They have a centerboard. Righting moment is mostly crew weight.
The only real design rule for these boats is that they have to be 18 feet long at the waterline and be a monohull. The Aussie league limits mast height too if I remember.
The 18 foot skiff started out as a resupply boat. They sold stuff to ships coming into the harbor. Whoever got there first made the sale. Faster boats meant more money. About 100 years later, they're race cars.