I just use the 1/4" braided polypropylene that I use as a bow line since it floats. Tied down with a truckers hitch. I've never had a problem with stretch.
If you need low stretch 1/4" braided polyester is a cheaper option than the exotic ropes.
After years (er, decades) of replacing inexpensive Home Depot poly rope I bit the bullet and started buying good quality rope. Piecemeal at first, 30 feet at a time from this outfitter and that. Some was more hand kindly, some held knots or hitches better than others. Some of the best was WTF black, which is a stupid color for painter rope that may be needed in a rescue.
I knew I want floating rope, for obvious snag-safety reasons. I knew I wanted high tensile strength. I knew I wanted it garish bright for easy visibility. I knew I want rope thick enough that it doesn’t hurt my hand when pulling the canoe while I trudged across a hundred yards of 2 inch deep sandbar, or for hauling it high and dry up a steep bank to some ledge site. I knew I wanted rope that holds firm with a knot or hitch.
Given those criteria Pblanc suggested this rope. I like it.
http://www.austinkayak.com/products/7241/BlueWater-Ropes-516-River-Rescue-Rope-BW-R3-300-ft.html
Ever helpful, Pete suggested it when it was both on sale and coupon discounted. At the (then cheaper) price I bought a 300 ft spool, which should last me a lifetime, and I will never use junk painter lines again.
I still have a box of Home-Crappo rope, some used, some still new. That’s now the give away “Here, use this” cordage.
One issue I have had is the boat moving forward on the racks some while braking hard, making the bow to bumper tie down slack.
Yeah, this bow/stern rope arrangement /______\ , typical on most vehicles, kinda lends itself to that. The bow and stern lines need to be tensioned in opposite directions, but I really prefer the belly lines tied off to the rack crossbars (with gunwale stops), and the bow and stern lines tied off to the vehicle. Belt and suspender tie downs, in case of catastrophic rack failure.
With the usual 2 canoe load that is 8 lines, 8 trucker’s hitches and 5 minutes tops to tie it all down.
The gunwale stops help prevent forward movement to some degree, but on long highway roadtrips I add a third painter line pulling the canoe backward. That line is unfortunately tied from thwart or seat to crossbar; in a real panic brake I’d rather have that emergency line going to some stout vehicle point, rather than risk tear off a crossbar or thwart.
But I have not yet figured out a way to tie down an oppositional stern springline that’s attached to the bumper or hitch without occluding access to the truck cap or van doors.