A C177; FLYING ONLY 500 FT MSL IN MARGINAL WX; ENCOUNTERED A BLIMP. FORCING HIM TO TAKE EVASIVE ACTION; WHICH RESULTED IN AN INCURSION INTO JFK CLASS B AIRSPACE.

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WE FLEW S THROUGH THE NEW YORK/HUDSON RIVER CORRIDOR AT 1000 FT. AFTER PASSING THE NARROWS BRIDGE; WE DSNDED TO 500 FT; TURNED L (E); AND FLEW ABOUT 1/3 MI OFFSHORE PAST CONEY ISLAND AND THE ROCKAWAYS (ENRTE TO FRG). BEING 1/3 MI OFFSHORE MEANS THAT WE WOULD BE TO THE R OF ANY WBOUND TFC HDG TO THE CORRIDOR. IT WAS OUR INTENT TO FLY E TO JONES BEACH; THEN CONTACT FRG TWR FOR LNDG INSTRUCTIONS. AS WE APCHED THE LONG BEACH AREA -- ALMOST TO WHERE THE MINIMUM 'B' ALTS CHANGE FROM +500 FT TO +1500 FT; A LARGE OBJECT JUST AHEAD EMERGED FROM THE HAZE. AT FIRST; WE THOUGHT IT WAS A BALLOON; THEN REALIZED IT WAS AN ADVERTISING BLIMP AT OUR ALT. IT WAS ABOUT 1/4 MI OFFSHORE AND ALMOST DIRECTLY IN OUR FLT PATH. WE IMMEDIATELY TURNED 90 DEGS R AND INITIATED A CLB. DEPENDING ON OUR EXACT POS; IT IS POSSIBLE WE BRIEFLY ENTERED CLASS B AIRSPACE WHILE WE AVOIDED THE BLIMP. AT 1200 FT; ABOUT 1/2 MI OFFSHORE; WE RESUMED AN EASTERLY COURSE. THIS RPT IS NOT JUST ABOUT A POSSIBLE CLASS B VIOLATION. IT IS ABOUT AN ADVERTISING BLIMP POSITIONED JUST OFFSHORE IN AIRSPACE USED AS THE SOUTHEASTERN FUNNEL OF THE NEW YORK CLASS B CORRIDOR. THE BLIMP; I REALIZE; WAS PROBABLY LEGAL. BUT ITS PRESENCE IS TANTAMOUNT TO ERECTING A BILLBOARD IN A TRAVEL LANE OF A HWY. THE BLIMP OPERATOR COULD APPLY FOR A WAIVER SIMILAR TO BANNER-TOW ACFT; ALLOWING IT TO BE LOWER (AND OUT OF THE WAY OF TRANSITING AIRPLANES). ALSO; THE OPERATOR SHOULD INFORM FSS SO WE COULD HAVE BEEN ALERTED IN ADVANCE TO ITS PRESENCE.