In this post we are going to talk about how a sailboat sails. It is pretty clear how a sail works when sailing downwind, like a parachute, but upwind might be harder to understand. There are a lot of similarities in how a sail works and how an airplane wing works. Both generate lift, but a sail uses that lift to propel it through the water and an airplane uses that lift to fly. Lets look in more detail about how lift is generated and then how that lifts actually makes a sailboat sail.
The secret is in the shape of the sail and the angle of the wind. When the sail is full of wind, it takes on the shape of an airplane wing. When it has this shape, there are certain aerodynamic principals that are working on the wing. It is these forces that create lift.
It is easy to understand this principle of you put your hand out the window of a car while moving. If you put your hand up and allow the palm of your hand to face the wind, your hand will be pushed back. This is like sailing downwind, the wind just fills the sail and pushes it. If you slowly start to tilt your hand down, pointing your fingers into the wind, you will come to a point when you will start to feel your hand lift up. This is because the way that the wind races across the top of your hand creates lift and pulls your hand up. This is the same principle used by a sail and an airplane wing.
A more technical explanation goes like this, as the wind flows over the outside of the sail, it increases its speed and creates a low-pressure area. This is Bernoulli’s law in action. At the same time, the wind that is on the inside of the sail decreases its speed and creates a high-pressure area. This difference in pressure creates the lift and helps pull the sail in that direction. This newly created lift tends to push the boat over (or heel), but it also creates a small forward thrust.
The really interesting thing is that under the water, the keel is fighting against the lift being created above. The keel is shaped like a wing and slices through the water easily when moving forward, but it resists the left to right, or heeling, movement of the sailboat. As it helps resist this heeling motion, it allows the sail to use that forward thrust and propel the boat forward. If it was not for the keel being deep in the water, having this shape, and weighing a lot, the boat would just fall over from the force of the wind.
How fast can you go?
One last thing to talk about is, “what happens when the wind speed keeps increasing, can you keep going faster and faster”? The simple answer is yes and no… Clearly you can increase your speed, but at some point you can’t go any faster. As a boat cuts through the water, there is friction being created against the hull. This is known as drag. As the speed of the boat increases, the drag increases, eventually they become equal and the boat cannot go any faster.
So now you know how a sailboat sails. Amazingly enough, this was not discovered for many years. Most boats sailed downwind only, then over time they learned to sail at 45 degrees to the wind, then eventually they learned to sail upwind. Maybe someday we will learn to sail directly into the wind….. Probably not, but sails and sail technology is always pushing the envelope on how close to directly upwind we can sail. If you watch some of the sailboat races, you will see technology being used that will blow your mind!!
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