How a Sailboat is Propelled

Video Transcript

Getting your sailboat to go upwind is a little more challenging. What we’re looking for here is to create something called lift and that’s the force that takes as upwind. What’s quite interesting is the same force that allows an aircraft to fly that causes the sailboat to move through the water. So, if I can just draw this up on the board here, hopefully, you recognize we’re looking at something resembling an airplane and what happens is as this aircraft accelerates along the runway, wind hits the leading edge of the wing. And as the wind does that the particles of air separate and some of the air comes up across the top of the wing and some of the air comes along underneath the wing. The interesting thing is physics dictates that these particles of air, they split at the leading edge. They arrive at the trading edge at the same time and that has happened, then the particles that went across the top of the wing that went further and arrived at the same time must have gone faster. And a fellow called Danny Bernoulli discovered that the faster a fluid flows then the lower the pressure. So what’s happening is above the wing we’re creating an area of low pressure. So, we have relative low pressure above the wing and higher pressure underneath the wing. It’s this low pressure that actually sucks the wing up into the air and we get lift and the results of all these lift being sucked up into the air is that the wing or the airplane is lifted mostly upwards and a little bit forwards. So, if I take this wing off the airplane and put it on my sailboat, I suppose we get the aircraft here… and imagine this to be my main sail. So, I’ve got a helicopter I view here as the mast and here is my sailboat, then we’re going up wind, so the wind is hitting the front of the boat. So, what we have here is the wind goes across the back of the main sail. It goes faster than the wind that comes across the front of the main sail, as a result, we get lift and the boat gets lifted mostly sideways and little bit forwards. We don’t want to go mostly sideways and little bit forwards, we want to go mostly forwards and little bit sideways. So what we have to do is introduce another wing and this wing is actually underneath the boat and is called the keel and as all boat tracks through the water and it doesn’t actually track in a dead straight line, it moves slightly sideways, that’s called a leeway. It’s the force of the wind causing the boat to move slightly sideways. So, as the water hits the keel, it actually hits at an angle. It doesn’t hit straight on, it hits at an angle and some of the water comes around this side of the wing and some of the water across the shoulder side, the stuff that goes further has to go faster and as a result we get relative low pressure forming on the windward side of the keel. And the lift that creates once again is mostly sideways and a little bit forward and it’s the resultant of the lift on the sails and the lift on the keel that actually allows the boat to sail mostly forwards and little bit sideways as we are going upwind.

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