After reading the threads on longboards and hard rails below, I was a little surprised at some of the comments by very knowledgeable people. Either all my understanding and experience of how rails work is completely backassward or we are misusing terms to the point they say the opposite of what they should. The following rant is based on years of riding experience, a little shaping experience and the Rocket Science and Balance articles in Resources. I maintain that hard edges slip and soft edges hold. Hard edges release (they shed water quickly and efficiently). Soft edges hold or grab water, bending it. Hard edges do not "take energy" from a wave, soft ones do. Try this experiment: Turn on the tap and put the convex side of a spoon up against the flow of water...the water will "grab" the spoon and pull in in the direction of the convex. Put a spatula in the same stream of water and it will be repelled (almost as efficiently as puting the concave portion of the spoon in the stream). Now imagine those shapes of edges on a rail. Which one holds (is literally sucked into the wave face) and which one planes on top of the water surface (almost repelling water). Hard edges on the tail allow water to escape from the bottom quickly but mostly the are counteracting the downward pull of the fin(s) (Loehr's Balance Article). If you put soft rails on the tail, both the fin and the rail are pulling the board's tail downward (excellent nose rider but no speed down the line). Hard edges will hold the tail up keeping things relatively neutral (you can really feel it when you just catch the wave and start to drop). The Hawaiians used round soft tails on finless boards using the downward pull to keep the olos on track. I will admit that under some circumstances a THIN hard edge will hold a sticky track (paipos and skim boards) but they will lose the "grip" unexpectedly due to slight changes in the angle of attack. My thought is that the thin edge is changing from a "planing surface" (Rocket Science) to a penetrating surface (fin). As soon as you add bulk to the edge the grip/tracking will disappear. Hard edges work great with boards that are surfed from the fins since the rails are not needed to get the board to do what you want it to do. You want a hard edged rail to release, not stick when you are making rapid direction changes. Soft curves are great for single fins that use the rail like hulls and fish. Best of all; attributes of each can be blended on a given board so that they enhance the other design elements. Mix poorly and the board is a dog. The edges can be blended on the same rail too; look at a tucked edge, chines or Greenough Edge bottoms. Bottom rail line: Soft = hold, Hard = release.