I am not entirely sure about the English names for the parts of a boat but I try my best: the Boat always tries moves towards where the bow is pointed because of the keel below the ship which acts like a guide rail because it creates a path of less resistance. And the hull makes it very hard to much it left and right. So even though the wind comes from a 45° the shape translates that in mostly forward movement and a little bit of sideways movement. Like a toy car you are stepping on: you are acting as a downwards force but the wheels translate it to forward movement. I hope that somewhat gives you a bit more insight!