I'm trying to make a computer simulation of ship. Currently i'm working on a force of wind on the ship sail. This is the page which describes the basics: http://www.animations.physics.unsw.edu.au/jw/sailing.html Basicaly, wind (W - vector) has some staring direction and then that direction is chaged because of sail (S - vector). So wind change vector (Fw) is: Fw = S - W. Because of Newton low, it means that force on boat is -Fw, or: Fb = W - S So far so good. But when the angle between the wind and sail becomes close to PI/2 my physics break apart. What happens then is that the resulting Fb is either down left or down right, both of which is wrong. As it is intuitively clear, when angle between wind and sail is PI/2, the resulting force on sail should be in the same direction as wind. Clearly, the problem is that wind current will split into two streams, left and right, and when their separate forces are summed, everything in fine again. But how to put it into simple equations?