Why the Apeks RK4 became my everyday sidemount fin: blade design, neutral behaviour, spring-strap ergonomics, propulsion, trim, drysuit compatibility and the situations where I choose something else.
Why the RK4 became my default sidemount fin. Fins affect propulsion, trim, foot position and even how easy the diver is to move on land, so I treat them as tools rather than fashion.
What the fin changes
The RK4 is my everyday sidemount choice because it combines a technical blade shape with near-neutral behaviour in the water. Neutrality makes it easy to use across my wet and dry exposure systems without forcing my feet strongly up or down.
The large, high heel strap is unusually convenient in sidemount, where reaching the foot can be awkward with a cylinder occupying part of that space. I also simply enjoy the design. It looks distinctive without sacrificing function.
The practical point
The right fin is the one that makes the complete diver easier to control, not the one that wins an argument online.
Why the RK4 lives closest to the door
The RK4 is the fin I reach for most often because it sits in a useful middle ground for my current diving: technical-style shape without the extreme negative weight of the F1. Apeks describes a short, wide one-piece rubber blade and steel spring straps with two adjustment points. The strap design is a practical favourite of mine because I can grab it easily even when sidemount cylinders make reaching the heel awkward.
I can use the RK4 with wet or dry exposure without feeling that the fin is trying to dictate my trim. That versatility, not a claim that it produces the most thrust in every kick, is why it became my everyday fin.
Fins are propulsion, trim and ballast at the same time
The blade produces thrust, but the fin also changes the mass and buoyancy at the end of the legs. That affects trim. A fin that feels wonderful with a thin wetsuit can make another diver excessively feet-heavy in a drysuit, while a deliberately heavy fin can be useful when a drysuit configuration tends to make the feet float. This is why I own several pairs instead of expecting one design to dominate every dive.
My Apeks RK4 is the everyday sidemount choice. I like the technical-style short blade, the near-neutral behaviour I experience in the water and especially the large spring-strap geometry: with sidemount cylinders beside me, reaching the heel can be less convenient, so a strap that is easy to grab matters. For demanding technical diving I may deliberately choose the much heavier, negatively buoyant Hollis F1—the “Batman fins.”
Technique matters more than catalogue language
Frog kick, modified frog, back kick, helicopter turns and careful positioning near a silty bottom ask different things from a fin than straight-line flutter kicking. A stiff, compact blade can translate small ankle movements into controlled thrust, but stiffness also costs muscular effort. The useful question is whether the fin works with the diver’s technique, suit and mission.
DPV use introduces another interaction
When a DPV is pulling the diver, the fins are still in the flow and still influence body position and drag. I have found the TUSA Tri-Force particularly pleasant in that context. That is a personal observation, not a laboratory measurement, but it is exactly the kind of detail I care about: not just how the product feels on a shop floor, but how it behaves inside the complete system underwater.
The fin test I care about
I want to hover, frog kick, back kick, turn and stop without the fin fighting my ankles or changing my trim in an unwanted way. I also want to be able to put it on and remove it in the real configuration, with cylinders or a drysuit limiting reach. That is why a strap handle can matter to me as much as a blade feature.
Current manufacturer context
Apeks describes the RK4 as a short, wide technical-style one-piece rubber fin with steel spring straps and two strap-position adjustments, intended for technical/confined-space use as well as general diving. The specification explains the design; my reason for using it daily is how that design works with my sidemount ergonomics.
Explore related topics
This article stands on its own, but these pages go deeper into the parts that connect directly with it:
