Inside the mind of Xavier #2 | Finding our vision in an ocean of possibilities

25 February 2022

In this article series, Xavier Lepercq, SP80 co-founder and technical manager, shares his thoughts on boat design, speed sailing, and the world speed record. Reaching 80 knots on the water with the wind as sole source of power is an incredible technical challenge. The pursuit to break the world speed record has led him to question architectural codes that are currently the standard within the sailing world.

In the first article, the very simple mathematics determining how fast a boat can go was laid out:

This means that:

  • Increasing power makes the boat faster.
  • Decreasing drag makes the boat faster.

The world sailing speed record has very few rules, so it is very easy to get lost in different designs. We started moving forward with the design the day we understood that you not only need a design but also a vision, a philosophy to refer to when you need to make critical choices and trade-offs.

Our vision that drove all our design is the following: First, prioritize stability even though it does not appear in the “speed maths”. And only then, increase the power as much as possible while reducing the drag as much as possible.

Among other things, stability can be ensured thanks to a well-thought design and a good understanding of  the regime of the foils.

1. Boat stability

First of all, two different kinds of stability will occur during a record run with the SP80 boat:

  • At low speeds, that is during the launch and recovery phases, stability mostly comes from buoyancy and weight forces.
  • At high speeds, during the run, hydrodynamic and aerodynamic forces are so high in comparison of the weight and buoyancy forces that they are governing the stability in a dynamic fashion.

The boat has to transition between these two stability regimes while accelerating during the record run. To ensure the reliability of the boat at low speeds – which is key for breaking the world sailing speed record – SP80 designed a planning trimaran with three floats always in contact with the water. 

At high speeds, stability is managed by a complex mechanical system that removes the moments created by aerodynamic (kite) and hydrodynamic forces (foil). In other words, this power module optimized by the team allows to counterbalance the kite’s power (pulling up the boat) with a foil that will anchor the speedcraft to the water.

Thanks to three floats, an optimized power module and the weight of the boat,  the SP80 craft will therefore not be flying, in contrast with what we usually see today in the sailing world.

But again, What if making boats fly is a dead end ? 

STABILITY EXPLAINED IN VIDEO

Find out more about our power module !

Now,  having a huge kite in the air and a foil to keep the boat in contact with water also adds stability uncertainties. Let’s focus on the foils and the complex physical phenomenons occurring underwater. 

2. FOIl stability

A slight variation of the hydrodynamic forces created either by the wave pattern or a transition in the flow regime can dramatically affect the stability of the craft. There are some major different regimes of flow around an hydrofoil, and all foils are not suitable for a target speed of 80 knots. Among the ones considered by SP80, let’s mention sub cavitation hydrofoils (well known in the era of foiling of Hydroptere, SailGP, AC), super cavitation hydrofoils, base ventilation hydrofoils (used by the actual record holder Paul Larsen in 2012) and full ventilation hydrofoils.

    • Sub cavitation hydrofoils are not an option for the targeted 80 knots. Indeed, cavitation is ultimately inevitable on these foils, limiting them to around 55 knots and making them very unstable above this speed.

    Cavitation of an hydrofoil in EPFL’s cavitation tunnel (Ecole Polytechnique Fédérale de Lausanne).

    • Supercavitating hydrofoils are the most performant foils. Unlike sub cavitating hydrofoils, they have a wedge shape with a sharp leading edge to favor the appearance of cavitation and only the pressure side of the foil is in contact with water.However, they are extremely unstable and prone to ventilation. If air from the atmosphere gets sucked into the cavitation bubble, pressure at the suction side will increase, and the lift will drop. The effect of such hydrofoil transition on the boat’s behavior is highly dynamical and can be unpredictable, thus making it too hazardous for the record.
    • Base ventilation hydrofoils are very stable and almost as performant as supercavitating foils. They have a shape similar to supercavitating hydrofoils but both the suction and pressure sides are in contact with water.

    However, in case of cavitation or ventilation of the suction side, there is a transition to a fully ventilated foil, which creates a loss of lift. This transition can occur at 75/80 knots, which was fine for Vestas’ record as Paul Larsen did not go faster than 70 knots, however making it hard for the SP80 boat to reach its targeted speed. 

    Vestas Sailrocket II was the first sailboat to use base-ventilation foils for the world sailing speed record.

    •  Superventilating hydrofoils are the least performing foil, but they will always deliver the lift you expect. They are very similar to supercavitating hydrofoils but water vapor is replaced by air. Once the suction side is ventilated (around 5 to 8 knots of boat speed), there will be no transition up to 200+ knots! Those foils are used by Hydroplanes to make sharp turns at 150 knots. 

    This foil is thus the most reliable at high speeds and was an obvious choice for SP80, in a pure speed record context. Now, a whole team is dedicated to optimizing those foils in order to gain in performance while maintaining high stability. 

    Hydroplanes are powerful motorboats that can reach 150+ knots! They use superventilating hydrofoils for their skidfins and propellers.

    Conclusion

    Following our vision to put stability first and ensuring the security of our pilot at all times, we validated a trimaran design, with three floats and a power module handling the forces alignment between a kite and a foil. This design asserts one key principle of our concept: the SP80 boat will stay in contact with the water and will not be part of the flying boat’s generation. 

    Regarding the foils, the fluid dynamics team of SP80 is set on increasing as much as possible the performance of full ventilation hydrofoils while keeping them stable, high stability being the main reason behind our choice of hydrofoil for this boat. Find out more about our researches and developments here! 

    Our key elements regarding stability (floats, power module & super ventilating foil) were tested on a scaled model to validate our thinking before getting real with the final boat’s construction.

    With the first point of our vision validated, we have yet to focus on the other two:

    • Increasing power as much as possible: the huge potential of the kite’s power will be discussed in a future article.
    • Decreasing drag as much as possible: the boat’s aerodynamics was optimized thanks to numerous simulations. With the construction advancing well and the first parts taking shape, we will soon be able to show you the final design. 

    Stay tuned ! 

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