as long as you support the kayak where it is strongest, in most cases this is in the bulk-head areas. In most cases this divides the kayak roughly into thirds. I store my kayaks on their sides.
If you use a hard mount bracket you may want to pad the bracket with carpet or closed cell foam.
Kayak Motor Kit
Touring Kayak Paddles
Cartop Kayak Carriers
Canoe / Kayak Anchors