect vessel designs we have made a significant step forward,” said Ulf Tudem of EFI. “Towards the bow, ridged parts of the lower hull seal the cavity without use of fragile rubber enclosures and towards the stern a special, ridged and solid air cushion enclosure arrangement is used. On each side of the air cavity, planing bodies housing the propulsion engines and the pod propulsors keep the pressurized air entrapped with a minimum of air ventilation. Combined with the IPS system enabling individual vectoring of the pods, the vessel obtains superb handling and manoeuvrability and in most cases thrusters will not be required.”

“It’s been a long voyage reach current state of developments. We are very pleased with the excellent support we have received from European Union, national RTD supporting institutions in Norway and Sweden and leading maritime industry partners.” commented Tudem.