-- Last Updated: May-20-12 4:55 PM EST --
I cartop a 17' boat with a rack with 30" spread and have been doing so for years. You might be surprised at how few cars offer a greater spread.
Use front and rear tie downs, and get an extender if it makes you feel safer; there are a few including one made for long surfskis that looks great, but I can't recall the manufacturer. Hopefully someone else jumps in with product info.