RedactedNarrative: Vessel 1 in front of Vessel 2, both travelling at 10 mph or less.  Vessel 1 Operator suddenly slowed down in front of Vessel 2 and Vessel 2 Operator was unable to avoid a collision.  Vessel 2 struck Vessel 1 on the right-hand side, causing damage.  Vessel 1 Operator was thrown from the seat and claimed to lose consciousness for approximately 5-8 seconds.