RedactedNarrative: Unit 2 was tied to dock at Lake Murray Dam with no one aboard.  Owner of Unit 2 was in parking lot when collision occurred.  Operator of Unit 1 was attempting to tie rope off to the dock at Lake Murray Dam behind Unit 2.  According to operator of Unit 1 mooring rope got tangled in the throttle of Unit 1 causing Unit 1's bow to collide with the stern of Unit 2.