Each pair of boats racing is followed by two umpire boats. If a Skipper thinks a foul against them has occurred he will hold up a flag. The umpire will immediately respond as to whether a foul has occurred.
If the umpire determines there is no foul they will hold up a green and white checkered flag. If there is a foul the umpire will hold up a yellow or blue flag (each boat will have either a yellow or blue flag flying off the back) which indicates that boat has been charged with a penalty and is required to complete a 360 degree turn before crossing the finish line.