-- Last Updated: Mar-07-13 7:48 PM EST --
Another alternative for any relatively flat roof is the tracks from Thule or Yakima. Probably the strongest, most secure mount of all.
I usually drive 65 regardless of boats, but the guys that pass me on the flat highway I pass going over the Altamont Pass, still at 65. I've never had the engine over 4K RPM (65 in 4th gear) but its usually at about 2200 (65 in 6th gear)
I 'spose you could put a playboat inside, but since my boats are all 8-9' long and I am averse to wet sandy stuff all over the interior, I haven't done it. Can readily carry WW paddles inside if desired.