When I bought CLC's Oxford Shell 'Classic' demo boat, CLC assured me I could car-top it for the 2+ hour ride home on my Jetta.I was dubious but they were right! The OS is very light and just needs to be secured.  I looped bowlines on stem and stern and ran them to the attachment points I could find under the bumpers, and tied the middle snugly (deck/coaming side down) to bike rack tracks I had on  my roof rack, with a little foam cushioning. The 20 foot boat perched on top of my 14 foot car looked a little weird, but I had no problems at all on the drive. 

I have not transported that boat but regularly do transport boats of similar length.  You can transport on foam kayak racks but if you do, bow and stern lines are a must.  Saddles would be better than foam because they provide more side to side support.  Widely spaced racks like those on my truck are better than racks close together on small cars.  The best is a bar/V-rack type system that most of us with skis use.  They spread the loads more over the length of the boat instead of concentrating it near the center.  Be careful to not overtighten your straps because they can damage the boat.  On V-rack systems, bungies are normally used so you don't have that worry.