– How does a sailboat travel faster than the wind?

Question

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

Answer 1

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.