Winter break

This is the second time this winter that we have a foot of new snow on the ground - time to take a winter break. Well, at least for sailing around here - we have a one-week trip to Bonaire with an ABK camp coming up soon :)

I bet that when we return, we'll be too spoiled to go out in freezing temperatures and snow, so there will be a few weeks without windsurfing afterwards. But when the withdrawal symptoms get unbearable at the beginning of March, days will be longer, and the weather should get warmer, so we'll be back on the water then.

The secret to warm hands

After 4 weeks without a good windsurf session, we just had to go sailing today - who cares that the temperatures were a bit below freezing? They were supposed to go above the freezing point, and it was sunny and windy.

We planned to sail at Skaket Beach in Orleans, a great spot for NW winds. When we got there, the wind was indeed great - and so were the waves. There were not very big, maybe chest to head high, but they were breaking everywhere. Not really surprising - Skaket does not have single break point like a reef, but instead lots of little sand banks, and the drop off is pretty slow. We arrived near high tide, and the wind averages were probably in the low thirties - two things we had not seen in Skaket before. As flat water sailors, we were a bit intimidated. It did not help that the wind was straight onshore; who knows if we would have made it out if we had tried. But the final nail in the coffin were large chunks of wood and trees in the shore break. I saw only a few, but I had no desire to run into one of these babies on the water.

So we decided to drive up towards Wellfleet to explore other options. The waves at First Encounter Beach looked a bit more managable, and the wind was side-on, but there was even more stuff floating in the water. We checked out a few ponds and inlets, but the road to the spot that look most promising on the maps was completely under water. You could see where the road was, because there was a bridge in the middle of the bay - rather funny, now why did we not take a picture again? Oh yes, we had windsurfing on our mind, and severe withdrawal symptoms after 4 weeks without.

We ended up at Indian Neck Beach in Wellfleet Harbor. Here's a picture of where we sailed:


The wind there was side-on, and lots of white caps could be seen a bit further out. However, the water was pretty flat, since the wind had only a mile or so over the water to build up chop. So we decided to go out. In view of the wind shadow near shore, we took the big boards and somewhat bigger sails - a 4.2 for Nina, and the 6.2 for me. As expected, the wind near shore was weak and gusty, but it cleaned up and got stronger after a few hundred feet. It actually got a bit too strong for my 6.2, but I kind-of managed.

After maybe 15 minutes on the water, my finger tips started to hurt from the cold. I was wearing neoprene gloves with the inside of the fingers cut out for a better grip, and mitt shells on top for warmth. But since I spend quite a bit of time in the water practicing my waterstarts, water temperature was maybe 39F (4C), and the wind chill was somewhere around 15F (-10C), that was not warm enough. So I went bag in and put re-usable hand warmers into my mitts, after activating them by twisting the little metal plates on the inside.

That worked beautifully! My hands were nice and toasty for the rest of the short session (maybe another 30 minutes), without ever having to stop and shake the blood back into my fingers. The heating pads were still nice and warm at the end of the session, and probably would have worked for another half hour.

Nina also used the re-usable hand warmers inside here Dakine mittens. She had complained about cold fingers in a variety of different gloves and mittens she had tried before, but today, she was toasty and warm the entire session. Needless to say, she looked great as usual while sailing, and did not get as much waterstart practice as I got because she just did not fall in when tacking. Oh well, as long as she'd having fun :)

Today's session was the coldest windsurf session ever for both of us, but we stayed nice & warm and had fun. Ok, I did get a bit cold at the start, but once I figured out that the beginning of January is not a great time for being in the water under the sail (at least here in the Northeast), and after I used the hand warmers, things got a lot better. We sailed cautiously because of this was a new spot for us, and because of the cold; and then, we had to cut the session short because Nina had an important appointment in the afternoon. But nevertheless, this was a great day of sailing.

For all of you who might be interested in trying re-usable hand warmers, here's a bit more information. We used the HotSnapZ reusable handwarmers, purchased from Amazon.com. They cost $20 for 3 pairs, and can be re-used many times. They contain a over-saturated solution of sodium acetate and a little metal plate. When you bend the metal plate, the sodium acetate starts to crystallize, and the bag gets really warm. It stays warm for about 30-60 minutes, depending on the size (we used the 4-inch red round bags). They can be re-used after putting them in boiling water for 10 minutes, which dissolved the crystals again. This process can be repeated hundreds of times if you are careful enough. And if the bags should burst, not really a problem - the stuff in them is food grade (not that I suggest that you eat it!). You may see some reviews where people complain that re-heating the bags was too much trouble - but that was probably a couch potatoe or maybe a kite surfer :-}
The one thing that's less than perfect about the re-usable hand warmers is that they get hard when the sodium acetate crystallizes. Here, the HotSnapZ warmers are better than a cheaper brand I have tried - they keep some flexibility for quite a while, and they also seem to be generating heat a bit longer.
Before trying the re-usable hand warmers, Nina first thought about using the cheap, non-reusable hand warmers. However, these have several problems. The non-reusable hand warmers are iron-based, and require oxygen to work (basically, you're just converting iron to rust). So you could not put the reusable hand warmers in a ziplock bag to keep the water out. But if you put them in water, they don't work, either. Even if you would figure out how to keep them dry and in air contact, you'd still have to worry about the bag breaking. If that happens, more oxygen can reach the iron, and the temperature can get really high really quick. That's not an issue at all with the reusable hand warmers.

So, if you have a dry suit or winter wetsuit, good booties, and a hood, but cold fingers kept you from windsurfing (or from windsurfing longer), get some mittens or mitt shells, put some re-usable hand warmers in, and go have fun on the water!

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!

