Question: What is a foot shelf and what is its purpose?

Answer: A "foot shelf" is an extra piece of "lens shaped" cloth (narrow at tack and clew and wider in the middle), attached to the foot of an "attached foot" mainsail. It might be as much as a foot wide, on a large mainsail. It is sometimes made from cloth that is lighter and stretchier than the body of the sail. Its name comes from the fact that when the outhaul is fully eased, this cloth folds out horizontally from the boom and forms sort of a "shelf". When the outhaul is tight, this cloth folds up in a bunch parallel to the boom.

Its purpose is to enable the belly, or deepest part of an "attached foot" mainsail to move horizontally away from the boom, when the outhaul is eased... much like the way a loose footed main does. It achieves this due to the extra material. It is a way to make an "attached foot" main act (trim) more like a loose footed main, but while still maintaining the attachment to the boom (which some long distance blue water cruisers use to catch rain water).

Question: If I want to go with a loose footed sail, would I still need a shelf foot?

Answer: A shelf foot used to be very popular, but it has now mostly been replaced by the loose footed main, which achieves the same result but neater, and with less material and labor.

"Shelf foots" are not used in loose footed mains.

Question: Are there any advantages of a foot shelf versus a loose foot?

Answer: The only plus with a shelf (attached) foot is that you can catch rain water with it.