Jet skis, PWCs (personal water craft) and jet boats derive their name from their type of drive; the jet drive. These recreational watercrafts are driven by a jet valve similar to the those of an aircraft, but use water force instead of air force. Water is sucked in at the bottom of the hull and discharged through a pipe at the stern. Therefore, a jet ski or jet boat cannot ride astern as with conventional propeller drives. The flow direction can not be changed, so the watercraft uses a curved flap as an aid for reverse thrust. An advantage of the jetski and jetboat is the shallow draft and the impossibility of ruining the propeller.