Research Notes: Comparing Fin Hydrodynamic Kick Resistance
When I talk about how "heavy" a fin feels, what I'm really pointing at is the hydrodynamic resistance the blade builds as it sweeps through the water. The raw feeling is easy to notice, but hard to quantify. To make comparisons across builds, I pulled together a quick benchmark-based model so every fin gets an apples-to-apples resistance score.
Step 1: Define the Benchmark¶
Start with a short training fin: 150 mm wide and 100 mm long, for a blade area of 0.015 m². That combination gets the baseline resistance score of 1.0 unit at both the 5 N and 10 N test points.
This keeps the arithmetic friendly because everything else scales relative to the trainer blade.
Step 2: Scale Other Fins Relative to the Baseline¶
In this simplified model the drag force \(F\) that a fin generates is proportional to three things:
- Area: the square meters of blade the water "sees".
- Projected angle factor: how much of that area is presented to the flow once the blade bends.
- Bend distribution: where along the blade the flex concentrates.
The baseline handles the constants, so every fin boils down to a relative resistance ratio:
Because the benchmark force \(F_\text{benchmark} = 1\), the ratio reads directly: a fin that generates twice the integrated force of the trainer comes out at 2.0 units.
Step 3: What the Ratio Buys Us¶
- No velocity assumptions. As long as we compare fins at the same kick speed, the water velocity cancels out of the math.
- Dimensionless scores. "This blade is 3× as heavy as a short trainer blade" is immediately understandable.
- Design clarity. Every experiment, whether you try longer carbon layups, different tapers, or alternative resins, boils down to a single comparable number.
Step 4: Worked Example¶
Say you build a carbon bifin that measures 0.20 m wide and 0.60 m long. The blade area grows to 0.12 m². On area alone that's an 8.0× increase over the benchmark.
Real blades bend. A long, soft carbon blade can feel about 5.0 units right as the kick starts, but as the stroke continues and the blade folds, the effective resistance can relax to roughly 2.5 units. That matches the in-water feel: heavier to initiate than a swim trainer, then quickly easing into the 2-3× range once the blade is moving.
Resistance Table¶
The table below applies the same logic to a range of carbon layups and a couple of well-known polymer fins. The measurements capture two points in the kick cycle: 5 N to show initial resistance, and 10 N to show how much stiffness the blade hangs onto deeper in the stroke. The \(\Delta R\) column highlights how quickly the blade drops off between the two loads.
| Fin type | Notation | 5 N | 10 N | ΔR |
|---|---|---|---|---|
| Swim training fin | (baseline) | 1.0 | 1.0 | 0.0 |
| Short fin 230 (4.5 kg) | C230-T45-R25-F02 | 2.5 | 2.3 | 0.2 |
| Medium 400 root (1.2 kg) | C400-T12-R29-F16 | 2.9 | 1.3 | 1.6 |
| Medium 400 mid (1.2 kg) | C400-T12-R34-F12 | 3.4 | 2.2 | 1.2 |
| Medium 400 tip (1.2 kg) | C400-T12-R37-F11 | 3.7 | 2.6 | 1.1 |
| Long 600 root (1.2 kg) | C600-T12-R45-F24 | 4.5 | 2.1 | 2.4 |
| Long 600 mid (1.2 kg) | C600-T12-R49-F20 | 4.9 | 2.9 | 2.0 |
| Long 600 tip (1.2 kg) | C600-T12-R55-F16 | 5.5 | 3.9 | 1.6 |
| CRESSI Clio fins | (not carbon) | 3.5 | 3.3 | 0.2 |
| Seac Sub Talent fins | (not carbon) | 6.3 | 6.1 | 0.2 |
Legend
- C### marks a carbon blade with ### mm of free blade beyond the foot pocket.
- T## gives the tip load at a 90° bend, reported as kg ×10 (≈ newtons).
- R## shows the initiation resistance at 5 N (×10, no decimal).
- F## notes how much the resistance drops between 5 N and 10 N (×10, no decimal).
You can plug your own fin measurements into the Predicting Flex page, which now outputs the same resistance score for quick comparisons.
Comments
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