Fin Propulsion, Physics and Material Properties
Authors: n/a
DOI / Source: n/a
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Reading level: Beginner
Why This Matters for Freedivers
Most freedivers obsess over “carbon vs plastic” or “soft vs hard,” but this guide explains the real point: a fin only works well if your technique can make it behave like an efficient “flapping foil.” If your fin is too stiff for your body/skill level, you often compensate with bicycle kicking, side-to-side sway, and ankle fatigue—wasting oxygen and making every dive harder than it needs to be.
Synopsis
This presentation explains fin performance using a simple but powerful idea: freediving fins work like an oscillating (flapping) foil, more like a wing producing lift than a paddle producing drag. When you kick well, your legs transfer energy into the blade so it oscillates smoothly and creates alternating high- and low-pressure regions along the blade. Those pressure regions generate vortices that travel toward the trailing edge and “shed” cleanly behind you—this is what produces efficient thrust.
Where many divers lose efficiency is vortex control and timing. The guide describes how poor technique can cause vortices to interfere with each other and create drag instead of thrust. Examples include bicycle kicking, mismatched kick timing between downstroke and upstroke, and pausing in ways that interrupt flow right when a vortex should detach. In practical terms: you feel like you’re working hard, but you don’t move as well as you should.
A major practical takeaway is fin stiffness selection. The author argues that diver weight (your body plus any gear/weight you’re pushing) is the primary factor, and that many new freedivers choose fins that are too stiff. A stiffer blade isn’t “better” if you can’t bend and recover it properly in your kick cycle. The result is wasted energy, poor vortex shedding, and technique breakdown. The presentation also emphasizes that reducing drag with streamlining often improves efficiency far more than small differences between fin models.
Finally, it explains material behavior in simple mechanical terms. It distinguishes elastic deformation (bend and snap back with little energy loss) from plastic deformation (permanent or sluggish bending that absorbs energy every kick). It argues that polymer/rubberized fins can lose energy if they flex into a sluggish region—especially as temperature changes—while fiberglass/carbon behave more like ceramics/glass: they don’t have the same plastic region, but can fail if pushed past their elastic limit. The main message: understand the physics, pick the right stiffness, and make technique the priority.
Abstract
This presentation introduces a practical framework for understanding freediving fin performance through oscillating (flapping) foil propulsion. It explains how thrust is produced primarily through lift-based mechanics and controlled vortex generation and shedding, and how inefficiencies arise from technique errors such as bicycle kicking, poor stroke timing, and excessive drag. Key selection factors are discussed, especially matching fin stiffness to diver weight and ensuring technique can properly oscillate the blade. The presentation also summarizes relevant material properties (Young’s modulus, elastic versus plastic deformation, and energy loss across the kick cycle) to help divers compare fin materials and choose equipment that supports efficient propulsion.