How do Althea’s sails work?
Imagine standing aboard Althea on Grand Traverse Bay. The wind is blowing from the side, yet instead of drifting downwind, the yacht glides gracefully forward. How is that possible?
The answer lies in Bernoulli’s Principle.
A sail acts as an airfoil. As wind flows around the curved sail, it travels faster over one side than the other. Faster-moving air creates lower pressure, while slower-moving air creates higher pressure. The pressure difference produces a force called lift. As wind flows over both sides of a curved sail, it accelerates around the leeward (downwind) side, creating a region of lower pressure compared with the windward side. This pressure difference, often explained using the Venturi effect along with airfoil theory, helps generate lift that pulls the sailboat forward.
Instead of pushing the sailboat directly away from the wind, that lift pulls the sails sideways and forward. Because the sails are attached to the mast, they transfer this force to the entire boat.
On Althea, you’ll often notice that the sails aren’t flapping. They’re carefully trimmed into a smooth curve so the wind flows efficiently across them, creating lift and allowing the yacht to sail efficiently—even when heading at an angle toward the wind.
A well-trimmed sail works much more like an airplane wing than a parachute. Most modern sailing relies on aerodynamic lift, not simply the wind pushing from behind.