Fin keels are a very broad category of keel which is characterized by a high aspect ratio appendage. Within this category are a vast world of variations and subtle differences that give rise to very different performance characteristics.
The theory behind a fin keel is the leading edge will produce lift which will carry the boat to windward. The whole keel is shaped like an air foil to aid in generating lift and make the entire system much more efficient.
To create more leading edge, you simply need to have a longer appendage, simple! The problem is a longer appendage directly translates into deeper draft. For yacht builders, a deeper draft means that buyers in shallow waterways will not buy their product. Yacht builders want to be able to sell their yachts to everyone, which means that they need to produce a yacht with a shallow enough draft that it will appeal everyone while still retaining the desired performance attributes that will make them competitive when compared to deeper draft competitors.
Yachts compete on windward performance and on their ability to sail into shallow waters. Naturally, these two camps are on opposite sides of the spectrum. You can either sail to windward OR get into shallow waters, not both with a regular keel.
This is where the wing keel shines. A wing keel is a regular fin keel with small wings projecting to the sides at the tip of the keel. These little wings serve two purposes: They provide more leading edge length and they create less drag from tip vortices.
Adding wings naturally adds more leading edge to the keel, but instead of adding more leading edge in a downward direction, the leading edges now travel laterally. This is supposed to produce the same lift generated as a longer fin keel without the added draft.
Tip vortices directly relate to drag, and reducing these will reduce drag. Reducing drag will directly result in an increase in speed and higher performance! Tip vortices occur at the end of every airfoil. On an active airfoil, there will be a high pressure side and a low pressure side. The air foil in the middle acts to separate the two pressures and the resulting pressure differential will drive the airfoil. At the tip, there is no airfoil separating the two pressures and the high pressure side will spill over into the low pressure side, resulting in a vortex that trails along the tip of the airfoil.
A small wing at the tip will help reduce the spillover of pressures and reduce the size of the vortex. Air planes use these winglets at the tips of their wings to reduce drag and improve fuel economy, and sailboats use them too in the form of a wing keel. The wings on the sides of the tips are supposed to reduce drag by reducing the tip vortex.
While a wing keel may sound like the perfect solution to a sailor who is searching for a high performance yacht with a shallow draft, but they do have an additional problem with groundings.
Fin keels are rather weak and can suffer great damage during a grounding, but the sailboat can usually work its way off a shoal with enough effort. A fin keel will dig into the bottom, and be easily pull free from the bottom as you work yourself off a shoal. A wing keel on the other hand will dig into the bottom and get really stuck! Those wings will act like anchor flukes which will dig in and hold if you run aground in soft mud.
If you plan on sailing a fin keeled yacht into shallow waters and are looking at a wing keel for the added performance, be weary! If you are going to explore into shallow waters, you will eventually touch the bottom! When this happens, getting off will be a much harder ordeal. You may think you will never run aground, but it will happen if you like to explore shallow waters.
Wing keels supposedly offer added performance benefits with a shallower draft, but don't abuse of the shallow draft and go looking for trouble in waters where you can't float.