Sail - Sail Aerodynamics

Sail Aerodynamics

Sails propel the boat in one of two ways. When the boat is going in the direction of the wind (i.e. downwind - see Points of sail), the sails may be set merely to trap the air as it flows by. Sails acting in this way are aerodynamically stalled. Drag, always parallel to the wind, contributes the predominate driving force.

The other way sails propel the boat occurs when the boat is traveling across or into the wind. The sails acting as airfoils propel the boat by redirecting the wind coming in from the side towards the rear. By the law of conservation of momentum, the wind moves the sail as the sail redirects downwash air backwards. Air pressure differences across the sail area result in forces on sails including drag and lift. A component of the lift is the main driving force.

The sails can also act as airfoils in some downwind situations, e.g. spinnakers and square-rigged sails can be trimmed so that their upper edges become leading edges and they operate as airfoils again, but with airflow directed more or less vertically downwards. This mode of trim also provides the boat with some actual lift and may reduce both wetted area and the risk of 'digging in' to waves. In stronger winds, turbulence created behind stalled sails can lead to aerodynamic instability, which in turn can manifest as increased downwind rolling of the boat.

Sail are often equipped with lightweight tapes or strands (tell-tales) to indicate the airflow in their area. They may be on both sides near the leading edges of the sail, or at the trailing edge of the sail. Horizontal strips sewn into fore-and-aft sails and V-shaped markings on spinnakers assist with judging their shape from on deck. These may even glow in the dark, using luminous tapes.

On a sailing boat, a keel or centreboard helps to prevent the boat from moving sideways. The shape of the keel has a much smaller cross section in the fore and aft axis and a much larger cross section on the athwart axis (across the beam of the boat). The resistance to motion along the smallest cross section is low while resistance to motion across the large cross section is high, so the boat moves forward rather than sideways. In other words it is easier for the sail to push the boat forward rather than sideways. However, there is always a small amount of sideways motion, or leeway.

Forces across the boat are resolved by balancing the sideways force from the sail with the sideways resistance of the keel or centerboard. Also, if the boat heels, there are restoring forces due to the shape of the hull and the mass of the ballast in the keel being raised against gravity. Forward forces are balanced by velocity through the water and friction between the hull, keel and the water.