Not shown is the drag force of water on the keel and hull, directed along the boat axis and towards the boat’s rear. This drag force increases with the boat’s velocity relative to the water until its magnitude equals the magnitude of the net force vector (in black), at which time the boat’s velocity relative to the water becomes constant.

Note that when the sail and boat axis are directed at the same angle relative to the wind $(\theta = \phi)$, the net force is zero. As $\phi$ is increased beyond $\theta$, the magnitude of the net force increases, but its direction becomes more perpendicular to the wind. This is the trade off a sailor must make as he takes a zig-zag course, called tacking, against the wind. An excellent simulation can be found at http://demonstrations.wolfram.com/SailingAgainstTheWind/.