Twin fin surfboards can feature either fins or keels.
While the term “keel fin” is sometimes used to describe any sort of fin, it describes a specific design.
A twin fin setup features two side fins that are similar in appearance to the side fins you’d find on a thruster or a quad – sometimes a little deeper.
Twin fins can comes in a variety of depth and rake, but often feature a narrow base and moderate to large rake/sweep.
These types of twin fins can be placed anywhere from 5″ to 12″ from the tail of the board and may feature a variety of cant and toe angles. The further up and more toe and cant the twin fins have, the loose and more maneuverable the board will surf.
Twin fin set ups like these are going to give you a lot of options as to how the board performs.
A twin keel describes a fin with a much longer base and much shorter depth.
These can sometimes appear as almost half-circles.
Keel fins feature minimal or zero cant, minimal or zero (Parallel) toe, and are often positioned 5″ to 10″ from the tail.
On swallow tail fish, keels are often placed in line with or slightly in front of where the swallow tail meets in the middle anywhere from 1-2″ off the rail.
Keel fin setups are going to have a classic drivey and drawn-out feel.