Boat Speed

How Do Sails Work: Sailing into the wind
 
 
Dinghies can’t sail directly into the wind. Instead, they have to zig-zag into it if they want to travel in that direction.
 
This all makes Olympic Sailing on the TV very confusing for the none-sailor types!
 
But it also makes our sport far more strategical than it would be if we could just sail in a straight line everywhere.
 
So how do we sail into the wind?
 
 
The science-y part (sort-of)
 
What is fascinating is that we can sail into the wind at all. If you’re walking round your local pond and happen to throw a dry & shrivelled leaf into the water the wind will catch it and blow it in the direction of the wind.
 
How that same wind could be harnessed to allow something to be blown towards it is, on the face of it, a bit mind-boggling!
 
How boats sail into the wind is hotly debated and the science is often simplified to help us lay-people comprehend it.
 
But if you had to sum it up in one word that word would be LIFT.
 
 
What is lift?
 
Lift is an invisible force created by air (or water) flowing around the surface of an object (such as an aeroplane wing).
 
Sails work similarly to aeroplane wings. Both use lift to get where they want to go.
 
As the pilots among you will know, planes like to take off into the wind. This creates more lift helping them “lift” off.
 
You can think of a dinghy just like an aeroplane turned on its side. The forces are the same.
 
The wings on a plane are shaped so one side is more curved than the other. The air particles hit the leading edge of the wing and are split, with some travelling over the longer, curved side and some travelling along the straighter side. This forces the particles travelling on the curved side to travel faster than their counterparts travelling on the straight side.
 
Then the famous Bernoulli’s law comes into play. In the 1700s Monsieur Bernoulli theorised that this higher speed creates lower pressure on that side of the wing/ sail.
 
And as higher pressure likes to move towards low pressure it applies a force to the sail. This force is lift.
 
But why does lift allow you to sail into the wind?
 
Lift creates a force that sucks the boat towards the curve of the sail. On its own, that force would mostly suck the boat to the side rather than forward.
 
But dinghies have wings above and below the water. That’s right, your centreboards have a purpose other than to give you something to stand on while righting after a capsize.
 
As the centreboard travels through the water it also creates lift. Like with the sail, that force is also mostly a sideways force with very little pushing the boat forward.
 
But here’s where the magic happens… it’s called the “squeezed pip effect’”.
 
 
What is the squeezed pip effect?
 
Imagine holding a slippery lemon pip between your thumb and ring finger. Your fingers are applying equal pressure on both sides of the pip. But as you squeeze, the pip will shoot out in a different direction to the forces applied by your fingers.
 
In the same way the opposing side-ways forces of the sail and centreboard mean the boat is squeezed forwards.
 
This is because the sail and board’s side-ways forces cancel each other out leaving a resulting forwards force.
 
 
 
The less science-y part
 
Ok, science lesson over. Here are the answers to some queries you might have about sailing into the wind.
 
What happens if you sail too close to the wind?
If a boat sails too close to the wind the sails lose their lift as they start to flap. This disrupts the airflow vital for lift. This angle and any angle closer to the wind is appropriately termed the “No Go Zone”.
 
What is close-hauled sailing?
Sailing close-hauled is the term used to describe the point of sailing that is the closest angle to the wind that you can sail which still generating maximum lift.
 
What angle can you sail the wind?
The angle that you can sail towards the wind varies from boat to boat. The range is between about 30 and 50 degrees off the eye of the wind. If you set your sails well or are a multi-sailed keelboat then you’ll be able to point near the 30-degree angle. But if you’re a novice sailing a single sail dinghy you’ll be pointing a lot lower. These differing angles make for interesting handicap racing!
 
How does tacking help us sail into the wind?
Tacking is simply the term for turning the boat across the no-go-zone. Going from close-hauled on one tack to close-hauled on the other allows us to reach any point into the wind.
 
 
Hopefully this helps explain how those expensive pieces of cloth work.