The Hobie Mirage ST Fin upgrades the fins on the Mirage Drive to increase the surface area leading to faster speeds and more efficiency. The Mirage ST Fin is smaller than the Turbo Fins but larger than the Standard Fins. The ST Fins use the existing masts.
This is an excellent item if you want more rigid and quicker speeds than the original but don't want to to give up the more shallow draw than the Turbos ( I have both) I use the turbos under sail and I use the ST's when I am kayaking in shallow water.
The fins do add a little bit of speed or resistance, depending on how you look at it, but there are some flaws. Primarily, the metal rod doesn't go all the way down to the bottom of the ST fins like they do on the standard fins. So the bottom inch or so of fin doesn't have any backbone. First time out I went through some shallow water and rubbed the fins on the bottom, which caused the fin to tear and the rod to poke through the fin. They are also kind of a pain to switch out. The original fins just have a hole in them were a pin goes through. The STs have a screw-type mechanism that is a lot more tedious to deal with. Bottom line, STs are great for deep water where you don't have to worry about rubbing bottom. Standard fins are better for river floating.
I bought these after using the longer Turbo ST fins for a while in my Hobie Adventure. I thought a higher cadence with less resistance might suit me better. But, when I tried these (non-turbo) ST fins, it did not seem nearly as efficient---definitely more work for me to cover the same distance. So, I'd strongly recommend that you go with the Turbo ST fins instead of these. It might take a while to get used to the turbo fins, but once you do, they far superior. You can go faster and farther, and you can always back off a bit and get a similar effect as with the shorter fins. Of course, the turbo fins cost more, but they're well worth it, IMHO.