I tried it once with a 27cm after one of the side biters fell out on the second go. Had two or three runs and the single fin sheared off 200 m out to sea. It felt alright but the tri fin set up was much better on all points of sail and i'm not convinced that the fin box would support going much bigger ignoring the fact the fin snapped off. So i don't think you gain any thing on an early planning front. The fin I used was a cheapo prolimit stop gap, but I couldn't really see the point of much single fin experimentation unless you were a freestyler going small?