Hey Guys!
Today I’m going to talk to you about sailing! Well more about how the sails work in sailing. How does this apply to you? Well a sail is pretty much like a plane wing sticking up in the air so you can learn about how planes fly as well in this article. I know it is going to be tough learning twice as much in one blog post but I’m sure you’ll survive.
When you picture sailing I’m sure you think of it how I originally thought of it. You picture a boat with its sails out with the wind behind it moving along. But have you ever stopped to think about how the boat gets back to where it came from? It can’t sail away from the wind forever, otherwise boats would get stuck at places for a long time. Let’s talk about how boats do the impossible and sail upwind.
A boat can’t sail straight upwind as the sails flap and become useless. However, they can sail about 45 degrees away from the wind. The boat must sail in a zigzag pattern to get upwind but eventually a boat will manage to reach the upwind location. The boat changes direction by tacking, which means the front of the boat must pass through the wind to change direction. So how does it work?
The curved shape of a sail. Credit to Nouhailler
The sail (or wing) has a curved shape which pushes the wind down either side of the sail. This creates two different paths for the wind, one of paths is curved. Now when the wind curves around the sail it accelerates. This is similar to the laws of circular motion if you did that in high school. As a result of the acceleration, one side of the sail has wind travelling at a higher velocity. I know this is very difficult to visualise because I had trouble picturing it when I was learning to sail. So here is a simulation on how air flows over a sail or wing. It applies more to planes than to sail boats but you get the idea.
The acceleration causes a low pressure to develop on the curved side of the sail and a high pressure on the other side. Now, basic science tells us that the air will want to flow from the high pressure area to the low pressure. This will put a force on the sail towards the curve. This will of course result in the boat moving in the direction of that force.
Now you are thinking, ‘Emma that force is going to be mostly sideways. Why doesn’t the boat travel mostly sideways and a tiny bit forwards?’ Well there is another set of forces taking place below the water. All sail boats have a thing called a keel. This is essentially a part of the boat and the water has to flow around it when it is sailing along.
A Keel on a sailing boat. Credit to Paul Schultz
The keel operates as an opposite force to the force on the sail. This is due to the keel be perpendicular to the force on the sail. The boat then has to push aside a large amount of water to move in that direction. It is the same idea as trying to walk through water yourself in the swimming pool. The result of this force on the keel? Forward motion of course! If you are sailing correctly you also get hardly any sideways motion. Well that is a win win situation isn’t it just!
Here is an interesting video about how this applies to a plane if you are still stuck on that.
I hope that wasn’t too much to learn in one blog post.
Let me know in the comments if there is anything you would like to hear about next.
Emma
P.S. Here is a video showing you exactly what you can do if you understand the forces involved in sailing a boat. These boats have solid wing sails and even “hydrofoil.” This is basically the same idea as plane wings but underwater. They lift the boat out of the water which makes it go faster by reducing drag. It is way cooler to having a flying boat than a flying plane.
The Seriously Silly Scientist 