Dense air and pumping

After almost a month without a nice windsurf session, thinking about windsurfing is taking over, as it tends to do in the winter. Browsing through web sites and forums, I learned a few things today:

1. Winter windsurfing requires less wind.
2. Sail size x wind speed = constant, even though the lift a sail generates increases with the square of the wind speed.
3. Getting good at pumping onto a plane is a worthwhile goal (even though it breaks the validity of the statement 2. above).

Winter windsurfing requires less wind
I had a couple of sessions early December where I noticed that I was comfortably planing, even though the wind was a bit too low for the sail size I used. In a couple of other sessions, the sail I used seemed a bit big, even though the wind readings were well within the range that I usually used the sails.
The common thread in all these sessions was that the air temperature was just above freezing. So when I saw this thread about temperature and sail size, things started to click. I used the air density calculator at http://wahiduddin.net/calc/calc_da.htm and discovered that the air at 0C (32F ) is about 10% denser than at 25C (77F). Therefore, instead of needing 19 or 20 mph to get going, I need only 17 or 18 mph when it's cold enough. Cool!

Sail size x wind speed = constant
In the discussions I mentioned above, some people pointed out that the lift generated by a sail increases with the square of the wind speed. I had heard this before, and it's easy enough to verify that this is the general rule. Nevertheless, it puzzled me, because it did not match my experience. I can use an 8.5 m sail in 15 knots to get planing. If the wind picks up to 30 knots, I'll probably be on a 4.2 m sail - about 1/2 of the size in 2 x the wind. The two sails seem to have about the same power in these conditions - enough for me to be nicely powered and in the harness. If I use Jim Douglass' sail size calculator spreadsheet, I get the same result - half the sail for twice as much wind. But the lift formula suggests that the smaller sail should have twice as much lift as the larger sail in twice the wind - what gives? I don't think the calculator is wrong, since the same relation of sail sizes to wind seems to apply quite generally. The only exceptions I can think of are very good windsurfers - and they tend to be even further apart from the lift formula suggestion when they use small sails in marginal conditions for tricks, or larger sails in really windy conditions for speed.
So for fun, I posted the issue on the iWindsurf forum. I got quite a range of answers, from "real life is different from models" to "there are many other factors". I particularly liked the "many other factors" response - it's used very often in response to gear questions, sometimes with the conclusion "this question cannot really be answered".
However, one response pointed into the right direction: to calculate lift, we need to look at apparent wind, not at true wind. Since I always sail with my GPS, I know that my average speed is typically about the same in 15 knot winds and in 30 knot winds - maybe around 22 knots. That may be surprising, and it certainly does not feel this way, but the GPS does not lie. In 15 knots, the water is flat, and it easy to go faster than the wind. In 30 knots, I'll be on a wave board with a lot of control, and my primary focus will be to keep things in control. More often than not this year, I was actually slower in 30-knot winds than in 15-knot winds.
But if we assume a board speed of 22 knots both times and plug the numbers into the "Apparent-Wind Calculator", we get apparent wind speeds of 26.6 knots in 15 knot winds, and 37.2 knots in 30 knot winds. So the apparent wind is only 40% stronger when the true wind is twice as strong! Now, the results of the lift formula fit the observations - 37.2 squared is 1383, about twice as much as 26.6 squared (708). To get constant lift from the sail when the wind increases from 15 knots to 30 knots, the sail size needs to drop by a factor of 2.

Pumping
The reflections about apparent wind at different wind strength led me to think about pumping (well, and links to pumping videos on the forum sites may have contributed). It has perplexed me for some years that really good windsurfers often get going on much smaller sails than "regular" windsurfers. Many trick surfers won't touch anything larger than a 5.x sail, but they plane just fine when I still need a 7.0 to get going, even if they are about my weight. I have gotten a bit better at being efficient in 2010, but the one thing these guys usually can do a lot better is pumping. I'm not talking about endless race-like pumping - I'm talking about a few quick pumps, and then they are off, fully planing.
I do not want to look into how to pump here. There are many different ways of pumping; Andy Brandt showed us about 7 of them at the last Hatteras camp. Instead, I want to look at what pumping does, and why it is possible to stay planing after getting onto a plane with a few good pumps. I'll be looking at the apparent wind again here.

First, let's look at the "lazy" way to get going: take a big sail, and wait for a gust that's strong enough. On a 7.0, I need perhaps 18 mph to get onto a plane without pumping. Let's say I schlog around at 16 mph winds when an 18 mph gust hits. I'll be going maybe 9 mph before, and accelerate to 10 mph when the board starts to get onto the plane. Once it starts coming out, it quickly accelerates to perhaps 18 mph, and then stays at that speed (for simplicity, we'll assume everything happens at a right angle to the wind).

So, at the transition point, my board speed is 10 mph, and the wind is 18 mph. This gives and apparent wind of 20.6 mph. Now let's assume I have learned how to pump, and want to get onto a plane in 15 mph winds. I pump a few times and get my board speed up to 15 mph - now the apparent wind is 21.2 mph, above the 20.6 mph threshold we calculated above. So the board releases and accelerates a bit more - maybe up to 17 mph, giving me an apparent wind of 22.7 mph. That's still clearly above the planing threshold, so the board should remain on a plane when I stop pumping. I've seen this happen, but I don't think I really understood why. Now that I do, hopefully I'll be motivated enough to do it more, and get better at it.

A funny consequence of better pumping is that it invalidates the " Sail size x wind speed = constant" rule above, since it enables you to use a smaller sail in light winds. This makes actual sail sizes deviate even further from the "velocity squared" rule. On the extreme end, there is the one-sail-size quiver that I have seen some great windsurfers use. However, in addition to being efficient and great at sailing overpowered, they usually lived at places with frequent and relatively constant wind.