This is one Hobie Turbo Square Top Fin (Replacement fin) Black and Grey color. You need two of these for each MirageDrive and designed for increased speed.
These ST designs add more surface area creating extra thrust and speed as well as slower cadence for cruising. The fin tension screw allows adjustment to achieve low end torque or top end speed to meet the desires of the peddler. This turbo fin is 14″ long. There is a shorter version of the ST fin, not listed as the Turbo. It measures 12″ long which we list also as a separate item. You can count the sharks teeth to make sure you are replacing with the right fin. If you are upgrading from the standard black short fins you will need the longer masts for these fins. It is recommended to order the entire ST turbo kit (either version 1 or version 2 depending on the age of your boat).
See more here: http://www.hobiecat.com/mirage/miragedrive/ . We carry both upgrade kits!