You mention that there is a point where aerodynamics and hydrodynamics are quite different. What is that point and why?
Wings in the air behave one way at sub-sonic speeds, differently at near-sonic speeds and differently again at super-sonic speeds.
Wings in the water operating at low speeds behave in many ways like slow-moving wings in the air. However, as speed increases wings in the water start to have problems like ‘cavitation.’ Cavitation occurs when water encounters an area of low pressure - generated by a wing, for example — and vaporises, thus affecting the lift generated by the wing.
There is the additional issue with hydrofoils that they have a strut (or ‘mast,’ as we incorrectly say) that connects the wings to the board. This strut passes through the surface of the water, which means we need an entirely different approach to evaluating its lift and drag. Froude number, ship hydrodynamics and ventilation can come into play here, none of which apply to wings in the air.
How do you test foil wing designs?
We test hydrofoil wing designs with CFD (computational fluid dynamics) software on the computer. When we have a design that looks good on the computer, we build it and test it on the water. We don’t test off the water as we don’t have the facility for that.
How has fluid dynamics been applied when designing and testing our foil range?
As mentioned above, we look at hydrofoil designs on the computer using CFD software. When we have a design that we think may be an improvement, we build and test it. For example, we looked at the pitch stability of wings on the computer, built the prototypes and evaluated them for pitch stability. This has led to some quite stable designs which can now be seen in the Duotone foil range.