How Your Sailboat sails in apparent wind.
When you’re out sailing, you’ll encounter three different types of wind. Let’s start by looking at the first scenario.
Imagine you’re on the water on a perfectly calm day—there’s no wind at all—and you’re simply motoring along. As your boat moves through the water, it generates what’s called boat wind. This is the wind that’s created by the motion of the boat itself. It always comes from straight ahead, directly along the centerline, and it’s the breeze you feel on your face when you’re underway under power.
Now let’s introduce the second type of wind: true wind. This is the actual wind that exists in the environment, independent of your boat’s movement. You can observe true wind by looking at flags fluttering, ripples and waves on the water, or how anchored boats swing.
Once you raise your sails, a third type of wind comes into play—apparent wind. Apparent wind is the combination of true wind and boat wind. It’s the wind your sails “feel” and the one that actually drives your boat under sail.
Apparent wind lies somewhere between the direction of the boat wind and the true wind. As a mainsail trimmer, your job is to trim the sail so that it aligns with the apparent wind. When trimmed correctly, the apparent wind will flow smoothly across the sail, generating lift and propelling the boat forward efficiently.
For example, if you’re sailing with the true wind coming from the side (a beam reach), the apparent wind will still fall between the true wind and boat wind. To keep the sail trimmed correctly, you’ll ease the mainsheet so the sail is angled with the apparent wind. This ensures that the wind flows cleanly across both sides of the sail.
As the true wind shifts further aft—say, between the beam and the stern—you’re on what’s called a broad reach. In this case, boat wind and true wind begin to cancel each other out slightly, which makes the apparent wind feel lighter. The apparent wind still lies between the two, but now it’s doing more pushing than lifting. The airflow is no longer flowing smoothly across the sail’s surface; instead, the wind is pushing more directly on the sail.
Finally, if you’re sailing directly downwind, known as a run, the true wind is coming straight from behind. In this case, apparent wind and true wind align, and because boat wind opposes the true wind, the resulting apparent wind is quite light. There’s very little lift, and your sails are simply being pushed by the wind rather than generating aerodynamic drive.
