This is sooo true. The Windrush was 75kgs when being used as a no trailer, roof top boat.
boat sat next to car with roof rack. Boat was flipped up and over to lean against rack. Boat lifted and pushed onto rack from side. spars etc tied into tramp section.
45kg full rigged boat will end up being a 30 kg lift. That is about 70pounds.
As I have already said, the roof set up is as important as the boat.
Any car can take heaps more weight than this. I reciently moved a 70kg sheet of glass on the froof of my old Honda Accord. Two pillows on roof to take pressure and a spectra haliard jammed in the doors!