Rail convergence or divergence for tail riding and nose riding

What we do is examine the angle between the fin or fins and the tail rail, this angle gives an indication of the angle of attack of the fin when the board is turning or trimming ( whenever the board is running on a rail rather than just the bottom )With a board with side fins this fin/rail angle can be altered with toe in, so that even a board with parallel rails in the tail ( like a Simmons for example ) can have an angle between the rail and the fin.Boards with a single fin rely on the rails converging in the tail to get an angle between fin and rail.When noseriding the situation is different because it's the nose rail angle which is determining the angle at which the fin is presented, and the nose rail angle is outwards rather than inwards. . . thus presenting the fin at a negative angle rather than a positive one.If we look at a trimming situation it can be seen that when on the nose the negative angle of fin presentation points the fin in towards the face . . .. a narrower 'pig' nose tends to angle the fin in more than a wider one, as well as more importantly providing a longer section of diverging rail .The converging tail rail section has much less influence as there is little or no rider weight on it.This also explains why boards which tend to have long converging tail rail sections and a wide point forward are not noseriders . . . the converging tail rail presents the fin(s) at a positive angle of attack which means that the fin(s) point out away from the wave face. Boards like these prefer to trim off the tail with a slightly nose down attitude.