The U shape and V shape boats with the open fronts are convenient, but in big surf the front of them can collapse if the bar gets pushed or pulled around. It happend to my buddy Jamon a couple times in his Caddis U-Boat. If you aren't going in the surf, then it's not an issue. Some U-boats are more stable than others too. The tube I was talking about above has a round innertube, but the back of the boat is shaped to make a V so it kinda points through the water. I wouldn't describe it as fast, but it might be a little better than just a regular round tube.