This cart is placed at the end of a canoe, and puts two wheels under the canoe... and two wheels on TOP of the canoe. The cart works fine when toting the canoe right side up. Where this cart really shines however, is when you turn the canoe upside down and use the other set of wheels in order to easily load the canoe onto the roof of a car. As you lift the inverted bow of the canoe up to the rack, the stern just follows along on its wheels. One person can cartop a heavy canoe without assistance and without any real strain. This device certainly opens up a lot of possibilities for those of us for whom loading the canoe was the worst part of the whole paddling experience.
The cart is very well-made and is easily attached to the canoe in just a few seconds. You put it on the canoe after parking, and use it to take the canoe off the car. Then you just flip the canoe over and use the other set of wheels to transport to the water--where you can remove the cart to stow in the canoe. After retrieval, you can install the cart (again, just by lifting the stern, placing the cart OVER the end of the canoe and snugging the supplied webbing and clip to a thwart) and ferrying the canoe back to the car where you flip it over and use the other wheels to load up. Remove the cart from the racked canoe before driving. I'm not aware of any other cart that is so versatile.
I would say that this is not intended to haul a canoe over a mile of soft sand or boulder and root strewn portages, but for the majority of distances and surfaces that most of us deal with, the Quantum Engineering Four Wheel Cart will leave you (and your back) smiling.