For years, I cartopped a Laser and then a Minifish atop my 16-year-old Camry... neither boat was that heavy, so cartopping did NOT affect the performance of the vehicle that much. My greatest concern was securement, so I developed a system for lashing down the hull and spars, and that system was good enough to haul either boat safely at freeway speeds. The Minifish made it from Coronado, CA, out to the Salton Sea, no problem, which included pulling the grade up and over mountain passes 4,000' in elevation. Of course, traveling that far with the boat atop the car, I stopped several times en route to check the lash job. When cartopping, I always flipped the hull atop the car with bow forward, and I used snap-links from the Depot (industrial carabiners) to clip those metal eyes under the car, or clip other points of securement. I also ran torque straps around the hull and through the door frames of the car, opening the car doors first so I could open them if necessary during the trip to water's edge (or back to the house). The dolly I built went into the trunk, so parking wasn't that big of an issue on the other (launching) end of the trip. I must say that the Mighty Obamanomic Megayacht Transporter DID receive a few scratches in the paint over the years, but that didn't bother me, I was happy just to avoid trailer registration fees, maintenance, etc. You'll need at least one more person to facilitate the cartopping, otherwise you'll have to ask passersby to help ya flip the boat up or down. Once you find a good launching area for your boat, cartopping is a viable way to get the boat down to the water and back to your house... flipping the boat will give ya muscles to work the sheets under sail, LOL. Be lookin' like Ah-nold Schwarzenegger in no time, AYE???
P.S. When sailing on San Diego Bay, I would launch from a small beach by the boat ramp in Glorietta Bay---perfectly legal with the construction of the new boathouse for kayaks & SUPs, the city changed the signage to permit the launching of non-motorized craft. Whenever I sailed offshore, I would launch from a small beach near the boat ramp on Shelter Island, which would save me time and effort beating to windward down the channel. Figure out the best (and legal) launch points for your boat, then cartopping becomes fairly easy, plus you'll be saving those trailer registration fees I mentioned.