Displacement monohull
For many centuries man has been travelling over the worlds seas at ever increasing speeds. New generations of ships are often faster than the ones they replace. New technologies were introduced in order to make the speed increase possible. Conventional displacement monohulls could no longer keep up and multihulls and planing hulls were introduced. Even higher speeds were achieved with hydrofoils and air cushion vehicles. The practical maximum speed of all marine craft mentioned so far lies around 100 km/h. The drawback of the recent trend for high speed marine craft is the increased power requirement and fuel consumption. It is very unlikely that any "conventional" marine craft will be able to operate at much higher speeds with acceptable fuel efficiency.
Displacement monohull
Catamaran
Planing monohull
The excessive power requirement of high speed marine craft is mainly caused by viscous drag, well over 50% of the drag is caused by water friction. The obvious solution is to minimise water contact. This approach works for hydrofoils, hovercraft and SES's. The speed of a hovercraft is bounded by the sea state and longitudinal stability considerations and the speed of a hydrofoil by cavitation of the foils.
Surface effect ships
Hydrofoil boat
Hovercraft
The ultimate low-drag marine craft would be a boat without water contact, a hovercraft without a fan: a WIG boat. A WIG boat is a boat with wings that cruises just above the water surface, it is floating on a cushion of relatively high-pressure air between its wing and the water surface. This cushion is created by the aerodynamic interaction between the wing and the surface, called ground effect. This is where a WIG boat is different from an aircraft, it cannot operate without ground effect, so its operating height is limited relative to its size.
WIG boat VT-01 in extreme ground effect
Two Russian Orlyonok WIG boats in ground effect cruise
WIG is an abbreviation of Wing In Ground-effect. A WIG boat can be seen as a crossover between a hovercraft and an aircraft. It flies just above the surface, usually the water surface, therefore others use the term WISE or WISES (Wing In Surface Effect Ship). The Russians use the word Ekranoplan (Ekran = screen, plan = plane), which is also commonly used in other languages nowadays. Some other terms are commonly used in literature.
A hovercraft is floating on a cushion of air that is created by a fan that blows in a cavity under its hull. The cavity is bounded by so called skirts, rubber balloons that restrict air leakage and more or less seal the cavity. The air cushion reduces the friction drag of the hovercraft with the water which would make it a very efficient vehicle if is wasn't for the fan that creates the cushion. A WIG craft also sits on a cushion of air, but this cushion is created by aerodynamics rather than by an engine. This means that it only exists when the WIG craft has sufficient forward speed. This is called a dynamic air cushion as opposed to the hovercrafts static air cushion. You can compare this to the relation aircraft-helicopter, the aircrafts wings generate lift because of their forward speed, whereas the helicopters rotor has work continuously for generating lift. A more indepth explanation of ground effect aerodynamics can be found in the aerodynamics section.
The Volga-2, an ekranoplan (PAR-WIG)
A WIG boat is especially designed to take advantage of the benefits of ground effect. Therefore it will always fly close to the surface. Although it is called ground effect, most WIG vehicles only operate over water, but some are amphibious. Some WIG vehicles have the ability to fly without ground effect as well, but inefficiently as compared to aircraft. Some aircraft are designed to use the ground effect only for take off, such as the VVA-14.
WIG boats have been around for decades, more about their background can be found in the history section. In the past 40 years a large number of different WIG craft have been designed and built, some of which are currently or will soon be commercially available.
In the Encyclopedia of WIG boats most known WIG boats are listed. Looking at all those designs it is remarkable how different they all are. However, when looked at in more detail one can recognise some recurrent concepts which all have their specific qualities. Many recent designs are based on the Lippisch or PAR-WIG design concept.
The Hoverwing, a Lippisch-based design
Despite more than 4 decades of development apparently WIG craft have not reached maturity since they are not widely in use yet. Only recently the step towards commercial application has been considered by some companies. Perhaps the most important reason for this is the fact that especially in the former USSR, where ground effect technology was the most advanced, the Ekranoplan programme was highly secret. In the years before perestroyka it was even forbidden to use the word Ekranoplan.
Apart from this there are a few other reasons for the slow development of this technology. The main problem is getting out of the water, since the required power for take-off is a number of times higher than that required for cruising. This is due to the high drag in the water just before leaving the water surface, also called "hump drag". Flying boats and seaplanes experience the same hump drag at take-off, but they can utilise the excess power to achieve a higher top speed, which is generally not possible for WIG boats due to longitudinal stability issues.
Another limitation for WIG development is size. A WIG craft that fulfills all efficiency expectations would be extremely big, hundreds, maybe thousands of tonnes. Only at this size the relative height will be sufficienly small to be more efficient than for example a 747 on a trans-Atlantic route and still be clear of the waves. It may be clear that the investors for a project to develop a craft this size will not be easy to find if the technology has not proven itself first on smaller designs.
Those smaller designs are not only important for proving technology, but will also initiate rules for certification and build up operational experience. Time will tell if small WIG craft are (commercially) attractive enough to be developed as the predecessors for the ocean liners of the future.
