-- Last Updated: Jun-25-12 11:08 AM EST --
As one engineer to another, I would like to respond to your thoughtful reply with some additional clarifications directed to all who have responded negatively to my cautions. It never occurred to me that a cross wind could topple a small car like a Mini, and although I suppose it is possible, such an outcome is quite unlikely. From your description of your setup it seems reasonable that the boat is quite secure to the car. I had assumed that this would be so, but hereby verify and accept that it would be possible to securely attach the boat to the car, no matter the size of the car, and that you have done so.
My concern and the reason I stated that small cars such as the Mini would not be safe under high wind conditions has to do with directional stability and control. A long boat mounted to a small car has a significant increase in sail area and the combination of the increase in overhang and the relatively light weight of the car make this combination unsafe in certain environments. I am an old guy (64), have been doing this paddle sport thing since I was a kid, and can attest that there are places all over North America where there are dangerous cross winds at times. States where the police might choose to intervene in my experience include New Hampshire, Washington, Wyoming, Florida and virtually all of the Canadian Provinces, especially out west. In addition to dangerous cross winds, there is the problem of high speed, two-way highways with one lane going in each direction. On these roads a large trailer truck approaching in the opposite direction can cause significant buffeting due to wind wash. A car that is quite small is much more influenced than a bigger one in these conditions. Adding a significant amount of sail area to smaller vehicles is not something that can be handled safely under worst case conditions. This is plain physics and as an engineer you know this. Additionally, when my wife and I were young and did not have a lot of money we had small cars, starting with a Volkswagon Beetle, probably the worst car for hauling canoes and kayaks ever devised by man. Yes we hauled our canoe on the roof of our VW, I learned what I am talking about in the school of hard knocks. We have also owned other small cars and still do (my wife's current car is a Subaru, we have owned and loved a couple of Mini Coopers for that matter) and can attest from over 40 years of experience that what I am saying here is truthful and accurate, whether you want to accept it or not. Stay safe out there and try to remember not to shoot the messenger.