Apparent wind speed — the wind you actually experience — is the real wind speed, plus or minus your speed against or away from it. So if I paddle my kayak at 3 knots* into a 9 knot headwind, I feel the resistance of a 12 knot wind working against me. “That’s OK”, I used to think. “I’m only paddling into it until I stop for lunch. Then I’ll be heading back, and I’ll get as much push home as I had drag on the way out.”
Sadly, it ain’t so. Once I turn downwind, the apparent wind is reduced to 9 knots minus my 3 knot paddling speed, or a mere 6 knots. So the wind pushing me home is only moving half as fast as the wind I fought against while outbound. Even worse, the wind’s resistance — or assistance — increases exponentially with its speed. So that 6 knot tailwind works for me with only a quarter of the power that the 12 knot headwind worked against me. The only way to offset this decrease in wind force is to harness a lot more of it. Time to hoist the sail.
*Knots are a measure of speed used for boats. Substitute MPH or KPH if you want — all the math will work the same.
Once the sail is up, I trim it, turning it to the optimum angle for the wind direction. Fluttering fabric is ironed into a taut curve as smooth airflow displaces turbulence. With the wind streaming steadily across the front and back of the sail, it both pushes and pulls me on my way. In a light breeze, I set a second sail and listen to the soft gurgle of water parting under the bow. In a strong wind, a single sail speeds the kayak as if the rudder pedals beneath my feet were accelerators. The boat cleaves through each successive wave or races up the fronts and falls with a splash onto the far faces.
For physicists, air is a fluid. Which means that in a very real sense, we live at the bottom of an ocean of air, earth’s atmosphere. What we usually think of as the ocean is actually the lower and denser of two layers of fluid that girdle the globe. So when I hoist my sail and seize the wind, I am riding the currents of an upper sea and dancing on the interface of two vast oceans.