In a recent Veritasium video, they discuss that a sailboat can travel further downwind than the wind speed. Steve Mould, in a reaction video, explained how a boat can travel faster than the wind at an angle, but not really how it can end up further down wind than the actual wind speed. I’m having trouble connecting these explanations…Links below for both videos.
https://youtu.be/u5InZ6iknUM
In: Physics
So, in the Steve Mould video, you should have learned from the stick wheel demonstration that the distance your boat moves can be greater than the distance the wind has moved in the same period of time, provided you’re at an angle to the wind, right? Well, as you might imagine, that’s great if you want to go in a different direction to the wind, but not so much if you want to go in the same direction.
What you do then is once you’ve gone a fair distance one direction, you turn your boat and your sail so that you’re at the same angle to the wind but in the opposite direction. This way, you’ve made two long stretches of movement in directions different to the wind, but the total distance you’ve moved just in the same direction as the wind is still higher than the distance the wind has moved in the same period of time. At 8:49, Steve talks briefly about tacking, the method of moving against the wind by making short movements at alternating angles to it, and the same principle can work downwind.
The sail acts like a wing that generates “lift” which adds more energy to the boat relative to the wind
You sail when wind is pushing you. Wind pushes you unless you’re travelling the same speed (assume there’s no propellor or anything just a sail). More particularly it’s the speed in the wind direction. So if the wind is blowing north at 3mph you can be going no more than 3mph north. However there is nothing stopping you going 3mph and 4mph east at the same time. This means you’re actual speed is 5mph (pythagoras). You receive the force pushing you east through the use of the sail as a wing. Even though the wind is blowing north, the sail’s shape makes it feel a force east (lift) in addition to the force north (drag).
The problem is that as you speed up, the wind slows down relative to you. If there is a 10mph wind and you move at 10mph, there is no wind from your point of view.
However, there is no reason why you can’t go faster than the wind. After all, if you imagined a large sail attached to a pulley connected via a gearbox, you could easily go faster than wind. Energy conservation is not a problem.
In this very particular case, if you take into consideration the angle and speed of the fan blades, they are moving slower than the vehicle as a whole, which means the wind can actually push at them.
My understanding is this:
A boat’s sail has two modes of operation: like a parachute, and like a wing.
In parachute mode, air just pushes at the sail like air would push at a parachute. Simple enough to understand.
In wing mode, lift (up) is generated on a wing based on air flowing (horizontally) around the wing, with the wing splitting the airflow in an asymmetrical way and thus generating a difference of pressure (a force). An airplane stays in the air not because there’s an updraft from below it, but because air flows horizontally over the wing.
So what they did with that propeller vehicle is: the propeller is a pair of wings, and whether air blows at the vehicle from behind, or from the front (when it’s going faster than the wind), the propeller spins fast enough to generate air flow across its wing-shapes and generate a “lift”.
To put it in terms of an airplane: initially there’s an updraft, air lifting up from below, gets the airplane in the air, and it gains horizontal speed, and gradually the wings start lifting the airplane because of its horizontal speed and because of air flowing over the wings.
And then, the airplane goes into a region where there’s a DOWN draft, air blowing down from above the airplane, but it still can fly because it’s moving forward and the wings still generate lift. Airplanes fly through down-drafts all the time.
On that vehicle, once it gets up to a speed, the propeller cuts through the air fast enough to make each blade function *like a wing*, and whether there’s an “updraft” (wind from behind the vehicle) or “downdraft” (wind from the front of the vehicle), the blades still function like wings and “lift” the vehicle forward.
Hope that makes sense.
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