I had this explained to me somewhere in my past. In Bethwaites book, High Performance Sailing, there is a diagram of water coming out of a tap onto the side of a glass and sticking to the glass side until it gets up a ways on the other side. This shows that water that is moving always wants to stick to a smooth surface up until the surface speed drops to zero, or a critical speed is reached where the corner is too sharp for suction to overcome the force required to bend the water quick enough.

The water wants to stick to the sides of the smooth foil until a certain speed is reached and the corner becomes too sharp for the waterspeed and the water can no longer stick. The hum is caused by the foil reaching the speed where the water can no longer stick to both sides of the foil. The nearly flat blunt back part of the board allows air to be sucked down at the back and the humming is caused by the water flipping back and forth between sticking first to one side of the board and then the other side. This is slower than a sharp trailing edge because it drags more water along behind the trailing edge of the foil. Rick White likes the sound of silence because it is faster. Any of you guys that think this is BS, I encourage you to leave your boards as they are, especially if I get to sail against you. Whatever turns your crank!

One other way of making the back corner less critical is to have a real rough surface on your foil. This makes the boundary layer thicker (this means you drag even more water along with your foil) and you do not get nearly as much suction on the sides. But this is not a good state because it causes your board to ventilate even sooner than having that blunt back edge. If you have ever had one of your rudders ventilate when you are cranking along at over 20 knots, it is a real adrenaline rush and is probably because you either picked up a weed, or you have rough boards, or you have a real blunt trailing edge. The leading edge is critical on your foils, andso is the polish. the weeds you pick up introduce roughness, and really lower the ability of the foil to give good turning ability. THe same with all those old hobie rudders that we have all seen with the black paint killed by the sun and the fiberglass fibers showing and real rough. Guess how good they work! Food for thought, isn't it?