Time for a post about actually
sailing and (even better)
sailing fast. I'll write a few sentences on the topic of
gusts (which many sailors already know) and then move on to talking about
the fan which is something that I kinda observed on my own, and then (when I read about it Frank Bethwaite's
High Performance Sailing) was confirmed and expanded upon. But it's not something I hear folks at the club talking about, and I don't recall reading posts about it on other blogs, so I don't know how widely known
the fan is.
Gusts
|
My naive view of gusts |
We all know how to spot gusts, right? Look for those dark, ripply patches on the water. These patches may be blotchy, they may be in lines or columns. They move along the water. The wind in the gusts is faster than the average wind. If you're on the race course, and see multiple "dots" of fast wind, you'll notice that the boats who win will be sailing from one gust to the next, trying to "connect the dots". Their boat speed is huge compared to those who just point for the mark. Avoid those flat, mirror-like spots on the water -- those are lulls with less-than-average wind.
For a long time, I thought that's all there was to it -- gust = faster version of ambient wind. Eventually, I noticed that the gusts often have a slightly different wind direction than the ambient wind, but that's it.
The Fan
The fan is a term I read in
High Performance Sailing. It describes an area of high pressure, and high wind speeds, around the perimeter of a gust, spreading out in a "fan" shape, especially in front of the gust. The wind in this fan is faster than the wind in the gust itself. In hindsight, I have often noticed that the wind feels very fast as I sail into a gust, but I always assumed that this was a trick of my perception -- it "felt" fast because I was coming in from slower air, but quickly got use to the new speed. It turns out I was wrong -- the wind actually is fastest at the transition.
You'll notice two things about the fan -- fast wind, and changing wind directions. All the wind in the fan is faster than the ambient wind, and much of it is faster than the "core" of the gust. But it's the change in direction that we want to exploit.
Consider two boats approaching a gust on a port tack. The red boat, on the left, heads for the centre of the gust. They are hit with the strong wind of the fan as they enter the gust, experience a header, and actually slow down, until they get to the normalized wind direction (where they sail faster than someone who didn't hit the gust at all).
The green boat, on the other hand, aims to skirt the gust, and ride the fan. The boat experiences a lift and can blast by the fleet (including the red boat).
The next time you're on the water, and see an approaching gust, try to feel out how big the fan is, and how much the wind changes direction in it. Feel the difference in the speed. Compare it with the wind in the centre of the gust (which is still better than the ambient wind).
One final note about the fan. The diagram shows using the fan when going upwind. Now imagine what happens when you go downwind. A boat like the Laser goes about half wind speed on a run, and much closer to wind speed on a broad reach, right? So, someone who hits a 10 knot gust on a run will be going about 5 knots (faster than the 4 knots the rest of the fleet is going). But, the boat who is right beside the gust, and trims his sail for a reach can ride the 12 knot wind of the fan at a speed closer to 9 knots -- passing the guy in the centre of the gust and waving as he does so!