The only way I can think that this would be possible is if the wind was blowing directly onshore?
Edit: thanks for all the well thought out replies. I think I’ve got it figured out know, I’ve always Been a bit confused about ships tacking into to wind and I think the key thing I was missing was the sail being using like as an airfoil creating pull, used in tandem with with the fins/keel .
In: 341
As others have said, the sail acts like an airfoil or wing that interacts with the “apparent wind” (true wind plus relative motion of the boat) to create a pressure differential between the two surfaces of the sail. Which generates a force.
The keel on a sailboat or sailboard is absolutely essential, as it *also* acts like a wing down in the water, which works to mostly “cancel” the force that would push the boat sideways as well as a righting moment to keep it from just tipping over, so that only the forward force remains to push the boat forward.
Remove the keel, and the boat/board will just blow mostly downwind!
But with the keel and the forward momentum of the boat, the forces on the sail allow you to sail across the wind, and even at an upwind angle. Some well designed racing boats can sail as close to 20-25 degrees off of directly upwind!
The “apparent” wind speed/direction due to the motion of the boat allowing it to sail perpendicular to the wind at faster than the “true” wind speed sort of seems like it should violate physics, but there *is* a mathematical solution that shows momentum and energy are still conserved. The key is to realize you are balancing wind forces, not wind speeds.
It is however, a surprisingly devious and non-intuitive physics problem! It starts to make sense when you realize an airplane operates on the same principle, generating lift by interacting with the “wind” from its own forward motion, even in dead calm weather.
To give you an idea of the complexity, my college physics class normally had a problem set each week with 5-10 problems to solve, but one week during the term, we were instead given a single physics problem – the infamous “sailboat problem” to solve, open collaboration with other students in the class, and It was *by far* the hardest problem “set” of the entire year, at least until we started into quantum physics!
Here’s a decent explanation if you want to get into the weeds.
https://www.real-world-physics-problems.com/physics-of-sailing.html
Also, if you want your mind blown by another nautical aerodynamics problem, look up “Magnus Sail” and the “Magnus effect”
Latest Answers