Wednesday, September 2, 2015

Push Tack

The Push Tack is a pretty cool tack:


I can do Push Tacks in light wind. Nina and Marty can do them in high wind, too. So the planing Push Tack is on my ketchup to-do list. Andy Brandt says it's a great tack to do when sailing into a lull. With many marginal days recently, I had plenty of lulls to push tack into. Well, maybe I should say push fall into. Since I am so good at finding ways to mess up the push tack, I made a little step-by-step video. Enjoy!

Friday, August 28, 2015

Sarah-Quita's movie


Sarah-Quita Offringa is making a windsurf movie. That's great - good windsurf movies are rare, and Sarah-Quita is perhaps the best female windsurfer in the world. Nobody has beaten her in freestyle competition since 2008. She's collected 9 world championships, including one in slalom. There's a good chance she'll add a 10th one this year, again in slalom. She is 23 years "old". And she has been knighted. Monty Python would be jealous!

So yes, I want to see her movie. As soon as possible. She's making it easy: on the Indiegogo campaign where she is raising funds for the movie, one perk gets you a download link as soon as the movie premiers. All you need is contribute 15 Euro (and select the perk). Cool! I'd have done it, but Nina beat me to it. You want to help her and get the movie, too? Hurry up and donate! Only 6 days left in the campaign!

Here's a Kuma movie featuring Sarah-Quita at the recent PWA freestyle event in Fuerteventura:

Thursday, August 27, 2015

Capers have more fun

Capers have more fun. It's true. Or is it Cape Codders? Well, anyway, I mean windsurfers living on Cape Cod. The last two days proved it. The wind forecast was dismal. The computers thought we might get 10 mph, maybe 12. Computers are stupid!

We got lovely sea breezes instead. Yesterday was the appetizer. Wind meters in Buzzards Bay showed mid-20s! Kalmus got up to 20. We expected the stronger wind from Buzzards Bay to come over, but it never did. It actually dropped a bit, to 18. But the Kalmus meter reads a bit low in SW, so we probably had 20. That would explain why Nina had fun on her 4.7 the entire afternoon.

Today was the main dish. Computer predictions were even lower than yesterday. But we already discussed the intelligence of computers. This is what we got:
Yes, that's 5 solid hours of wind. We started around 3, so we rigged big - 5.0 for Nina, 6.5 for me. Very nice at the start. Very well powered later. It was sunny. Air and water were around 75ºF (24ºC). Can you spell perfect? Purrfect?

I did not sail that much. It come out to about 110 km between yesterday and today. But that's ok. I tried to imitate Nina, and fooled around with things that made me fall. A lot. "Try" is the right word, too. Nina has the Duck Tack nailed now - light wind, fully powered, flat, chop - it does not matter much. She'll get more than 50%. She also gets the duck while planing switch in both straps a lot, although the usually jibes out. She needs some ABK help here. But pretty soon, she'll have the hardest part of the Funnel and Switch Kono. She also got another Shove It. I did not see it, but I saw some tries that she did not count, and they looked good.

When I said I tried to copy her, you did understand that I meant the falling part only, right? The stuff I am working on is easier. I tried a few Push Tacks, but I seem to be going backwards there, so I'll hold off a couple of weeks until someone can tell me what to change. I'm still working on 360s in the straps. I get about one per session (if I'm lucky), out of many tries. But I am not worried. Nina was at this stage with her Duck Tacks a few months ago. There is hope. Just got to keep trying. I feel I am getting close to exhausting all the things that I can do wrong. Sooner or later, I'll have to do the right thing on a regular basis.

One of the wrong things in a Carve 360 is fun: when you almost have it, but right at the end, the nose of the board goes way up in the air, and the board flips over. I'm good at that! Today, I managed to keep the sail above water while this happened, and water-started out of it clew first. Looked like the what the board does in a jump jibe, anyway, so I might as well use the jump jibe ending! I'll call it a wet carve 540. Or maybe a Carve 360 Jump Jibe? It's more of a Carve 360 fall jibe, but jump jibe sounds better.

Falling is funny. I'm good at doing it when I'm not supposed to, but I can't do it when I am. Seeing Nina's Shove It tries, I tried a few myself. Seeing the "Step-by-step tutorial" video before only encouraged me - I can do silly little things like backwinding the sail while sailing, and think I'm working on a cool trick. But hey, maybe it works! I can even carve and then backwind a little, and if a wave just happens to roll under my board at the right time, I might even get a bit of air. Freestylers told me the trick here: use a short fin! So much easier to get it out of the water! But when you are in the air, you are supposed to fall onto your sail. That looks really cool. It also looks like fun. I should be able to do this - I fall onto my sail all the time! But when I think "Shove It", I can't fall anymore. I can come to a complete stop. Maybe that's good? Probably. I get lots of practice starting to plane again, with both feet still in the straps. That's bound to be a useful skill, right?

