The lift of sails explained (finally) well

Why are sails the engine of a sailboat? It seems like a simple question, and yet, about lift and the effect of wind on sails there has always been a lot of confusion, even bringing up-wrongly-the Venturi effect. What better “guide” is there, to illustrate lift and its practical effects, than a university lecturer? We asked Professor of Naval Architecture at the University of Genoa, Paolo Gemelli*.

What is lift

The reason why sails behave as we are used to observing is one of the first doubts that seize the novice sailor but not only. Given, however, that the operation of fluids (such as air) around contours (such as sails) is governed by rules that are not always readily understood, explanations that are not always correct have settled over time. For the sake of simplification, theories have been constructed that have little or nothing to do with observed reality.

In brief:

• The aerodynamic force acting on sails is composed of two terms: bearing capacity and strength; the lift Is perpendicular to the apparent wind direction (The apparent wind, the wind with which sailboats sail once they are in motion, is the vector sum of the wind produced by the headway and the true wind) and directed from upwind to downwind side, the resistance is directed and oriented according to the apparent wind direction.
• The force that enables the sail to provide propulsion to the boat is due to the greater fluid pressure on the windward side than on the leeward side; the pressure difference is closely related through Bernoulli’s equation to the different fluid velocities: greater downwind and less upwind.
• The different velocity values are associated with the presence of fluid circulation around the sail, which, through the composition of upwind and downwind velocities, results in a respective decrease and increase in velocity.

Why does the sail carry?

When a profile, even a perfectly flat one, is immersed in a moving fluid and forms an angle with it (angle of attack) a thrust F is generated (see figure). This force is composed of two terms: the first is called the lift P, directed perpendicular to the apparent wind direction; the second is the drag R, directed parallel to the flow.

The existence of these forces is due, to the different fluid velocities on the upwind and downwind sides. As a result of Bernoulli’s equation this difference in velocity results in a difference in pressure: downwind velocity greater than upwind velocity therefore lower pressure.

Why does this happen?

For the fluid, the wind in our case, to behave as observed in reality, it is necessary to consider its motion as consisting of the superposition of two terms: the first represented by flow lines oriented according to the direction of the latter; the second, the circulation (Fig. 2a in red), consisting of closed flow lines around the profile and directed from the trailing edge to the leading edge on the upwind side and in the opposite direction on the downwind side.

The superposition of these two contributions produces a lower resultant of velocities on the upwind side than on the downwind side due to the different orientation of the vectors, partially discordant upwind (velocities subtract), concordant downwind (velocities add up). Application of Bernoulli’s equation relates the increase in velocity to the decrease in pressure on the downwind side.

Fig. 2 – If viscosity-free conditions are assumed in the fluid (ideal fluid), its motion around the profile is as depicted in Fig. (a) in black. The points indicated by the letters R represent the stagnation points at which the fluid velocity is zero and the static pressure is maximum. Under these conditions, the behavior of the fluid, especially at the trailing edge (the right end) of the profile, is quite different from that observed in reality. Only by switching to real fluids (which have non-zero viscosity) and introducing circulation (Figure (a) in red) the situation changes and well represents what is observed in reality (Figure b) ). The situation depicted in Figure (b) relative to the trailing edge is said to satisfy Kutta’s conditions.

Fig. 3 – Kutta conditions. In (a) Kutta’s conditions are not met and the fluid near the trailing edge of the profile should, unrealistically, make a counterclockwise rotation by briefly resalting the upwind slope. In (b) Kutta conditions are verified and the fluid exits more ‘smoothly’ from the profile.

Bibliographical references

Arvel Gentry; A review of modern sail theory ; Proc. of the Eleventh AIAA Symposium on the Aero/Hydronautics of Sailing, 1981

Fabio Fossati; Aero-Hydrodynamics and the Performances of Sailing Yachts.

Who is our “prof”

*Paolo Andrea Gemelli is a lecturer in Naval Architecture in the Nautical Product Design degree program at the University of Genoa. From 1999 to the present, he has been involved in maritime security with a focus on weather routing and naval intelligence. He is a member of the expert panel of the European Maritime Safety Agency (EMSA) and the Italian Association of Intelligence and Geopolitical Analysts.