General aviation pilot reports NMAC with blimp while transiting beneath New York Class B using the 'shoreline transition'. Large size and slow speed caused perception difficulties for the reporter.

Narrative

My route of flight required transition of JFK Class B/Class G airspace. Due to ceilings; elected to do 'shoreline transition' west to east; at or below 500 FT; outside of Class B but within Class G. This is not my usual route; which is a VFR transition of the Class B. The corridor is actually fairly narrow; bounded to the north by the shoreline; to the south by safe glide distance to the shore from below 500 FT; to the top by the 500 FT floor of the Class B; and to the bottom by safe distance from the ground. Traffic was a banner towing plane to my left and a blimp to my right. Passed the plane uneventfully; easy see-and-avoid. The combination of the unfamiliar size and slow speed of the blimp created an illusion that I was closing much more slowly than I actually was - assuming blimp was moving at perhaps 30 KTS. I was probably closing at close to 100 KTS. As the blimp loomed in front of me; I descended rapidly to insure passing safely. Probably I should have deviated to pass the blimp on the right; even though this would have taken me further from the shore. I definitely should have slowed down as I approached the blimp. Were it another airplane or helicopter; the closing speed would have been much slower; and more easily judged.