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The science of sailing is not rocket science. Sailing is more complicated. The maximum speed of a rocket powered car on a perfectly flat and level surface with no wind is limited by the total drag (aero + wheel resistance). The car reaches its maximum speed when the thrust of the rocket equals the total drag.

When sailing on a water surface, the maximum speed is never limited by the total drag because the thrust and drag are coupled. On the air side, both thrust and drag derive from the apparent wind flowing over the sail, the sailor and the board. The lift on the sail must be balanced on the water side by the lift generated by the board and its fin. Because the angle of the apparent wind is never zero, the thrust propelling the board is only a fraction of the lift generated by the apparent wind flowing over the sail. Under a small range of conditions the maximum speed of a windsurfer can be limited by the overall (L/D) resulting from both the air and the water side; where
1/(L/D)overall = 1/(L/D)air + 1/(L/D)water. When these conditions apply, it is important to maximize L/D on both the air and water sides. More often, however, windsurfer speed is limited by instability considerations.

In windsurfing vernacular, 'you spinout or capsize - and crash, dude.'

Kenney, B.C., 2001. Hyperwind Sailing. Catalyst, J. Am. Yacht Res. Soc., 6, 24-26.
Kenney, B.C., 2017. Hyperwind Sailing on Snow, Ice, Land and Water. Catalyst, J. Am. Yacht Res. Soc., 52, 4-18.

Dont get what? ... restating something technical, in a domain-specific analogy is perfectly ok.

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The science of sailing is not rocket science. Sailing is more complicated. The maximum speed of a rocket powered car on a perfectly flat and level surface with no wind is limited by the total drag (aero + wheel resistance). The car reaches its maximum speed when the thrust of the rocket equals the total drag.

This statement is mostly wrong. While it might be true that rocket science is "less complicated" because it is "just burning some fuel"... the phrase "rocket science" when used in everyday vernacular implies "rocket engineering", which is almost the hardest thing humans have ever built.

Generally speaking rockets keep accelerating - even underwater... mostly because chemical-rockets have very large ISP. The limiting factor is almost always fuel-availability (ie: fuel-weight vs fuel-burn-rate).

To put that into context, most rockets are more than 90% fuel, vs scram/ram-jet at about 60-70%, vs commercial airliner at around 30-50% vs cars at about 3-5%. Rockets have a very fast burn rate, so the rocket gets lighter very quickly relative to its drag, so that also helps it to accelerate.

The same thing happens with rocket-cars -> it is the wheel-bearing tolerances which limit top speed, not air-drag. It urns out that if the wheels are not quite perfectly aligned or they vibrate even just a little bit, there is enough energy stored in the angular momentum, that it will destroy the car.

Hi Barney,

Are those quotes from the papers you reference at the bottom?

I am also not quite sure of the context of your post. What is it you think windsurfers 'don't get'.

I would say that is a long time since I have felt limited in speed by 'instability'. Spinouts at top speeds are extremely rare for me on 'fast water' (Flat enough to not have issues with spinout - say following chop less than 15-20cm high)

That has me puzzled as well. I don't get it.

Rockets are not that hard. I built hundreds in my youth, and not many blew up. Of course, some didnt get off the ground either. But plenty did!

It helped that may dad had a shed full of lots of interesting Nitrate fertilisers (that don't seem to be widely available anymore - strange that....)

I only managed to clear the power lines a couple of times. I think the gunpowder was falling out of the rocket before it ignited. I wasn't game to ram it too hard.

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sailquik said..
Rockets are not that hard. I built hundreds in my youth, and not many blew up. Of course, some didnt get off the ground either. But plenty did!

It helped that may dad had a shed full of lots of interesting Nitrate fertilisers (that don't seem to be widely available anymore - strange that....)

Hi Andrew,

I was trying to make a joke but it blew up on the pad. Clearly, it wasn't rocket science. FWIW I was not talking about rockets but about the use of a small rocket motor to propel a vehicle horizontally through still air at STP until the constant thrust is equal to but opposite the total drag and the inertial forces have gone to zero.

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sailquik said..
Hi Barney,

Are those quotes from the papers you reference at the bottom?

I am also not quite sure of the context of your post. What is it you think windsurfers 'don't get'.

I would say that is a long time since I have felt limited in speed by 'instability'. Spinouts at top speeds are extremely rare for me on 'fast water' (Flat enough to not have issues with spinout - say following chop less than 15-20cm high)

The quotes are a mistake. I wrote the post on a small text editor and it was subdivided into quotes by the forum software when I submitted it. I don't know why. The only thing taken from my papers verbatim is the equation for L/D. You should study it because it is relevant to what you are trying to do.

From what I've read, you are a very experienced speedsailor. In my opinion you don't spinout at top speed because you know exactly how hard to push. The question that you should ask yourself is this, 'Am I limiting my top speed by avoiding spinouts or would a more optimum, spin resistant fin allow me to sheet in more and go faster?' Similarly on the air side. You use twisty sails to prevent catapults and capsize in gusts. How much faster could you go if you converted the gusts into thrust?

