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Encoding Fin Properties - V1 - Hydrodynamic Resistance Codes

Proven

Goal

Create a repeatable four-field code that captures fin length, tip stiffness, and hydrodynamic resistance so different blades can be compared at a glance.

Time needed

Type Hours
Implementation 0.25 h
Waiting 0 h

Inputs to capture

  • Free blade length beyond the pocket shoulder (mm)
  • Tip load that holds the blade at a 90° bend (kg)
  • Hydrodynamic resistance score at 5 N kickoff load
  • Hydrodynamic resistance score at 10 N follow-through load

The resistance scores come from the benchmark model in the research note on hydrodynamic kick resistance. The short training fin (150 mm × 100 mm, 0.015 m²) is fixed at 1.0 unit at both loads; any other blade is reported as a multiple of that benchmark.

Code structure

Field Format What it captures How to calculate Example
C### Three digits Free blade length in millimetres Measure shoulder-to-tip distance and round to the nearest 10 mm C600 → 600 mm free blade
T## Two digits Tip load at 90° bend (kg ×10) Weigh the mass that keeps the tip vertical; multiply kg by 10 and round T12 → 1.2 kg tip load
R## Two digits Relative resistance at 5N (×10) Compute \(R_{5N}\); multiply by 10 and round R49 → 4.9 units at 5 N
F## Two digits Resistance drop between 5N and 10 N (×10) \(F = R_{5N} - R_{10N}\); ensure minimum of 0, multiply by 10 and round F20 → drop of 2.0 units

The final code concatenates the fields with hyphens: C600-T12-R49-F20.

Instructions (step-by-step)

  1. Measure the free blade length

    • Lay the blade flat and measure from the pocket shoulder to the tip.
    • Round to the nearest 10 mm to produce C###.

  2. Record the 90° tip load

    • Use your flex rig (see the measuring flex techniques) to find the weight that holds the tip vertical.
    • Convert the mass to kilograms, multiply by 10, round to the nearest whole number, and format as T##.

  3. Estimate resistance at 5 N and 10 N

    • Use the hydrodynamic benchmark model (bench test or the flex predictor tool) to obtain the dimensionless resistance scores at 5 N and 10 N.

  4. Encode the hydrodynamic behaviour

    • Multiply the 5 N score by 10, round to the nearest whole number, and format as R##.
    • Subtract the 10 N score from the 5 N score (bottoming out at zero), multiply the result by 10, round, and format as F##.

  5. Publish the code

    • Join the four parts with hyphens.
    • Include the code alongside measurements, build notes, or test tables so others can compare builds quickly.

Example

The long carbon blade that bends mid-span in the research note measured:

  • Free blade length: 600 mm
  • Tip load: 1.2 kg to reach 90°
  • Resistance: 4.9 units at 5 N, 2.9 units at 10 N

Encoding:

C600-T12-R49-F20

References