What is a Sail?
Sails are flexible wings that are typically built with malleable materials to allow the sail to collect wind on either side to allow tacking. A sail plays a critical role in the sense that a sailboat cannot support itself or even move forward without a sail. Again, a sail cannot be made in just any shape. This is because not all shapes can support themselves in the wind. As such, most sails are triangular with the smallest point on top of the mast.
Understanding how a sail works is integral in learning how to propel a sailboat. Like wings, the pressure at the top near the mast is less than the pressure at the bottom of the sail. This is because the air moves a lot faster at the top than at the bottom. For this reason, the difference between the air pressure at the bottom and the top is principally what propels the sailboat. In simpler terms, the curve that is created by the air inside the sail travels a longer distance at the top and a shorter distance at the bottom thus propelling the sailboat forward.
Sailing Downwind and Upwind
We do not want to complicate things by using a lot of scientific terms but you’ve probably heard the terms downwind and upwind. It’s generally very easy to see that a sailboat can move forward when sailing in the same direction as the wind. This is what is called a downwind sail as the sail catches the wind that pushes the sailboat forward.
But what happens when the boat sails against the wind? Known as an upwind sail, it revolves around aerodynamics of the forces of the wind and the hydrodynamic forces of the underwater part of the boat combining to propel the boat not only through the water but also against the wind. The wind that blows across the sail triggers an aerodynamic lift that contains a sideways force and a small forward force to propel the sailboat against the wind.
The Importance of a Keel
Located underneath the hull, a keel plays a critical role in ensuring that the sailboat is stable and doesn’t slide sideways with the wind. In most cases, the flow of the water will lift the underwater surfaces of the boat, which essentially counter the sideways force of the wind, thereby propelling the boat forward.
In essence, a keel must have a heavy concentration of weight, usually a ballast, to pull the boat back down and prevent it from capsizing.
Learn more about sailboat keels here.
Point of Sail
The point of sail is fundamentally the difference between the direction in which the sailboat is heading and the direction of the boat. This angle of sail can change when the sailboat changes its course. At this point, the sail must be altered to harness the wind as perfectly as possible.
When the sailboat is very close to the wind, you should trim in the sail all the way to maintain an appropriate speed. As you sail away from the wind, you should ease the sails onto a close reach, beam reach, and then broad reach. If the sailboat is sailing directly into the wind, the sails should be eased all the way out before turning back up to a closed hauled.
Wind Flow Conditions
Identifying the wind flow conditions that sails operate in is of great importance in understanding how the sails work. In most cases, the wind will blow over the surface of the earth but with friction. This friction generally slows the air that is closest to the earth’s surface. This effect continues in a domino effect going upwards until the entire wind moves at a similar speed in what is known as the boundary layer. This is essentially why the wind speed in a sail will increase from the bottom of the sail and increase up the entire height of the mast.
Given that the sail is constructed using a flexible material, its triangular shape can be supported by the pressure difference that it produces from the bottom and at the top point of the mast. The sail also has a leading-edge entry angle that must be harnessed with the incoming wind flow angle. The sail will luff if the wind entry angle is too high while it will stall if the wind entry angle is too low.
Bottom Line
The main aim of the sail is to use the wind to create the force or the power that is used to propel the sailboat. If the weight of the crew and the sailboat is at a stable point, the sail should produce enough force to propel the boat. In essence, sails are flexible wings that produce the force needed to propel the sailboat by accelerating air over the curved leeward side of the sail while lowering the pressure on the other side of the sail that acts to move the sailboat forward.
