Ah, good! It should not have moved at all, if you cinched it down right.
See if this makes sense...
1. loop each tiedown around middle of cross bars.
2. load boat.
3. flip both free ends of each tie down over boat. You should have 4 free ends hanging over the boat.
4. run the non-buckle free end of tiedown, UNDER the inside of the main rack (the part that runs along roof), and out.
5. Buckle should be resting on boat about 12" above rack.
6. Feed free end through buckle and tighten to the point boat just begins to "squish" or deform....not too tight but not loose.
That way you have a solid connection between boat and roof rack. Should be no movement in wind at all.
If you don't have running boards on your SUV, stand on the tire!
paul, texas KD5IVP