In reply to Greenbanks:
If you are only doing it once or twice, pad out your roof rack bars (pipe lagging foam and gaffer tape is good), put the boats on upside down (decks are normally flatter than hulls and sit better, plus being upside down they can’t fill with water if it rains), then tie down on both bars with luggage straps. Make sure the cockpit combing or hatch rim isn’t sitting on a bar. For additional security run a rope from the bows to the front towing point and similar to the rear (doesn’t need to be tight, it’s a last resort to keep everything connected to the car) , Stick some dangling flags/ rags on the stern so that if you reverse you can see them and know how far behind they extend. Check your hatch covers have retaining cords, if not consider removing for the journey, Check after a few miles. Job done.
But before you start read your car max roof load limit in your manual, some cars roof attachments points are little more than cosemetic.
if you intend to transport them regularly specialist cradles give costly peace of mind, but may not be the easiest to load, and cockpit covers are good not only to stop rain getting in but also to stop grit/pebbles getting out and potentially chipping other road users windscreens.
Alternatively a cheaper option is to fashion mini cell foam blocks to sit on your bars and carve to match the hull shape so the boat fits better and the hull is evenly loaded, then tie down as above.
Post edited at 16:13