Apparent Wind and Velocity
Now that all chores of Opening Day are behind us it’s me to get back on the water. Here is a great question from a couple months back.
“Hey Krabby remember the dumb question I asked you last month?” asked one of my regular Sally crew.
“Sure I do, the only dumb questions are the ones that don’t get asked”, I replied.
“Well, here’s part two,” they said... I have been sailing for years and I understand the basic theory behind what’s going when there is a lift or a header, but it seems like there is more to it than the wind going forward or aft. Why does the wind on the sails change direction when the course of the boat remains constant and the wind direction is constant? I have also noticed that sometimes the boat changes course and the angle of the wind on the sails does not change and the wind direction stays the same. Can you explain this in short and simple way?
Definitely not a dumb one either. Maybe I am smart enough to be able to answer it. If I succeed I will surely deserve a “long swig at the halyards”.
To start out with, what you are describing are the dynamics of sailing a boat that make it interesting and challenging at the same me. If the wind blew constantly from the same direction and speed all the time then you would just trim them to your desired course, sit back and drive. This is rarely the case with the wind and even if you sail a constant course there is current that varies your actual course over the bottom of the sea.
The short answer to your question is that you are experiencing the effects of apparent wind direction and velocity on the boat. Understanding the concept of apparent wind is simple and complex. Let’s start simple: If the wind is blowing 15 knots and you are sailing dead downwind at 7 knots the wind you feel on the boat is blowing by you at 8 knots. That is your apparent wind speed because you are moving in the exact same direction as the wind. At 7 knots the wind you feel at your back is only 8 knots, not the 15 knots of true wind you would feel if you suddenly ran aground and stopped. The true wind angle and apparent wind are the same because you are running with the wind on the same course and the wind speed is greater than your boat speed. As you change course the apparent wind speed and direction changes. As you might expect the closer you turn your boat toward the wind, the greater the apparent wind speed. This also changes the apparent wind angle because influence of your boat speed relative to the angle you are sailing to the wind. Now this is where it gets tricky because as you turn towards the wind, the wind speed you actually feel on your boat (apparent wind) increases as your boat goes faster and then, you guessed it, the apparent wind speed and direction changes.
If I haven’t lost you so far, imagine what happens if there is a subtle change in true wind speed or direction. You guessed it again, and as the wind changes that’s what we feel and see on the boat; we feel a lift or header. If the wind gets light suddenly and we are sailing a heavy boat, like Sally, the wind feels like it has moved forward or headed us, because the wind slowed and we did not because our momentum kept us going for a moment. As we start to slow down then the relative wind changes again. If you watch carefully the next me you are out you will notice the subtle changes in direction and speed of the true wind and how it changes the apparent wind speed and direction on the boat.
Keep the “dumb question” coming...they keep me on my toes until that drink comes.