Physics of Sailing Video 2: Drive in Sails

This video explains the physics of how the wind drives your sailboat forward. This understanding is essential to understand the basic principles of sail trim that will generate the most efficient driving force from your sails. This lesson introduces Newton's Third Law of Motion, and explains how this is the physics behind lift in aircraft, and drive in sails.
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[@humlakullen]

Awesome video, thanks! However; a curved airfoil DO create a low pressure (suction/lift) on the curved side as well. Just hold the curved side of a spoon under running water from a faucet, and you’ll see. Better yet, next time hold that piece of paper stretched out in front of your mouth and blow on top of it. Same result.

[@chrisp8856]

I’ve been trying to find these exact videos for a week now. I can’t believe it took this much time since I was searching the terms “physics” and “basics of sailing”. Anyway, they’re perfect, and I understand exponentially more now that I’ve watched a few of them. I don’t have PayPal, but I could use Zelle! Thank you!

[@JG27Korny]

I think that Bernoulli effect plays a significant role especially when the sail is well trimmed. And the carbon based foils are specially designed to harness the Bernoulli effect in particular.

[@mstaufik3254]

Terima kasih informasinya dan arahan yg sangat bermanfaat untuk belajar.

[@kenfry2664]

Granted, you are trying to simplify here, but the incidence angle is the angle of the wing to the aircraft. The angle of attack is the angle to the apparent wind. So, for example, an aircraft with an incidence of two degrees must be pitched up 8 degrees to achieve an angle of attack of 10 degrees. Lift is perpendicular to the angle of attack, by definition.

[@kenfry2664]

Sails are not "flat sheets". They are curved, and are crude airfoils. Windsurfers often use RAF (rigid air foil) sails. Hang gliders use single surface wings... which are also airfoils.

[@bp-cb2gd]

Owen, thank you. I too was skeptical about the Bernoulli principle on a sail (or on a flat hand held at an angle out the window); intuitively and physically it didn't feel right. I'm going with your explanation re vector forces. thanks!

[@daviddoherty4429]

Think I'll go with the NASA engineer. You gave a good explanation of a boat which moves sideways. And a sail is not flat when full of air it looks remarkably like a wing shape when full of air.

[@shainowen2520]

Ty

[@cadenorris4009]

One question. This doesn't explain how the drive force in sails equates to a forward motion. The drive force is mostly oriented sideways, not along the longitudinal axis of the boat. So while there IS a forward component, it is usually not the major component. How does the shape of the boat, and the implementation of a keel factor in to make your boat go forward, instead of sideways? In aviation, a similar phenomenon takes place, the reaction force isn't oriented perfectly perpendicular to the wing, it is also oriented back. This is induced drag, drag created because of the production of lift. What is that sailing equivalent?

[@keithlittlebury2986]

Fascinating videos thank you. This leads me to ponder the role of the keel ?If the wind vector pushes the boat sideways how does the boat move forwards?

[@BasicCruisingwithOwen]

The wind can be divided into 2 components, a sideways vector plus a forward vector. The keel only inhibits the sideways force component vector. There is still a forward component that pushes the boat forward. Think of a bar of soap sliding sideways at a 45 degree angle towards the side of the bathtub. When it hits the side of the tub, all sideways motion is stopped, and the bar only slides forward. The keel doesn't perfectly stop all sideways motion however, there is still some sideways drift called leeway.

[@brettharman8921]

owen, thank you for clearly explaining angle of attack and newtons principles as the main lift component of a sail. it is disappointing to see and hear 90% of both sailors and pilots consider Bernoulli's principle to be the "main" lift component.

[@rcq2f]

I was about to comment on that, I have saw other videos on the subject and they try to argue that the lift force is due to bernoulli/venturi/magnus effect, whereas brilliantly explained in this video if that were true then it wouldn't be possible for planes to flight upside down. The true lift force comes simply by the change in the momentum of the wind as it passes through the wing/sail, old Newton's second law. Thanks for the great video.

[@HaibaneKuu]

I don't think those are actually contradictory to be honest. Bernouli's principle essentially explains the physics of airflow, while Newton's principle is a more broad look at the system as a whole. Newton's principle alone does not really explain airflow separation and stall conditions - stalled wing will still deflect the airflow downwards after all, and same with contaminated upper wing surface (such as icing conditions), and yet those conditions have severe effects on lift and drag. Now regarding planes flying upside-down, please remember that airfoils are producing lift at a certain angle of attack, and for asymmetrical airfoils, you'd require higher angle of attack to produce the same lift on the "wrong side", and it will produce more drag in those conditions. It doesn't disprove the Bernouli principle, it just disprove the idea of "air molecules have to travel further on the top side to meet up with the molecules at the bottom" - which is in fact incorrect. In both normal and inverted flight, airflow will be faster on the upper side of the wing.