There are two different chartered ferries to return to Stockholm with. One is on Monday evening at 21:30 from Utö, to a waiting bus at Årsta Brygga, which will take you to Stockholm City Terminal with an arrival time at about 23:30 on Monday night.
The second option is on Tuesday morning at 09:30 from Utö to a waiting bus at Årsta Brygga that will take you to Stockholm C. at around 11:30.
The Monday night ferry needs to be booked no later than Friday August 30th to be guaranteed a spot. It is of course free for you as a racer.
The Tuesday morning ferry does not need to be booked.