My house is a new house with what looks like a standard 2 car garage. I have 19'6" from the rear wall to the door. My 98 speedster is 20'6" on the trailer. I push the trailer in and turn it slightly and it fits with about a foot and a half in front of it that I can walk around easily. Works great if you dont want to park your car in the garage. My avalanche wont fit anyway since it is so long, so I dont mind.
So I know a 16.5 ft boat will easily fit. I am 99% sure a 18 ft challenger would fit this way also.
I really want a 20 ft Speedster and think it may possibly fit using this same method and a break away tongue. I just havent found anyone around who has one I can test the theory with.