I'd say The Cove Marina in Tanyard Branch would be the closest. (it's the marina formerly known as Red Gate Marina) But I don't know if they still have a launch ramp, and I fear it's become more of a "party place". You can call them @
(859) 304-1430
Other than that, it's Chimney Rock, Pandora, or Kamp Kennedy .... all of which are miles downstream from Herrington Marina or The Cove Marina. All the marinas upstream are even farther away (by several miles).
I'm curious, though ... what does having a 6hp have to do with launching at Herrington Marina ?? If you only have a 6hp motor on your boat, then you'd be better off launching at a ramp closest to where you intend to fish. That being said ... I used to fish out of the flat bottom wooden row boats that you could rent at Red Gate Marina & put a 10hp motor on it and fish Tanyard Branch & McKecknie Branch back when I was a kid fishing with my grandparents. It wasn't really that long of a ride, even though it's idle speed in both creeks.
On a personal note : I gave up even considering launching a boat at Herrington Marina ... not only because of the sharp curve launch ramp, but the people running the marina were never really all that "angler friendly" IMHO. They were the first to raise the launch fee from $5 to $10 ... then the other launch fees eventually were raised. If the other launch fees follow suit, then I won't be fishing Herrington anymore. (not that I fish it more than 1-2 times a year, as it is)
(oops ... sorry for the rant. LOL !! )