Helpful Calculations

On this page, skin thickness refers to 'T' as shown below.
  • Maximum thickness of a foil is equal to the maximum thickness of the core plus twice the thickness of the skin.
  • The chord length of the foil is equal to the chord length of the core plus the thickness of the skin plus the taper of the skin beyond the core (designated by '?' below).

Calculate Core Dimensions

Simplified approach - reduces foil dimensions based on skin thickness, and recalculates offsets. NACA 4 Digit foils only. Enter external dimensions: (use same measurement units in all fields)
chord max thickness
skin thickness:

Calculate Reynolds Number

Used in calculations of lift.
Velocity Chord length Fluid constant
Reynolds Number:
Giving you an idea of some of the forces at work (and how strong your foils have to be)

Calculate Righting Moment

Very simple approach - neglects righting moment of hull itself, and bases all calculations on 'W' - the half-width of the hull.
half-width of hull:
crew weight height
Righting moment:
  • Center of Gravity of seated crew is 6" inside gunwales
  • C of G of hiking crew is 25% of crew's height outside of gunwales
  • C of G of trapezing crew is 50% of crew's height outside of gunwales.
Why should you care about righting moment? This is one factor in determining the maximum lift your centerboard needs to generate, and how stiff and strong it should be. You may wish to multiply the result by a weighting factor > 1 to account for crew dynamics and pumping.

Calculate Lift

Provides an estimate of the lift generated by your foil.
Velocity Planform area (wetted)
Coefficient of Lift Fluid density
Calculated Lift:

Sample Coefficients of Lift

These are to be used only as general guidelines - coefficient of lift varies according to aspect ratio (and other planform factors) and Reynolds number, to name just some of the variables.
1.50 NACA0012 at angle of attack of 15 degrees (max before it stalls)
0.25 NACA0012 at angle of attack of 3 degrees (typical leeway angle upwind)
0.80 NACA0009 at angle of attack of 10 degrees (max before it stalls)
0.30 NACA0009 at angle of attack of 3 degrees
For maximum lift generated by your centerboard, think in terms of hullspeed upwind (if you're planing, you're probably also retracting the centerboard). For maximum lift generated by your rudder, think in terms of hgh steering angles at maximum speed downwind. Multiply the lift generated by your centerboard by one half of its submerged length, and determine if that exceeds the righting moment that your crew can generate. I'll break my rule about not including links here - check out Martin Hepperle's JavaFoil for online foil analysis.