Air Cavity System is a commercially offered technology for reducing drag of ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s.

Principle of operation

The idea behind ACS is to trap air beneath hull recesses and thus reduce wetted area with associated viscous drag. Since some air will be inevitably carried away by water stream, a pump connected to cavity must constantly replenish the losses. In small vessels engine exhaust might be used instead.

This is somewhat similar to surface effect ship

Surface effect ship

A Surface Effect Ship or Sidewall Hovercraft is a watercraft that has both an air cushion, like a hovercraft, and twin hulls, like a catamaran. When the air cushion is in use, a small portion of the twin hulls remain in the water...

 but ACS air cavity is not connected to the atmosphere which potentially allows for much higher air pressure to be maintained with less expense.

Efficiency

Netherland based DK Group, a company offering ACS as of 2011 promises up to 15% fuel savings. An engineer of DK Group even claims up to 40% savings are possible on purpose-build vessels.

However, according to the article in Scientific American more than 10% of drag reduction with ACS has never been yet independently confirmed.

See also

• Surface effect ship
  Surface effect ship
  A Surface Effect Ship or Sidewall Hovercraft is a watercraft that has both an air cushion, like a hovercraft, and twin hulls, like a catamaran. When the air cushion is in use, a small portion of the twin hulls remain in the water...
  
  
• Skjold class patrol boat
  Skjold class patrol boat
  The Skjold class patrol boats is a new class of superfast, large stealth missile craft, formerly known as MTBs . From 2009, the Royal Norwegian Navy officially label them as coastal corvettes because their seaworthiness is seen as comparable to corvettes. They are built at the Umoe Mandal yard...
  
  s are the fastest armed boats in the world as of 2011.
• Future boat developments
  Future Boat Developments
  Rising oil prices and environmental concerns are driving boat development towards better fuel efficiency and reduced pollution. Global warming impact of shipping also begins to be of concern. According to Professor James J...
  
  
• Supercavitation
  Supercavitation
  Supercavitation is the use of cavitation effects to create a bubble of gas inside a liquid large enough to encompass an object traveling through the liquid, greatly reducing the skin friction drag on the object and enabling achievement of very high speeds...