Since the 2016 Vendée Globe, foils have appeared on IMOCA boats. They replace daggerboards on new boats. While they didn't really prove their effectiveness on the 2020 edition due to very specific weather conditions, the first month of racing on the Vendée Globe 2024 shows that the fastest yachts are indeed foiling yachts. But how do these foils work on an IMOCA monohull?
What are foils used for on an IMOCA?
The foils on an IMOCA boat function like submerged wings, generating a lift force that partially lifts the hull out of the water. Here are the main aspects of how they work:
1. Structure: An IMOCA foil is made up of several parts, including a central barrot, a rectangular structural element, foam blocks on the sides, and carbon leading and trailing edges.
2. Principle: The asymmetry and camber of the foil's profile create a pressure difference between its upper and lower surfaces as it moves through the water, generating lift.
3. Effect on navigation :
- Foils reduce drag by partially lifting the hull out of the water.
- They increase boat speed by reducing friction with the water surface.
- The IMOCA boats start to partially "fly" from 14-15 knots of wind.
4. Special features of IMOCA boats :
- Unlike America's Cup boats, IMOCA boats don't have T-shaped rudders (a choice made by the rules), which limits their ability to maintain stable flight over long periods.
- IMOCA foils are designed to cavitate at around 35 knots, a compromise between performance and efficiency over a wide speed range.
5. Adjustments: The foils can be adjusted in terms of incidence to optimize their performance according to sailing conditions.
Hydrofoils have revolutionized IMOCA sailing, delivering significant speed gains, particularly on beam and downwind points of sail. They do not perform well in light airs and upwind sailing, although the speed difference between foiling and non-foiling boats tends to diminish in these conditions.
What are the advantages of foils on an IMOCA?
Foils on IMOCA boats offer several significant advantages:
1. Increased speed: Foils can increase boat speed by up to 10-15%, particularly on beam gears (between 70° and 120° of true wind).
2. Drag reduction: By partially lifting the hull out of the water, foils reduce water resistance on the hull, which can translate into a speed gain of up to two knots compared to a conventional IMOCA without foils.
3. Improved stability: foils increase the boat's righting moment, which in turn increases the power in the sails and thus overall performance.
4. Versatility: Some foil models, such as C-foils, are retractable and modular, offering greater flexibility according to sailing conditions.
5. Hull lift: Foils take the load off the hull by partially lifting it, thus reducing the wetted surface and increasing speed under equal power.
6. Anti-drift function: In addition to vertical lift, foils can create anti-drift lift, a task normally performed by traditional transverse daggerboards.
These advantages translate into significant performance gains, with estimates of up to 5 days gain on a race like the Vendée Globe.
What are the disadvantages of foils on an IMOCA?
Despite their performance advantages, foils on IMOCA boats have several drawbacks:
1. Limited effectiveness under certain conditions:
- Less efficient upwind and downwind.
- Need a minimum of 12-14 knots of true wind to be fully effective.
2. Control difficulties :
- The skippers are struggling to cope with the rapid acceleration of the boat with the foils.
- The absence of T-shaped rudders makes stable flight difficult, unlike the America's Cup boats.
3. Design compromises :
- Foils designed for high speeds are less efficient at low speeds.
- Foil thickness influences cavitation speed, creating a dilemma between strength and performance (the thinner the foil, the better it performs).
4. Problems in difficult conditions :
- In heavy seas, skippers often have to reduce speed to protect the boat and themselves.
5. Management complexity :
- Skippers have to constantly adjust the foils to suit the conditions, which can be both physically and mentally demanding.
6. Potential fragility :
- Foils are additional appendages that can break or be damaged, as the experience of some skippers has shown.
7. Performance in degraded mode :
- When conditions are not optimal for using foils, boats can find themselves at a disadvantage compared to models without foils.
These drawbacks underline the need to further develop and optimize foil technology for IMOCA boats, particularly in terms of retractability and the trade-off between performance and versatility.
