Sandy Goodall - Sailmaker, Designer and Sailing Principles Instructor, Victoria, British Columbia, aboard the Sif, Discussing how control lines effect the power of a sail
You can see from where you're looking that the sail has a certain thickness, from here to here [indicates width], which is another illustration of its 3-dimensional shape. All the many control lines that you see on the boat are to control the shape of the sail and to change the shape of the sail. For example, this line here, running under the boom, if I tighten this [tightens rope], watch the thickness of the sail here disappear. See how that became flat? If I loosen that [loosens rope], the sail becomes much fuller, or more baggy. In aerodynamic terms, the fuller, or more 3-dimensional curvature a sail has, the more horse power or driving force it will develop. So these are controls to help shape the sail and determine how much power you let the sail develop.