They have  a skirt around their bottoms.
New Zealander Dr. Alastair Kenneth Senior died in an attempt to use his homemade machine of this type.  Designs of these devices are based of A.A. West's single layer theory. These machines are sometimes confused  for devices that operate on the WIG effect. The enormous Aist and Zubr classes of these machines were  employed by the Soviet military. The father of this type of machine is Christopher Cockerell and his efforts  lead to the development of the SR.N1 . The U.S. Military uses the LCAC type of these to land troops. They have  a skirt around their bottoms. For 10 points, name these vehicles that ride over land or water on a cushion of air.
New Zealander Dr. Alastair Kenneth Senior died in an attempt to use his homemade machine of this type.  
hovercraft [or air cushion craft before mentioned]