Howdy Redtug,
Rye Harbor Ramp, Pierce Island Ramp in Portsmouth, Wells Harbor Ramp are the three I use. There is one at Traip in Kittery but I don't think we can make the swing with the length of the trailer for the R21. There is also Cashman Park in Newburyport on the Merrimack.
Wells is the easiest at all tides and currently is free. Pierce Island is good from about half tide and up, although you can do it at any tide there is not very much dock at low. Also, a bit hairy to get there through downtown Portsmouth and you want to make sure you are on the up tide side of the dock as I have seen many people damage their boats thinking the current isn't as strong as it is. Rye is good except 1 hour before/after low. Years of people power loading have eroded the bottom of the ramp and if you go off, you may get stuck.
There is also one in Elliot and I believe in Dover up by great bay but I forget the name.
Did we meet at the Standish ramp on Sebago a few weeks ago?
Be well.