Loose Foot

Most of today’s mainsails are loose-footed with nothing attaching the sail to the boom other than the tack at the gooseneck and the clew on the outhaul. As seen here, a loose-footed mainsail allows the sail to keep its aerodynamic shape all the way to its foot.

In the past, the bottom edge of the main was attached to the boom with slides or a boltrope, but the sail could not achieve a proper aerodynamic shape without building in an expensive shelf foot.

An extra advantage of the loose foot is that with just two attachment points, bending on the mainsail and removing it are much easier. 

Not attaching the foot of the sail to the boom  provides a much better aerodynamic shape without reducing strength.