Saturday, January 1, 2011

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 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.

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.


  1. Two subtle but important technique tweaks that increase efficiency and are important to pumping:

    1. Weight distribution between front and back foot. Efficient sailors rock slightly forward in lulls to maintain board trim and keep up speed...this is also crucial in pumping (in which you're really pumping the board, not the sail.). If you keep your weight static over the board you'll need 2-3 more mph to get planing.

    2. No matter how flat the water there is swell that can help or hinder you. Pump down hill if possible.

  2. I just came across your blog and found your explanation about the link between sail size and wind speed.

    Are you still convinced it is right?

    It get's me to think about the fin sizes. I always tought that I needed a smaller fin with a smaller sail because I was going faster. My weight does't change, so the force I apply is the same. But again, the apparent wind is different and I think in more wind, the lift of the sail will directed more forward, and less sideways, requiring less fin lift to counteract it.

  3. Hi Sailboarder,

    Yes, I still think the link between sail size and wind speed is correct: sail size * wind speed = constant.

    I think you got a good point about the fin size and apparent wind. In light winds, we need the big fins to convert the sideway pressure into lift. In higher winds, very good slalom or speed sailors will go to a smaller fin since they are going faster, and the fin lift will be proportional to the square of the speed. Regular windsurfers, however, will replace most of the lateral pressure on the fin with forward pressure from the more open sail. The high-wind boards will also be narrower in the back, so fins have to be smaller due to the lower leverage we have. But fin sizes are not as straightforward as sail sizes - I think it's easier to use the same size fin in 20 and 30 mph winds than to use the same size sail. Not a surprise if the board speed is about the same.

  4. I agree with you that you often don't need to reduce fin size with sail size since the increase in chop slows you down. However, if your speed increases too, you might get overfinned. To me, tailwalking is more difficult to handle than being overpowered.

    I have large gaps between my most used sails (6.0-7.5-9.0) and my current line of thought is that I need one fin for each sail that will fit a board, within the boards limits. On my Kona One, with 49 cm OFO, a 46 fits both the 7.5 and 9.0. A 40 fits the 7.5 and 6.0 completed by a Onshore wave 28 to fit the 6.0 in suitable conditions. I also think a powerfull 48 would get me planing earlier with the 9.0

  5. Sailboarder, I agree that with fins the size you are using, you need to downsize when the wind picks up. I am not sure how much of that is from speed, though. As wind increases, the sideways pressure in the big sails will go up, so you push the fin harder sideways. It will push back, and with the high leverage the larger fins have, will at some point try to throw you off.

    That said, tail walking often has a different cause: opening up the sail in gusts. This reduces mast foot pressure and puts more weight on the tail of the board, leading to tail walks. The best local speedsurfers can hold a much larger sail and fin than I can in most conditions, without tail walk problems. They are also much better at staying "pedal to the metal" - fully sheeted in and all weight in the harness.