My papers show what speeds are theoretically possible when various stability constraints are removed. The main point is; top speed is not limited by drag or L/D but by stability constraints.

Thanks for clearing that up Barney.

For what it's worth, I feel that I am limited by my ability to produce power by leverage: If I wear weight I usually go faster. The taller and heavier guys go faster again.

I may be limited by the L/D of the sail, but when it is blowing 40+ knots, I have to keep reducing sail size, and sail a broader course, to be able to simply hold the power I have in the sail. The is a limit to this though. I found at Luderitz, on the very broad course, in the strongest winds, reducing sail size only left me with less power. On a slightly less broad course, it may have been a different matter.

Even, or especially, in 25 to 30 knots, the larger guys go faster than me most of the time.

I am not so sure about the fin being a L/D or control limit. At my tops speeds in strong winds I don't feel there is much pressure on the fin at the angles we run, and i dont feel in any danger of overpowering it as long as it does not ventilate over large chop. And I tend to run much smaller fins than most sailors, even those of similar size to me. Sheeting in more does not seem like a good option, as there is an optimum angle before the sail stalls. Usually. I am trying to stay as sheeted out as possible for what feels to be the best L/D. on a broad run in strong winds. In lighter winds, below 30 knots, I have found I need to run a tighter course and have more pressure on the fin to get best speeds. This is the wind strength area where it feels like better L/D from both the sail and fin would definitly help.

My feeling is that sail twist helps to keep the sail stable and the CofE lower were I have more leverage over the power, and can direct it the most effecient vector. Likewise, lower aspect sails seem to do the same, for a net gain, even if they may be theoretically lower L/D.

BTW, the URL www.arys.org returns a page with 'Welcome. This site is under construction. Please come back soon"

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sailquik said..
BTW, the URL www.arys.org returns a page with 'Welcome. This site is under construction. Please come back soon"

Try: www.ayrs.org

Theres a lot of science to windsurfing, both equipment and the way we use it.

But i think its more of a question of brain wiring, or lack thereof that permits the better of us to reach the highest speeds.

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Subsonic said..
Theres a lot of science to windsurfing, both equipment and the way we use it.

But i think its more of a question of brain wiring, or lack thereof that permits the better of us to reach the highest speeds.

I hope this isn't leading to another brawn over brains argument!
Delicate little Mathew will have a conniption.

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Subsonic said..
Theres a lot of science to windsurfing, both equipment and the way we use it.

But i think its more of a question of brain wiring, or lack thereof that permits the better of us to reach the highest speeds.

HEY! I resemble that remark!!!

I have said it often:

"You need to be in the right place, at the right time, on the right gear."

I have been known to add "With and attitude of a considered disregard for ones own safety"

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sailquik said..

Subsonic said..
Theres a lot of science to windsurfing, both equipment and the way we use it.

But i think its more of a question of brain wiring, or lack thereof that permits the better of us to reach the highest speeds.

HEY! I resemble that remark!!!

I have said it often:

"You need to be in the right place, at the right time, on the right gear."

I have been known to add "With and attitude of a considered disregard for ones own safety"

Hi again, Andrew,

I have some answers for you but first I have a question. When you are 'in the right place, at the right time, on the right gear', what is the value of beta (i.e. the angle of the apparent wind)? Have you ever measured beta? If not, it is easily done by sticking a tell-tale and a protractor on the centerline of your board near the front.

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barney831 said..

sailquik said..

Subsonic said..
Theres a lot of science to windsurfing, both equipment and the way we use it.

But i think its more of a question of brain wiring, or lack thereof that permits the better of us to reach the highest speeds.

HEY! I resemble that remark!!!

I have said it often:

"You need to be in the right place, at the right time, on the right gear."

I have been known to add "With and attitude of a considered disregard for ones own safety"

Hi again, Andrew,

I have some answers for you but first I have a question. When you are 'in the right place, at the right time, on the right gear', what is the value of beta (i.e. the angle of the apparent wind)? Have you ever measured beta? If not, it is easily done by sticking a tell-tale and a protractor on the centerline of your board near the front.

Hi Barney. I have not measured apparent wind. I am not sure any device like you describe would survive a run in ideal conditions, or that I could monitor it accurately during the run.

But it should possible to calculate it, approximately. I say 'approximately' because the problem is that we dont know exactly the angle of the run to true wind and often don't know the exact true wind speed when at top speed. I have high confidence that the course angle from true wind direction will be between 130 and 135 degrees - lower end if the wind is lighter and higher end if it is stronger. True wind strength will be between 35 knots and 40 knots, (steady - which it seldom is). The problem with estimating wind speed is that at some venues (like Luderitz) it can vary considerably during the run - up to 15 knots! At Sandy Point, it is less gusty. On a couple of the best days, it has been between 40-50 knots - 2009, and 35-40 Knots - 2014. But on both days I have personally done almost identical top speeds of 47 knots. On the first I was on the edge of out of control, meaning I felt I was about to be literally blown (lifted) off the water at any moment. In the second it all felt relitively easy, and I was actually surprised at my speed.
Given that, I would estimate that I need about 36-38 knots @ 135 degrees off true wind to reach over 47 knots speed. I am 74 kg and 175cm tall. Bigger people (>85-90kg) will do over 50 knots in those conditions.

