some ideas

• you get a week to play the game 
• each person has to put at least 24hrs in 
• this can be 6hrs one day 4hrs another then 14hrs another
• while the persons not there their boat stops
• each person only races against AI not each other (to keep the competitive feel to the game)
• at the end of the week who ever completed the race in the best time wins the game
• may be each person that dose the race has to have the exact same boat so that no one has the advantage. (so no one feels cheated out of their money)