NMAC BTWN A HIGH WING C150 MAKING A STRAIGHT OUT DEP ON RWY HDG; ABOUT 3 MI FROM THE ARPT; AND A LOW WING PIPER CHEROKEE; ALSO ON A STRAIGHT OUT DEP; TAKING OFF BEHIND THE C150; AND PASSING THE C150 ABOUT 25 FT ABOVE. THE RPTING C150 PLT MADE AN IMMEDIATE TURN TO THE R TO HELP INCREASE FUTURE SEPARATION.

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I WAS DEPARTING RWY 23 AT LWM IN A C150 SBOUND; WHEN I HAD A NEAR MISS WITH A PIPER CHEROKEE ON CLBOUT BEHIND ME. IN THE RUNUP AREA I RECALL A C172 AND A CHEROKEE BESIDES OUR C150. THE C172 DEPARTED FIRST WITH A L TURN; THEN WE WERE CLRED FOR TKOF WITH A STRAIGHT-OUT DEP. OUT CLBOUT SPD WAS 75-80 MPH. I RECALL HEARING ANOTHER ACFT BEING CLRED FOR TKOF WITH A STRAIGHT-OUT DEP BEHIND US. I ASSUME IT WAS THE CHEROKEE I SAW IN THE RUNUP AREA. PROBABLY BTWN 3 AND 4 MI OUT; I ASKED TWR FOR A FREQ CHANGE; SO I COULD PICK UP A SQUAWK CODE AND RADAR ADVISORIES FROM BOS APCH. TWR APPROVED MY FREQ CHANGE AND WE CONTACTED APCH. WE WERE IMMEDIATELY GIVEN A CODE AND I REQUESTED FLT FOLLOWING; DIRECT PUT. AFTER BEING ON THE BOS APCH FREQ FOR A SHORT TIME; PERHAPS 2 MINS; A PIPER CHEROKEE TYPE ACFT FLEW DIRECTLY OVER US EXACTLY ON OUR HDG. I RPTED THIS TO APCH; AND TOLD THEM THAT THE OTHER ACFT HAD COME WITHIN 50 FT OF US. IT IS PROBABLE THAT THE SEPARATION VERTLY WAS CLOSER TO 25 FT. LATERAL SEPARATION WAS NEARLY ZERO. I LEVELED AND BEGAN A SLIGHT R TURN. THE CHEROKEE BEGAN SLOWLY TURNING TO MY L AND APPARENTLY ON COURSE. THE TIME WAS APPROX XA10Z. FROM THE ACFT ACTIONS; I ASSUMED HE HAD NEVER SEEN ME. I FURTHER ASSUME THAT HE WAS STILL ON THE TWR FREQ AT THE TIME. I TOLD APCH THAT I WANTED TO FILE A NEAR MISS RPT. THEY TOLD ME I NEEDED TO DO THAT WITH THE FSDO. I SAID THAT I JUST WANTED THAT REQUEST ON THE TAPE. I ASKED APCH IF THEY SHOWED THE OTHER TARGET ON THEIR SCREEN. INITIALLY HE DID NOT RPT THE OTHER TARGET. PERHAPS 30 SECONDS LATER HE RPTED A TARGET AT 1900 FT AT MY 11 O'CLOCK POS. I TOLD APCH THAT THAT WAS THE OTHER ACFT. I HAD VISUAL CONTACT WITH THE OTHER ACFT AND I HAD ALREADY CLBED ABOVE IT. THE RANGE WAS PERHAPS 1 MI. THIS INDICATES THAT BOTH ACFT WERE SQUAWKING MODE C AND THE RADAR TAPES SHOULD TELL THE WHOLE STORY. WE REQUESTED AN ALT OF 4500 FT; BUT WERE INSTRUCTED TO REMAIN CLR OF BOSTON CLASS B AIRSPACE. I REMAINED AT ABOUT 2200 FT FOR SEVERAL MI; UNTIL THE CTLR CLRED US INTO BOSTON CLASS B AIRSPACE AT 4500 FT DIRECT PUT. I BELIEVE THAT THE OTHER PLT FAILED IN HIS DUTY TO SEE AND AVOID MY ACFT ON HIS DEP. THE FACT THAT IT WAS A 'HIGH WING/LOW WING' SIT MAY HAVE EXACERBATED THE PROB. THERE WERE SPECIAL EVENTS SCHEDULED AT THE ARPT THAT DAY; AND THERE WAS A HIGH NUMBER OF ARRS AND DEPS TAKING PLACE.