Lest someone starts thinking about me as a freestyler, I must post my GPS tracks from today:
Sure looks like good old Back-And-Forth sailing to me! Indeed, lawn mowing was desperately needed today - very shortly after low tide, the small chop turned into sizable voodoo chop, coming at you from all direction at once. Kalmus at its finest! Even the kiter who joined us commented that it was "quite rough" today.

Well, I just wanted to tell you why I think Caper (or Cape Codders) have more fun. Yesterday, I counted 7 windsurfers on the water: the 4 usual locals from Centerville, ranging in age from the 30s to the 70s; one "permanent vacationer" who visits every summer; and two tourist from Quebec (for all lurking trolls: guess where they rigged!). Today, I saw 5 windsurfers out: the same 4 locals, plus a brave soul struggling a bit on old gear. Neither day did we see any of the usual suspects from Boston with a non-existing or "flexible" work schedule. So: Capers have more fun. Just don't call me a Caper to my face!

Sunday, August 23, 2015

Maximum Speed

My apologies. If the title of this post attracted you, you've been lured here under what you may consider "false pretenses". Yes, this post deals with speed while windsurfing. But it's mostly a geeky article that looks at some of the physics underlying windsurfing. So you may want to stop reading now.

But let's put the blame where it belongs. It started when someone asked the question:
Can we really say that a longboard cannot exceed by much its hull speed, NO MATTER the size of the sail? 
That seemed to be an important question for him. He asked on both the iWindsurf forum in the US, and on the Seabreeze forum in Australia. How that developed was typical for the two forums.. but let's not go there.

Instead, let's ask the questions:
  • Does the concept of "hull speed" (very similar to maximum speed") apply to windsurf boards?
  • If "hull speed" has some relevance, what is the hull speed for a typical longboard? How about a shortboard?
  • What can we learn from this?
To get started, let's look at GPS speed tracks from a shortboard session. The wind in this session was marginal, and dropped from about 17 mph (where I was just able to plane) to about 14 mph, not enough for me to plane anymore. Here is what the speeds looked like:
When planing, my speed was about 15 to 20 knots. When the wind dropped a few miles below my planing threshold, the board speed dropped dramatically, to about 4 knots. Instead of going about as fast as the wind, I was now going a lot slower. Even with what felt like plenty of power in the sail, the board did not want to go faster than 4 or 5 knots. I tried to pump up onto a plane a couple of times, which cost a lot of energy, but the speed dropped again as soon as I stopped pumping.

So from this, there seems to be a "hull speed" that is very hard to exceed when not planing - at least on a shortboard. Before we discuss longboards, let's have a closer look at the "hull speed" concept. It comes from traditional sail boats, where boaters and builders observed that the top speed of displacement boats was limited. Past a certain point, more win or adding more sail would not result in any significant speed increases. What this maximum speed or "hull speed" was depends on the boat length - it higher for longer boats.

The reasons behind the hull speed limit are easy to understand. As a boat travels through the water, it creates a wave at its bow (front end), and a second wave at its stern. These waves are traveling forward at exactly the same speed as the boat. Here is an example of what the waves look like, seen from the side:
The laws of physics describe the relationship between the speed S of a wave and the distance between subsequent wave crests (the wave length WL): 
WL = (S/1.34)^2 
The formula above give the wave length in feet when using the speed in knots. When a boat doubles its speed, the wave length will increase four-fold.

When the boat speed increases so that the wave length of the bow wave equals the length of it's waterline, we get the following picture:

 The boat has dug itself into a hole! Quite a bit hole, too: the second crest of the bow wave is where the stern wave crests, so the waves add up and become extra tall. It the boat goes any faster, the wave length of its bow wave increases further, and the crest of the second bow wave falls behind the boat. As a result, the stern (back) of the boat will sink deeper into the water, and it will look as if the boat tries to climb up its bow wave - sailing uphill! Just like walking uphill, that takes a lot of energy, so it becomes nearly impossible.

Turning the wave formula from above around, we can calculate the speed at which this will happen:
S = 1.34 x SQRT(WL)
This is the "hull speed". For typical displacement boats, it is a good indication how fast a boat can go. For a 15-foot boat, it gives a hull speed of 5.2 knots; for a 60-foot boat, we get a hull speed of 10.4 knots.

If this formula would apply for windsurfers, it would be bad news: a typical 8 foot shortboard would have a hull speed of 3.8 knots, and even a 12 foot longboard would have a hull speed of just 4.6 knots. So adjustments are needed!

Let us look back at the waves a boat creates. Looking at similar boats with different weights (or "displacement"), we can see that the height of the wave the boat creates is proportional to the boat's weight. A lighter boat will have to "climb" a slope that is not as steep as a heavier boat, and therefore has an easier time exceeding the "hull speed" that was calculated with the simple formula above. Indeed, it is well known that lighter boats are able to go faster than heavier boats of a similar shape (note that we are still only talking about displacement boats - we'll get to planing designs in a little while!).

