I fiddled around with building my own, a quad-line, delta/flying wing shaped foil. Like most of the others I've seen, it was slow and cumbersome, and tended to collapse suddenly in our twitchy Midwest winds. Any sudden changes in speed and/or direction tend to make a foil turn into a lifeless bag heading earthward. Don't get me wrong, I really enjoyed the challenge and I was flying one of my own creations, but it eventually became obvious that its limitations outweighed the benefits, benefits as in fun as opposed to work, especially in freaky winds. I couldn't fly it often enough because it needed the "right" wind. I watch videos of other quad-line foils, and see that they move very similarly to what I had built. I have nearly given up on the idea of building a "perfectly controllable" quad-line foil because of the difficulty of keeping the sail inflated to an adequate degree of "stiffness" to give the sail enough consistent reaction to inputs, especially sudden stops and reversing direction, punch-pull turns, spins and overall precision.