From our experience…
– Last Updated: Apr-15-07 10:58 AM EST –
We have had either Mercury/Ford sable/taurus wagons, and most recently a Subaru Outback, since we have had our own boats. So we've had long low rooflines in a resonably heavy car - adjust whatever I say for the situation you have with the Scion.
1. Plastic boats are always best carried on their edge, via a stacker or a Jbar, because that's where the boat is strongest. I suspect that a stacker would be easier for you to get the boat set into, but I might be wrong. If you can find these - the older Yakima stackers were actually an upside-down narrow U shape. They hold a boat very securely and, for a shorter person, are a little easier to reach and run a strap thru than the vertical ones.
We carried our first sea kayaks, one 16' and one 16'7", on these stackers mounted on Yakima rails supported by Yakima towers. We had this combo in pretty high winds - 30 mph plus - many trips over two hours and an annual one of 8 hours. The boats never budged. In that position the boat is real solid.
2. Saddles take up more space than stackers to carry a boat. In order to get more than two boats up, or two on saddles and a couple of bikes, we had to go to the longest bar length - something in the 70's as I recall.
3. (should be) fine to heft 36 pounds into a hullraisers for even a small person. You just might need to get a stool and push the boat around a bit to finish things off. If you can't do it you should hit the gym because you'll beat up your shoulders etc that much with regular paddling and hurt yourself.
4. We've carried four 16' to 17'8" glass boats for short trips on (padded) stackers and cross bars, and have carried two glass 17' plus and one plastic 16' boat on a 10 hour drive to a symposium in Maine on stackers. Not a problem.
One alternative to rollers in back is gliders - small flat feet that spread the weight out but till are easier to slide a boat into than regular saddles. I think the gliders are Thule - but you can mix these things up. I would recommend double straps in back and altogether very careful strapping of tying down with them - they offer a little less in the way of comforting surface to lock a boat down onto than saddles or stackers.