Fortunately for us windsurfers, a boat designer has developed a better formula that takes the weight of a boat into account when calculating hull speed.  A typical sail boat like the Catalina 22 weighs in at close to 2500 pounds, even before we add the crew; a windsurf board with sail and sailor will typically weigh in at less than 1/10th of that, while having about 1/2 of the waterline length. So if we use the "Better Way to Estimate Hull Speed", we get quite different results. Here's a graph comparing the calculated hull speeds for windsurf boards with a total (board+sail+sailor) weight of 200 lb:
For an 8-foot shortboard, the difference is not big, but the numbers match better what we seen in the GPS data. But for a 12-foot longboard, the numbers differ a lot: instead of a maximum speed of 4.6 knots, we now get a maximum speed of 8.4 knots!

This is where things get interesting. When I analyze my jibes, I take 8 knots as the minimum speed for a jibe that I cleanly planed through. The weight-adjusted hull speed formula predicts that I should be able to reach 8 knots, the planing threshold, easily on a longboard in displacement mode, but not on a shortboard! And that's exactly what it feels like. Here is a GPS track from a longboard session in marginal planing conditions:
Just like in the shortboard session above, the wind dropped shortly after I started sailing. But instead of a graph with two clearly separated speed ranges, you can see a wide range of speeds. There are runs with top speeds of 6, 8, 10, 12, 14, 16 and 18 knots - the entire spectrum is covered. The maximum speed in the second half of the session, when I was not planing anymore, was around 10 knots - reasonably close to the 8.4 knots predicted "hull speed", but still a bit faster. During the non-planing parts, I was going about twice as fast on the longboard as I was on the shortboard, in similar wind and with the same sail. The fun-factor was way more than 2 x higher!

Another way of looking at this is by plotting the board speed against the wind speed:
In this example, the longboard is faster until the wind reaches 18 mph; after that, the shortboard is slightly faster. The speeds diverge the most between 5 and 15 mph. In this range, the shortboard reaches a plateau, and speed barely increases, even though wind speed increased 3-fold. In contrast, the longboard keeps picking up speed over the entire range. The shortboard sailor feels a lot more pressure in the sail, but the board feels "stuck", unable to convert the pressure into speed. Frustrating! In the same range, the longboard sailor sees the increasing wind nicely converted into higher board speeds, and the longboard slowly starts to switch from displacement mode to semi-planing and finally planing. Fun instead of frustration!

I think the two curves above explain why windsurfing has been on a long downward slope in many parts of the world, including the US. Yes, planing is more fun than sailing in light wind, but we have been way to single-minded in our pursuit of planing. We have made our boards shorter and wider, so that they plane earlier and turn better once planing. Not a big problem at first, when most windsurfers had a longboard in addition to the "funboard". But 10 years later, the longboards were mostly forgotten. Windsurfer drove to the beach with their short funboards on weekends, only to discover that 5 to 15 mph wind is far more common than planing conditions - and that funboards are absolutely no fun if they don't get enough wind to plane.

Even if we have a new windsurfer who has mastered light wind sailing, and is now wants to plane, shortboards can make life more difficult. Instead of gradual acceleration (the blue curve above), she has to first learn to handle a lot of sail pressure without going much faster, and then be willing to go at least three times as fast as before. Not a problem for young daredevils - but I have seen more than one brave soul struggle quite mightily with this transition.

There is clearly a big bump to overcome when getting a shortboard to plane. Whether you think about it as "climbing up the bow wave" in the "hull speed" paradigm, or as having to climb the plateau and steep slope in the diagram above, I don't care; but climb it you must. We need some extra power to get over the bump, so a lot of intermediate sailors end up using a sail larger than what an efficient sailor would need. That is not per se a big problem; but more often than not, it creates problems later on. For example, one logical thing to do when sailing with a larger sail than necessary is to not sheet in fully. Again, not a big problem per se - until you want to learn a planing jibe. As you enter the jibe with a partially open sail, oversheeting to spill the power becomes impossible. Nor are you going at full speed, so you probably are sailing slower than the wind, even at your fastest point in the jibe. Therefore, the sail will alway keep some power in it, and you'll have to lean back to counter the power, instead of adapting a balanced surfing stance. When you then flip the sail, the tail sinks ... and so do your chances of planing out of your jibe.

Finally, back to the question: What can we learn from this? Several things, methinks.
  • If you are living at a "normal" windsurf spot (that is, not the Gorge or the Rio Vista / SF area), consider getting a longboard. The Kona One is a great board with a soft top, wonderful for learning, light wind freestyle, or racing; but old longboards from the 80s and 90s can be just as much fun. Get one, and "no wind" days will become a thing of the past. Sailing a longboard in 10-15 mph wind is a lot more fun than desperately seeking to plane.
  • If you are struggling with planing comfortably and in control, despite having had good instruction, also consider a longboard - it might make your life easier. But I'd suggest to try before you buy :-).
  • If you are on a shortboard, especially in marginal conditions, learn ways to get over "the speed bump". Got chop? Use it! Going downhill is always faster.  Learn to pump! Yes, it's hard when you start, but you may well save energy in the long run. Learn a few ways to pump; learn to really get a feeling for what it takes to get planing, both with effort and with minimal effort; and you may just see that you'll need a knot less wind to get going on the same gear. This can go on for a few years!