Wind, Course and Sail: A First Look
Sailing - matching sail and course to wind - is surprisingly easy.
[NOTE: For this article I am conflating course with heading, for simplicity's sake. For now, be advised that I am using it 'improperly', and that there's an important difference between the two.]
What I'm going to do here is skip all the jargon and terms that tend to make it seem complicated. Words are useful, and a few are introduced, here, but don't get hung up on them... they'll fill in over time. What we want is a framework to plug them into. Once you have the basics, it's much easier to see how all that fits in. So for now, let's look at the big picture.
Let's take a look at the above illustration. Picture your vessel able to spin like a dial at the center (heck, print this out and make one out of paper!). There are a few ways to divide this circle up, all of which are variously useful.
The line perpendicular to the wind divides our courses into on and off the wind.
The labels divide them into Irons, Reaches and Run.
But for simplicity, lets ignore lines and labels and focus on color.
Red is no-go; yellow is proceed with caution; green is go-go.
Red means you can't get there, directly. A rock in that quadrant is no danger (barring strong current)... unless you take action, it may as well be on the far side of the moon. To run on to it, or reach any other point in red, you MUST zig-zag, sailing in yellow.
If you set your course into red, no one will punish you, but your sails will flutter and flog, and your boat cease to move forward.
Yellow is sailing on the wind (aka into or toward the wind or to windward). Sails are hauled toward the centerline (how is a detail for another time).
This is the region you must sail in order to move the boat toward the general direction of the wind. The boat heels (leans over) and develops leeway (side-slips away from the wind) in this quadrant. If the wind blows hard enough, you will find yourself beating (smashing through waves).
Because of leeway, you're never going where the boat is pointed but always at an angle downwind of that. Thus, dangers on your downwind side deserve attention and avoidance. Ergo, caution.
Green is sailing off the wind (aka across or down the wind). Sails are eased out and away from the centerline, usually toward a maximum of 90deg.
Life is good in this hemisphere! Lots of power, even if your sails aren't optimally trimmed. Lots of choices in course - all of green, and yellow if necessary - good for approaching a buoy, say, or running a reef.
The only green concern is a jibe (read up on it, elsewhere in your further studies).
Now that we have this basic understanding, we'll venture a step into the 'how'. Let's focus on our labels, now...
Reaches require matching the angle of the sail to the wind and course. In doing so, we trim sail.
If the sail's out too far, it starts to flutter and won't generate power. If it's in to far, the wind blows flat into it... the boat heels more than it should (wasting power) and the sail stalls (losing power). Like Goldilocks, juuuuuust right is right. We have two methods.
To trim a sail, hold your course. Ease the sail out until it luffs (forward edge starts to flutter). Haul it back in, slightly. There is a sweet spot where you can learn to feel the power kick in. Practice makes perfect!
To sail full and by (sails full and by the wind's whim), trim sail as before. Whenever the wind shifts, alter your course - using the same indicators - until the sails hit the sweet spot.
Irons is still no go... you can't trim a sail there. Done.
Run has the sails out at about 90deg, so no further trimming there. Done.
With only this much info, you can sail most any small boat in wind from any direction. It's that simple! Early sailors had no clue about the Bernoulli Effect, say, or vector physics. Not needed to make the boat go.
This is not to say that continuing our education - a lifelong pleasure - won't increase our understanding and efficiency, and our abilities, options and safety. It will. Learning the language of sail, understanding the mechanics of sail and hull; your rigging, sail controls and the physics involved; the effect of currents and weather; and countless further adventures will endlessly enrich your seamanship.
But it's all building on simplicity!