PLT OF PA28 EXPERIENCES NMAC WHILE DEPARTING BFI.

Narrative

I WAS ON A DAYTIME VFR FLT FROM BFI TO FHR. I DEPARTED BFI ON RWY 31L; CLBING 'STRAIGHT OUT' TO THE NW (HDG 310 DEGS). THE FLOOR OF CLASS B AIRSPACE OVER MOST OF BFI IS 1100 FT MSL. N OF THE ARPT IT IS 1800 FT MSL. FIELD ELEVATION IS 18 FT MSL. ACFT ARE SUPPOSED TO AVOID FLYING OVER OR NEAR THE BASEBALL AND FOOTBALL STADIUMS N OF THE FIELD; JUST E OF THE STRAIGHT-OUT DEP PATH. BTWN THE LOW CLASS B FLOOR; THE BALLPARKS; THE OVERLAPPING AIRSPACE OF 3 TWRED ARPTS (SEA; BFI; AND RNT); HIGHER TERRAIN AND A FEW SETS OF TALL TWRS N AND NW OF DOWNTOWN SEATTLE (741 FT; 1015 FT; 1049 FT; AND 1071 FT MSL); AND FLOAT PLANE TFC ON LAKE UNION AND LAKE WASHINGTON; THIS TENDS TO BE VERY CONGESTED AIRSPACE. ACFT MIGHT BE ON ANY OF SEVERAL RADIO FREQS (SEA TWR; 2 BFI TWR FREQS; SEATTLE APCH OR DEP) OR NO RADIO CONTACT. I CLBED TO 1000 FT MSL TO STAY BELOW THE CLASS B AIRSPACE; AND MAINTAINED RWY HDG (310 DEGS) TO AVOID FLYING UNNECESSARILY CLOSE TO THE BALL PARK ON MY R AND TO STAY CLR OF ANY TFC INBOUND TO THE BFI DOWNWIND (SHOULD BE ON MY L FOR RWY 31L; ON MY R FOR RWY 31R). AT ABOUT 3 NM N OF BFI; APCHING THE S END OF ELLIOT BAY; I REQUESTED AND WAS GRANTED A FREQ CHANGE TO CONTACT SEA DEP FOR FLT FOLLOWING. I HAD PRE-PROGRAMMED THAT FREQ ON 1 RADIO. I LOOKED AT THE RADIOS AND THE COM PANEL TO VERIFY THAT I WAS SWITCHING TO THE RIGHT FREQ ON THE RIGHT RADIO. THIS TOOK A FEW SECONDS. AS I LOOKED BACK UP AND OUTSIDE; MY BACK SEAT PAX (A RETIRED PROFESSIONAL PLT SITTING DIRECTLY BEHIND ME ON THE L SIDE OF THE ACFT) TAPPED ME ON THE HEAD; LEANED OVER MY SHOULDER; AND POINTED AHEAD AND TO THE L AND DOWN (HE WAS NOT ON THE INTERCOM). I SAW WHAT APPEARED TO BE A BEECH KING AIR COMING ALMOST DIRECTLY AT ME; MOVING QUICKLY FROM MY 10-11 O'CLOCK POS; WING UP IN A R TURN. CLBING AWAY FROM THAT ACFT MIGHT CAUSE ME TO VIOLATE THE CLASS B AIRSPACE. A R TURN AWAY FROM THAT ACFT MIGHT PUT ME TOO CLOSE TO THE BALL PARK. A CLBING R TURN; MY BEST COURSE OF EVASIVE ACTION; MIGHT DO BOTH. THAT CAUSED ME TO HESITATE MOMENTARILY; THEN MAKE A MODERATE COURSE AND ALT CORRECTION. HAD IT NOT BEEN FOR THE AIRSPACE RESTRS; I WOULD HAVE MADE A MUCH MORE SIGNIFICANT AND MORE IMMEDIATE CORRECTION. THE OTHER ACFT PASSED WITHIN 100 FT OF MY L WING; ABOUT 20 FT BELOW ME; AND CONTINUED ITS TURN BEHIND ME. I DID NOT HEAR THE OTHER ACFT ON THE RADIO (THEY MIGHT HAVE BEEN ON A DIFFERENT FREQ; AS DESCRIBED ABOVE). BASED ON THE OTHER ACFT'S CONSTANT BANK ANGLE AND ALT AND THEIR LACK OF ANY EVASIVE ACTION TO MISS ME; I DOUBT THAT THEY EVER SAW ME.