French aeronautical engineers have formulated an innovative high speed transport solution to emerging maritime requirements while building on existing technologies. The result is a new generation of cost-effective vessels employing both ground effect and the air cushion technique. The so-called Grefco (ground effect vehicle) concept has been developed to provide competitive alternatives to existing vessel types in the ferry, supply and feeder sectors. Grefco offers a promise of substantial capital and operational cost reduction to a level that can stimulate growth of waterborne transport by attracting traffic away from other forms of transportation.
The main advantages of this new concept are reduced power requirements at service speeds up to 50kt or more, impressive stability and sea-keeping properties as well as a fully amphibious capability. The design is based on standard marine propulsion systems with diesel engines. The initial range of Grefco designs includes 60m and 135m vessels suitable to operate as ferries, ro-pax and cargo vessels. A 15-year study and development programme carried out in France has included construction of five prototypes up to 15m in length and 18-passenger capacity to validate design and production aspects.
Grefco's sponsors claim that air-cushion craft have to date generally failed to attract a wide market in the civilian marine sector because of shortcomings in operating performance and comparatively high costs. It is contended that the use of an aerial propeller affords poor efficiency in relation to speed, and that using gas turbine power units, although these are light in weight, imposes a cost and noise penalties. Further drawbacks attributed to the previous generation of air-cushion craft have included substantial power requirements, high operating costs, and impacts on the environment.
The new Grefco vessel operates completely out of the water, with only the propulsors immersed and incorporates suspended skirts. To realise the full air cushion efficiency potential, the overall width of the cushion is much greater than that of the hull. The air support system is divided into a minimum of four cushion chambers with separate air inputs. The skirt suspension and control system is based on pressure fields, restricting air leakage, and curbing the amount of power needed to maintain the air cushion. The very low skirt and surface friction factors reduce propulsive power requirements for a given speed.
It is claimed that Grefco vessels will provide better passenger comfort than other types of high-speed craft in adverse weather conditions. An active control system will dampen vertical motions, contributing both to passenger comfort and vehicle or freight security. A preliminary study was conducted into the craft's response to various sea states, and at vessel speeds between 10 and 50kt. It was determined that Grefco responses, notwithstanding high waves and high speeds, were well below prescribed seasickness curves.
Immediate promotional efforts focus on a standard 330dwt design (58m structure length and 62m vessel length; 37m overall beam) powered by two diesel engines producing a total 14,700kW. The vessel will be able to achieve cruising speed of 50kt at less than 11,000kW output. The SCF56-6 vehicle-passenger ferry model is designed for 680 passengers and 120 cars, with an alternative ro-ro load consisting of eight buses and 80 cars, or four trucks of 30m plus 80 cars. A pure ro-ro carrier within the S56 family is the STT56-6, designed to transport 18 trailers (20 tonnes each). At a later stage, the Grefco family will be expanded by S135 vessels with deadweight capacities of 3,000-4,000t traveling at speeds up to 70kt.
