Skyhawk 172/Cutlass 172
Air Traffic Control Fully Certified
Communication Breakdown; Situational Awareness
ATC Issue All Types; Conflict NMAC
Flight Crew Took Evasive Action
Human Factors
I was briefed and took the position. The sequence was set up as the Cessna 172 number one midfield; followed by an experimental sea plane by a mile and a half; all aircraft were sequenced to Runway XX. As the C172 was midfield; I cleared him number one for the option which he then read back. I then told the experimental to follow Skyhawk ahead in the downwind and he replied that he did not have traffic. I then saw that his traffic was in the base turn and told him traffic ahead and to your right a Skyhawk in the right base. He then said he had the traffic in sight and I cleared him number two to land. As the C172 joined the final I noticed the experimental a little tight on the base but he was 40 KTS slower than the preceding traffic and if he flew a normal pattern would fall in place behind the Skyhawk that he had already reported in sight. He then continued in the right turn and cut off the C172. The C172 then made a turn to the south to avoid being hit by the experimental. The pilot reported later on the phone that he thought it may have been less than 50 feet. After the C172 reported southbound and said that the experimental had descended into them; I instructed him to fly eastbound and enter a left base to Runway XX. The experimental and the C172 both landed without incident. There are no recommendations that I can make as this would have been avoided if the pilot of the experimental had been honest and told me that he didn't have the traffic in sight.
Local Controller described pattern conflict when one aircraft was instructed to follow traffic that was reported insight by the pilot; but turned base inside of the preceding traffic requiring evasive actions.