system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do sailboats / ancient ships steer if the wind isn't blowing in the direction they want to go?

Forgive me if my terminology is off. But I've always wondered how long voyages were made given the unpredictable pattern of winds. How are ships able to go where they want if the wind is, say, blowing in the wrong direction?
gpt
A sail can catch wind and push the boat forward from just about any angle greater than 70 degrees from directly in to the wind.  A sail boat has a big fin on the bottom that keeps it from being pushed sideways, and the sail is at an angle so the wind is redirected, which all together pushes the boat forward.  So as long as the boat isn't going directly into the wind, it can often just sail straight forwards in the direction it wants to go.

If the boat wants to travel in to the wind, then what the boat does is it picks up speed going across the wind, and then turns into the wind.  While the boat is facing in to the wind, the sail is not catching the wind, so there is much less force trying to slow the boat down as it turns.  AS the boat loses speed, it will turn far enough that it is going sideways to the wind again, but in the other direction- and once it picks up enough speed again, it will turn back in to the wind again, which creates a zig-zag pattern that slowly moves them in the direction they want to go.  This is known as Tacking in to the wind.  Depending on how close to directly in to the wind they want to go, they could just go in mostly the right direction, hoping that the wind will shift in their favor and then tack in to the wind to correct their position once in a while if the wind doesn't change.