The way I figure it, the easiest way to implement a whip is to treat it like a thrown weapon that returns to your hand. You aren't throwing the entire whip, of course, just the tip. The length of the whip is the range. This length is not used for any weapon vs. armor comparison; whips are effectively no bigger than a short sword, for those purposes.
Whips, however, are made of leather. They are treated as edged weapons when used against bare skin, but as blunt weapons against any kind of armor. When used against metal armor, roll standard damage, but the whip only does damage on 5+, and only 1 point. If you must use a whip on a knight, you need to make a called shot at an exposed area.
Whips can also be used to grapple a limb, disarm an opponent, or strangle, using the grapple rules I posted previously.