First, my limited credentials. I've used sporasub HDs, Cressi Garas and Omer Ice fins.
I decided to part with my sporas - which are very stiff fins, but also very clunky compared with my Ice fins. Foot pockets seem boxy on my feet, and the blade stiffness makes them awkward for surface swimming, or maneuvering to take pictures. I wanted a replacement backup fin that was also stiffer than the Ice fins.
The Ice fins are fantastic, by the way. Relatively soft, very comfortable and efficient. Very very good across the board. There have been times, coming up from depth, when I've felt like I overpowered them and I've been wanting a fin that might provide a little more oomph from deeper dives.
Beautiful day here yesterday. The water is clearing up and visibility is probably in the 50 foot range. Temp is around 50f. Just lifting the blade off a flat surface the Runners look to be about twice the stiffness of the ice fins.
In the water the difference was immediately noticeable. Still great for surface swimming, but my feet were tired for the first 45 minutes and I was definitely moving faster. In general feel they are very much like firmer Ice fins - with a nice even return on the blade. Very smooth and efficient feeling. After a good long swim to get to some deeper water I checked them out in about 10-12 meters. Not too deep but that distance takes me about 1/3 mile out into serious boat country. Definitely a difference on the way up and down. More thrust, but very even and comfortable. The fins are also good for maneuvering and long swims. I was out nearly 3 hours and took them into more close quarters in the nearby breakwall. Great power and control with these fins and with the same really precise, smooth feeling return in the blades.
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