Foils on a sailboat. How they work, advantages and disadvantages

What happens below the waterline of a traditional sailboat? What is hydrodynamics and why do hull and appendages affect the speed of a boat? Starting with the basics, Professor of Naval Architecture at the University of Genoa, Paolo Gemelli*, former author for us of the articles “The Carrying Capacity of Sails,” “Mainsail and jib. How to regulate them with the help of science“, “Mainsail and jib interaction, boundary layer and forestay,” which have been widely read.

In past episodes we introduced you to the hydrodynamics of a sailboat, explaining the concepts of hull and drag, then we dealt with keel and rudder. Today it is the turn of the latest evolution in appendages, namely foils. The “wings” that allow a boat to fly over the water (in this case we speak of “full foiling” sailing) or otherwise reduce the wet surface area of the hull by generating a bottom-up thrust (lift). How do they affect a hull and how do they affect the design of a boat?

Foils: operation, advantages and disadvantages

Foils work by primarily exploiting lift-a force that is generated when an airfoil moves in a fluid. This force allows the hull to be lifted out of the water and a reduction in drag resulting from a decrease in the submerged surface area of the hull. It should be mentioned that the effectiveness of foil action becomes apparent above the minimum veliocities that allow hull lifting. Below the minimum values, foils do not generate sufficient lift representing, on the contrary, an increase in wetted surface area, which makes it necessary to retract them.

The origin of lift can be explained by the Kutta-Joukowski theorem: a fundamental principle of aerodynamics that links the lift generated by a body immersed in a fluid (such as an airfoil) to three key factors:

1. Circulation (Γ): Measures the rotation of the fluid around the body (representable as a vortex spinning around the airfoil).
2. Fluid density (ρ): Indicates the amount of matter present in a given volume of fluid (e.g., water).
3. Undisturbed velocity (V∞): The speed of the fluid approaching the body from a distance.

The Kutta-Joukowski theorem states that, in an ideal fluid, the lift generated by a wing is proportional to the product of the three quantities just mentioned and is directed perpendicular to the undisturbed velocity of the fluid.

Under real-world conditions such as when the profile moves in seawater, the fluid is characterized by the presence of viscosity and rotationality (i.e., the presence of vortices), and this causes the appearance of phenomena that would otherwise be absent: such as energy dissipation and the formation of different flow regimes (laminar or turbulent) that affect the generation of lift.

When foils are added to a hull, such as an IMOCA 60, the balance of forces changes as shown in Figure 1.

Starting with the analysis of the forces on the appendages, it is immediately evident that the vector representing lift (black arrow) is decomposed into the two components: lateral (orange) and vertical (green). On the sail plane (also a lift-generating airfoil) the vertical component is shown in blue and is oriented downward.

Thus we have two force systems: one directed vertically and one directed horizontally.

To the first system belong the vertical components of lift (shown in green or blue), the weight force (denoted ‘Newton’ in Figure 1 and shown in blue) and Archimedes thrust (applied in the center of the hull and shown in yellow).

To the second system, however, belong the lateral lift components (shown in orange), directed to the right or left depending on whether they relate to the appendages or the sail plan.

Putting all this together we can finally analyze the balance of this particular vessel. If we decompose the possible movements into translatory and rotatory, we can easily apply the rules of elementary physics and obtain the following.

Foils – Translational movements in the vertical axis

There are mainly two forces acting vertically on the hull: the weight force (in blue denoted ‘Newton’), which tends to make it sink, and the Archimedes thrust (yellow), proportional to the volume of the living work, which, on the contrary tends to push it upward and make it float. In the absence of other forces, the hull finds its own equilibrium point that equalizes the two forces according to a certain plane of buoyancy (corresponding to a certain immersion).

In the case depicted in the figure, we have some additional elements consisting of the appendages and the sail plan that generate effects that overlap with those just indicated. To analyze them, however, the horizontal axis must also be considered.

The lift generated by both the sail plan and the appendages produces both vertical contributions that overlap with Archimedes’ thrust and weight force, and horizontal contributions that tend one to drift the boat, the other (the horizontal component of keel and foil) to keep it on course.

Now, however, it is also necessary to analyze the second type of motion: rotational motion.

Foils – Rotational Movements

The cardinal equations of statics state that in order for a body to be in static equilibrium (somewhat inaccurately, one might say ‘does not translate and does not rotate,’ but it would be more correct to speak of a body that has zero accelerations) it is necessary for both the resultant of external forces and the resultant of external moments to be equal to zero. All forces and all moments must be balanced.

If the concept of force is fairly intuitive that of moment is somewhat less so. It can be helpful to imagine having to open a door by pushing it open. If our hand acts on the handle we will be able to open it without difficulty, however, if we instead apply force near the hinges the force required will be much greater.

This happens because in order to set the door in motion, which moves by rotary motion hinged on its hinges, it is necessary to apply a force at a certain distance from the axis of rotation (the hinges), the greater this distance (force arm) the less force is required. The product of the force by its arm is called the ‘moment of force.’ The moment is considered positive if it generates a clockwise rotation (seen from the axis).

In our case we have two distinct moments: a lurching moment and a straightening moment. Again, the boat finds its own point of equilibrium between the moment of forces that tends to heel it transversely and the righting moment that tends to bring it back to its original position.

If we imagine (more or less correctly…) that the axis of rotation passes through the bottom of the hull and try to imagine what rotation, clockwise or antriorclockwise, generates each of the forces considered above, we can easily understand the mechanism just indicated.

Specifically, as observed in Figure 1, the lateral component of the lift generated by the sail plane and the vertical component of the keel lift produce heeling.

Contributing instead to the righting moment are: the weight force, Archimedes’ thrust and the vertical component of the lift of the submerged foil.

It is therefore clear that the alteration of any of the forces acting on the hull affects its overall equilibrium in the context of a highly dynamic situation such as that of a sailboat sailing in often highly variable wind and wave conditions.

The advantages

Speed: Boats with foils can reach significantly higher speeds than traditional boats, up to double the wind speed. Think of America’s Cup catamarans hurtling across the water at over 50 knots, a true spectacle of speed and adrenaline.

Efficiency: Reducing water resistance increases the efficiency of the boat, allowing it to sail with less wind and lower fuel consumption. An advantage not only for racing boats, but also for cruising boats that can sail with greater range and reduced consumption.

Stability: Foils can improve the stability of the boat, reducing heeling and making it more controllable.

Disadvantages

Complexity: Boats with foils are more complex to design, build and maneuver. It requires a highly skilled crew and in-depth knowledge of the technology to make the most of the foils’ potential.

Costs: Foil systems are expensive and require specific maintenance. The use of this technology is still the preserve of racing boats and wealthy owners.

Safety: Foil sailing can be riskier in case of collisions or wrong maneuvers. Lack of experience and lack of knowledge of risks can lead to serious accidents.

Paul Andrew Gemini