Apeks RK3 vs RK4: Which Fin Should You Buy?

A fin that floats at your ankles mid-safety-stop, or one that leaves your legs aching after a single wreck dive — either mistake is easy to make when you're choosing between the Apeks RK3, RK3 HD and RK4. These three fins look similar at a glance and share the same vented blade heritage, but they behave very differently once you're in the water. This guide breaks down the real differences in weight, buoyancy, and handling, so you can match the fin to the diving you actually do.

All three fins share Apeks' vented blade design, which reduces drag on the upstroke and boosts thrust on the downstroke — the same underlying principle that makes split fins efficient, without losing the directional control a solid blade gives you. From there, the family splits into two philosophies. The RK3 and RK3 HD are close siblings: same shape, same foot pocket, different blade density. The RK4 is a different design altogether, built for divers who want precision control over raw thrust. Independent diver testing has also put some real numbers behind the feel: a medium RK3 weighs in at roughly 982g, against roughly 1.1kg for a medium RK4 — a difference divers notice both in a kit bag and underwater.

The RK3 has been the standard-issue fin for military, special ops, coast guard and public safety divers for decades — a track record built on being tough and predictable rather than flashy.

The RK3 features an over-sized foot pocket built to accommodate drysuit boots, paired with a spring strap for quick donning and doffing, even with cold hands or thick gloves. This matters more than it sounds — a rigid drysuit boot doesn't compress the way a neoprene wetsuit boot does, so a foot pocket sized for wetsuit boots often won't close comfortably over a hard-soled boot. The short, wide blade delivers strong forward thrust while staying manoeuvrable, and divers consistently describe the RK3 as an energy-efficient fin — you don't tire as quickly compared with a stiffer blade, which is part of why it suits everyone from beginners to technical divers.

Key RK3 features include:

• Oversized foot pocket designed to accommodate drysuit boots comfortably
• Spring straps as standard, with a custom-designed buckle to hold them in the optimum position
• Vented blade that reduces vortex drag and boosts downstroke thrust
• Two large grommet holes sized to fit most karabiners, so you can clip your fins on for the walk to the water
• Simple push-pin design that makes replacing a spring quick and easy if one ever needs swapping out
• Ultra-rugged thermoplastic rubber blade built to take abuse in harsh environments

The RK3 HD swaps in a higher-density rubber compound, which makes the blade stiffer and gives it negative buoyancy rather than the standard RK3's slight positive lift. That's a genuine trim consideration: a fin that floats at the ankles pulls a diver out of horizontal trim over a dive, while a negatively buoyant fin helps keep your legs settled — which is why the RK3 HD is a strong pick for drysuit divers, wetsuit divers who want a firmer kick, and photographers who need to hold position without fighting their fins.

Everything else about the RK3 HD carries over from the standard RK3 — the same oversized foot pocket, spring straps and custom buckle, vented blade geometry, and the same four sizing options.

The RK4 takes a different approach entirely: a compact, wide-blade technical fin built for energy efficiency and precision rather than raw power. Its buoyancy sits closer to neutral-to-slightly-negative, giving balanced handling whether you're in a wetsuit or drysuit, and it's purpose-built for frog-kicking and back-finning — the kicks technical and cave divers rely on to avoid disturbing silt or turning tight passages into a cloud of zero visibility.

Divers who've used both fins side by side consistently report the same pattern: the RK3 feels stronger and more relaxed on open-water dives, shore entries, and swimming against current, while the RK4 comes into its own the moment the environment gets tight — wrecks, swim-throughs, caverns — where its shorter recovery movements barely stir up any silt at all. The trade-off is that the RK4 is heavier and takes more technique to use well; it's not usually the fin you'd hand a brand-new diver on their first open-water course, and shore entries can feel clumsier with the extra weight and stiffness.

RK4 highlights include:

• Delivers an energy-efficient dive with strong performance on both the upstroke and downstroke
• Two adjustment points with stainless-steel spring straps, allowing quick changes between wetsuit and drysuit setups
• Integrated stainless-steel buckles that reduce snag hazards and stand up to hard use
• One-piece rubber construction that's durable and easy to recycle at end of life
• Easy to put on and take off, even with gloves on