I should have added that what I'm unsure on is just how to strap it down.  Do I need to have rope at the bow and stern, or is it enough to strap it down at the roof rack

That depends on how far apart the racks are. I don't use bow or stern lines with mine. The stern extends 1 foot beyond the rear rack. The front rack is just at the front of the coaming. In front I use a ratchet strap and in the rear a bungee with hooks. And before anyone gets excited, it's been 13 years and the WD12 has neither flown off, come loose or cracked from the ratchet strap.

Finally, once you've paddled that boat a few times, you may find yourself cartopping it a lot more than you expect.

Laszlo

I recommend a kayak foam blocks set. It comes with straps and the whole thing is less than $40.  I have a nice Thule or something rack that does not work as well when hauling only one kayak.

I place the boat cockpit side down and cinch across the beam before and aft of the cockpit coaming.  I really cinch it down, but then I use the remaining straps to ensure the boat doesn't move fore and aft, strapping from rack to padeye and back-again.  It is very secure with this setup.

Regarding your specific question about bow and stern lines I consider them to be standard car topping equipment and strongly recommend them for kayaks of any length.

Under normal driving situations bow and stern lines provide:

1. Redundancy in case some part of your main straps come loose or break. While rare, it happens, and these lines buy you some time to stop and address any problems with main straps. This is especially important at high speeds.
2. An easy visual indicator/proxy of how stable your kayak is at all times while driving (relates to #1). You can't always see the boat itself, but you can easily if the bow or stern lines are moving in an unusual way.
3. Some extra stability for the whole tie-down system.

But probably most importantly, in exceptional situations, such as a crash, they may be the only thing keeping your boat from becoming a projectile. While those main straps provide tremendous fore-aft grip, inertia is a wild thing if you decelerate fast enough.

All this said, I've never personally tested my bow and stern lines in strap failure or a crash (and hopefully never will), but I'm always very happy they are there. They really don't take long to put on either one you get a good system and habit in place for your specific vehicle.

   Laszlo - how do you pad between the kayak and the roof/rack?

   If you have the cross bars on your Explorer, I'd just get a set of these from CLC:

https://www.clcboats.com/shop/products/new/aero-roof-rack-pads.html

They work fine on my Mazda for a SUP and were fine when I cartopped the Skerry (getting it on there was the trick).  

And yes, I'd put a bow and stern line on if you have handles/tiedowns on the WD.  Just in case.

For padding I use forms I cut to individually match the bottom of my boat. The forms are made from the 3" thick minicell foam sold by CLC. They're contact cemented to wood which is then bolted to the racks. That eliminates the possibility of the pads moving.

The 3" wide foam surface absorbs the force of the ratchet strap and provides a high friction surface when the straps are tightened. It also cushions road bumps.

Laszlo