<p>Fig. 5. Similar to an airplane wing (Fig. 4), the shape of a sail allows a pressure differential to create "lift" in the horizontal direction. This simplified diaram shows the counteraction from the keel that allows the boat to have a forward momentum.&nbsp;</p>

Fig. 5. Similar to an airplane wing (Fig. 4), the shape of a sail allows a pressure differential to create "lift" in the horizontal direction. This simplified diaram shows the counteraction from the keel that allows the boat to have a forward momentum. 

Image by Emily Sesno