This is an old topic, but I got locked out of the forum for a while and then was travelling. Now I’m back I maybe can pass on my ideas. I car-top my boats a lot, to and from the paddling pond down the road and also on vacation. I always carry them upside-down.

My lightweight, somewhat delicate canoe sits on top of my minivan’s roof rack with foam blocks on the gunnel to protect the wood; the foam blocks deform nicely over the rack and also grip the canoe well.

For my 2 plastic kayaks I have a pair of 2 x 4 beams that I mount across the roof rack, each has a couple of dowels in the underside that fit into holes already in the rack, to keep the beams from sliding around while loading the boats. The beam topsides are notched to fit the edge of the cockpit coamings, 4 notches per kayak. There is a big hook bolt in the center of each beam, the rachet ties run over the first boat, under the hook, over the second boat (if there is one) and across to the far side of the rack; works for one or two kayaks.

The 2 x 4's also serve to transport my small sailboat which I built with flat side decks with this in mind.

With any boat on top of the van, I take the painter line back around a rack crossbar and forward again to the painter hook or handle then down to a bracket that I have attached permanently and unobtrusively to the hood (bonnet in the UK) to hold the boat if it slips. This serves as a reassuring tell-tale in case the boat works lose, which saves me stopping to check the ties. I don’t use a rer strap, but a rag hanging from the rear of the boat is visible through the rear window which reassures me the back end of the boat is still where I put it.

The only downside is vibration from the ties in the slipstream, but a single twist solves that problem. The difference in fuel consumption with 2 kayaks on top is no more than 3% over the same journey repeated many times.