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I was also concerned that about my size (6'3", 240 lbs) - as long as I stayed on the pedastal with my feet flat back I felt pretty stable, after I got a feel for the thigh straps. It did not feel at all stable, even sitting still, when I tried to move my legs to give my feet a break. Some ankle rests will eliminate the need to rest my feet.
The boat turns on a dime, and I can't wait to take it out on the kind of water it was meant for. I think that it accellerates just fine, if you know how to paddle.
Having tested what are the supposedly weakest points of the boat and being pretty happy with them, I'll assume my opinion on the whitewater performance will match the previous favorable reviews.
Addendum (4/17/01) - Took it out on whitewater - I was amazed at its ability to catch eddies. I still am not able to keep my feet comfortable and still keep my center of gravity low enough to keep from going swimming. Great boat, which I could say the same for my tired old feet. Anybody out there have any ideas?