Sunday, January 2, 2011

Sheet in!

"Beim Funboard fahren ist das Segel immer dicht!" (roughly translates to "The sail is always fully sheeted in when sailing a short board"). Those were the words of wisdom my windsurf instructor Tuple tought me about 6 years ago in a stance lesson. I'm finally starting to understand this now...

Have you ever had a day where you were fighting in really gusty wind, barely planing, while other guys were planing all the time, apparently without being affected by the gusts at all? I just had such a day a few weeks ago, during my first trip to Ninigret. Here's the wind graph from that day:

We sailed from about 1 pm to 3:30 pm. The windmeter readings are probably about 10 mph to low, but it was really gusty. A few windsurfers were out there, struggling as I was, but two others (Bart and Walt) were constantly planing and apparently having a real ball.

To my defense, I have to say that I have seen days where things were reversed - I was out there planing, hardly noticing any gusts, while others were having a hard time, often complaining about how gusty the wind was. What's going on?

On the day in Ninigret, one point of difference was that the guys having fun were out with bigger and faster equipment than those of use who struggled. At least in my case, I had been a bit fooled by the forecast, which had predicted steadily increasing winds - so I rigged small. Most of the time, I was not planing, but in the gusts, even the small sail felt too big. Note that Ninigret is a shallow pond, so the water was very flat.

To understand what was going on, let's first look at the apparent wind at the time when a gust hit. We'll start out with 20 mph wind. I'm not planing, so the board is going about 10 mph; the faster guy is fully planing, going 25 mph. We are both sailing at a 90 degree angle to the true wind. Then, a gust of 30 mph wind hits. Here's a graph of the apparent wind:


For the low guy on the left, the apparent wind suddenly increases from 22 to almost 32 mph - a nearly 50% increase. For the fast guy on the right, the apparent wind increases less, both in absolute and in relative terms: from 32 to 39.5 mph, about 20%.

Ok, that's interesting, it explains why the wind seems less gusty once you are nicely planing. Instead of feeling the 50% increase in the true wind, the planing windsurfer just feels a 20% increase. But this is not the whole story.

To understand the second part, we need to remember that a windsurf sail works basically the same way as the wind on an airplane - as an airfoil (or aerofoil in British English). How much lift (or, for a windsurf sail, drive) an airfoild provides depends, among other things, on the "angle of attack". Easy enough to understand - on a windsurfer, that's how far we sheeted in. When we start sheeting in, we get a bit of pressure, then more as we sheet in more, until at some point we hit the optimal angle; after that, sheeting in further will start to make us go slower. If we plot the lift (or drive) against the angle of attack, we get a curve that will look like this one:

The actual curves for windsurf sails will be a bit different, but the general form will be the same. Note that near the maximum (around 20 degrees in the plot above), small changes in the angle of attack will only lead to small change in lift. However, at smaller angles (when only partially sheeted in), small changes in angle lead to larger changes in lift.

Why does this matter? Well, the fast guys in Ninigret were nicely sheeted in, at an angle near the top of the curve. I, as one of the slow guys, however, was only partially sheeted in, so that gusts would not catapult me. Let's say the fast guys were at sheeted in at 20 degrees, and the slow guys at 10 degrees. Here's a diagram that shows the sail position relative to the apparent wind before and in the gust:

As the gust hits, the apparent wind direction changes, and therefore, the angle of attack changes, even though we did not (yet) move the sail. If we look at the affect of these changes in the angle of attack diagram above, we see that:
  • The slow surfer who was only partially sheeted in now is fully sheeted in, even though he has not adjusted the angle of the sail to the board. The lift coefficient of his sail has increased by about 70%, from 1.0 to 1.7.
  • For the fast surfer, the apparent wind direction has also changed by a similar amount. But since he was near the optimum before, the lift coefficient of his sail has dropped, from 1.7 to 1.4.
The airfoil formulas state that the lift generated by a sail increase proportional to the lift coefficient, multiplied with the square of the wind velocity. Let's plug the numbers in and see how this feels to our surfers:
  • Slow surfer before: 1.0 x 22.4 x 22.4 = 502
  • Slow surfer after: 1.7 x 31.6 x 31.6 = 1697
The wind speed has only increased by 50%, but to the slow surfer who has not yet sheeted out, this feels like a more than 3-fold increase. Shall we say "catapult"?
  • Fast surfer before: 1.7 x 32.0 x 32.0 = 1740
  • Fast surfer after: 1.4 x 39.5 x 39.5 = 2184
For the fast surfer, the lift only increased by 25%! No surprise these guys looked like they were unaffected by gusts and having fun!
--
Obviously, the discussion above is a bit simplified. I'm not taking drag into account at all, and windsurf sails are not fixed wings, either. For example, the fast surfers probably had some additional benefits from the tops of their sails opening up in the gust. Nevertheless, I think that the discussion above does indeed explain observations that have puzzled me for a long time.

What do we learn from this? Rig for the lulls (or, better, get good at pumping yourself onto a plane), and sheet in. It will make gusts disappear, using some good old airfoil engineering magic. It took me about 6 years to understand this - no wonder windsurfers stick to the sport for decades!

2 comments:

  1. Do you have any data for the actual CL for various sails? That would make for some really interesting analysis!

    ReplyDelete