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Research Notes: Monofin Layer Stack

This note records the monofin layer stack derived from the Bluewater Freediving reference photo and cross-checked with the Predicting Flex technique.

Reference photo: Bluewater Freediving monofin internals.

What the reference image shows

  • Six ribs rise off the foot plate with progressively shorter carbon caps. The staggered rib height maintains rocker even with a softer tip.
  • The outer two ribs blend into the side rails, indicating a load path shared between rails and central ribs.
  • Carbon cloth at the foot is visibly thicker and overlaps the rails before tapering, confirming at least 6 layers near the foot pocket plus the rib caps.

These points match the earlier DIY monofin BOM notes: wide blades combine lateral ribs with an aggressive taper.

Predicting Flex setup

The geometry was entered into the tapered cantilever beam calculator described on the Predicting Flex technique page with the following parameters:

Input Value
Free blade length 520 mm
Blade width 700 mm
Layers at foot 6
Layers at tip 2
Minimum layer length 100 mm

Model output:

  • Angle at tip (under max load): 90.1°
  • Load required for 90° deflection: 46.6 N (≈4.75 kg)
  • Second moment of area at foot: 452.81 mm⁴
  • Second moment of area at tip: 16.77 mm⁴
  • Hydrodynamic resistance score at 5 N: 23.11 units
  • Hydrodynamic resistance score at 10 N: 21.81 units
  • Hydrodynamic resistance score at 46.6 N: 12.55 units

These values keep the load at the tip at 46.6 N, which is roughly four times the stiffness of a single long freediving blade.

Current working stack: 6→4→2 layers, 100 mm minimum overlap, and rib tapers only after the cloth count steps down.

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