RedactedNarrative: Vessel 2 was travelling on plane north on the Wisconsin River.  Vessel 1 was travelling on plane behind vessel 2.  Vessel 2 decreased speed to navigate to a fishing spot closer to the west shore.  Vessel 2 did not look behind to see how close vessel 1 was following.  Vessel 1 was not paying attention and didn't observe vessel 2 slow down.  Vessel 1 turned hard to the left.  Vessel 1 collided with vessel 2.  Vessel 1's engine came out of the water causing prop damage to vessel 2 rear and left side.  vessel 1 also suffered collision damage on the rear from the impact.  No injuries were sustained and the incident was reported 5 days after the fact.