Hydrodynamic Drag During Gliding in Swimming
Authors: Daniel A. Marinho, Victor M. Reis, Francisco B. Alves, João P. Vilas-Boas, Leandro Machado, António J. Silva, Abel I. Rouboa
DOI / Source: https://doi.org/10.1123/jab.25.3.253
Date: 2009
Reading level: Beginner
Why This Matters for Freedivers
Gliding is “free distance” — the less drag you create, the less oxygen you waste. This paper backs up a simple technique cue with physics: keeping the arms extended in front (streamlined) reduces drag compared with keeping them by your sides, so you move farther for the same effort and stay calmer.
Synopsis
This study looked at a very practical question: when you’re underwater and gliding (not kicking), does your arm position change how much water resistance you face?
The researchers used computer simulations (computational fluid dynamics) with a full 3D model of a swimmer and compared two common underwater gliding positions: 1) Streamlined: ventral (face down) with arms extended forward. 2) Arms by the sides: ventral with arms along the trunk.
They tested these positions at underwater speeds typical of elite swimmers during starts and turns. The result was consistent: the streamlined “arms in front” position produced lower drag coefficients at every speed tested.
They also broke drag into two main parts: - Pressure drag (the “wall of water” effect from shape and flow separation). - Skin friction drag (water rubbing along the surface).
In both positions, pressure drag was the dominant contributor to total drag. Skin friction was smaller but still meaningful. The key reason streamlined wins is that it smooths the body shape (especially around head/shoulders), reducing the pressure differences and wake behind the body.
Practical takeaway: if your goal is to travel farther while using less energy (whether in swimming starts, freediving descents, or underwater glides), streamline with arms extended in front is the more hydrodynamic choice.
Abstract
This study used computational fluid dynamics to compare hydrodynamic drag during submerged underwater gliding in two ventral body positions: arms extended forward (streamlined) versus arms along the trunk. Simulations were run at flow velocities typical of elite underwater glides. The streamlined position produced lower drag coefficients than the arms-along-trunk position across the tested speed range. Drag component analysis indicated that pressure drag accounted for the majority of total drag in both positions, with skin friction contributing a smaller portion. The findings support adopting the arms-extended streamlined position to minimize drag during underwater gliding.