Does that help?

I will see if I can devise a device to get a more accurate measurement of apparent wind angle. Can't use the centreline of the board for accuracy though as it is angled to the direction of travel, although it might be close enough. And aparent wind angle at virtually water level will probably be quite different from the mid sail level.

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sailquik said..

I will see if I can devise a device to get a more accurate measurement of apparent wind angle. Can't use the centreline of the board for accuracy though as it is angled to the direction of travel, although it might be close enough. And aparent wind angle at virtually water level will probably be quite different from the mid sail level.

How about a Helmet Vane with an indicator where you can see the apparent direction in front of you? I know that when you sail without a helmet it is easy to find the apparent direction, you're facing it when the wind sounds the same in both ears.

Nice idea, but how do I ensure my head is exactly lined up with the direction of travel? Normally, I don't think it is, and it may be quite a neck twist to get there.

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sailquik said..
>>>>

Therein is another question, apparent wind angle, relative to centerline of board, or direction of travel?
I think the telltale and protractor with a mast cam, would work, you can see the angle relative to board, and probably get an estimate of yaw from the wake.

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decrepit said..

sailquik said..
>>>>

Therein is another question, apparent wind angle, relative to centerline of board, or direction of travel?
I think the telltale and protractor with a mast cam, would work, you can see the angle relative to board, and probably get an estimate of yaw from the wake.

Ahh,, mast cam. Great idea. I think I will try to mount a little device I aquired about 25 hears ago. If I can find it. It is a mast mounted wind vane intended to show apparent wind direction. I hope to find a way to attach a protractor to it. The vital part of the puzzle is your idea to use a sports cam to monitor it!

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sailquik said..

. And aparent wind angle at virtually water level will probably be quite different from the mid sail level.

wind-data.ch/tools/profile.php?h=5&v=20&z0=0.0002&abfrage=Refresh

If you put in a roughness length of 0.0002 ( or 0.002) in this web calculator you find there's only about 5% difference in the wind speed close to the surface versus at at 5metres. ( The graphs are pretty coarse at less than a metre, don't show the fine detail down there) . My rough vector diagram gives that as accounting for about 5 degrees shift in apparent wind from bottom to the top of the mast when 135 degrees off the wind.. But keep in mind that these wind profiles only emerge if you average the wind readings at each height for at least a couple of minutes. The sail sees an instantaneous profile which is most likely anything but logarithmic. And it takes a little while for a new steady profile to develop as wind moves from one surface to another of differing aerodynamic roughness. Plus profiles are distorted passing over an undulation as there often is sailing in the lee of an exposed sand bank. The nose of the board will influence the wind direction also so you'd have to put the tell tale on a little pole of at least a couple of inches.

It's all a bit academic, would take some serious instrumentation to figure it out with any accuracy at each speed venue. Sailquick's estimates of board speeds, wind speed and direction are as good as we'll probably get. Go with those estimates and do the vector diagram on your beer coaster.

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sailquik said..

decrepit said..

sailquik said..
>>>>

Therein is another question, apparent wind angle, relative to centerline of board, or direction of travel?
I think the telltale and protractor with a mast cam, would work, you can see the angle relative to board, and probably get an estimate of yaw from the wake.

Ahh,, mast cam. Great idea. I think I will try to mount a little device I aquired about 25 hears ago. If I can find it. It is a mast mounted wind vane intended to show apparent wind direction. I hope to find a way to attach a protractor to it. The vital part of the puzzle is your idea to use a sports cam to monitor it!

Why don't you use one of these Andrew, then you can say you invented them.

Thats pretty close to what I have - or had. I know I saw it somewhere recently......

Yep I don't get it!! I just rig up sail and have a great time why clog your mine with vortexes,angle of attack,apparent wind,foil sections..............may as well stay home.

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sailquik said....."You need to be in the right place, at the right time, on the right gear." I have been known to add "With and attitude of a considered disregard for ones own safety"

Subsonic said..
Theres a lot of science to windsurfing, both equipment and the way we use it.

But i think its more of a question of brain wiring, or lack thereof that permits the better of us to reach the highest speeds.

That's right. If a sailor is thinking about their personal safety, they are never going to sail fast. This has been the undoing of many potentially good sportspeople.

I think about concentrating forces into drive and visualise forward momentum. Only then, do the forces balance into that forward-sucking feeling we love as speed sailors.

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choco said..
Yep I don't get it!! I just rig up sail and have a great time why clog your mine with vortexes,angle of attack,apparent wind,foil sections..............may as well stay home.

That is quite a valid way to approach your windsurfing Choco! It's also probably a lot more fun to clog up your shed with new sails, boards and fins!

I am interested to find out how I can actually go faster. I have not had a PB for quite a few years and I seem to be bumping up against a ceiling when I am in the best conditions. The technical side of it interests me, and I am guessing, there are others it interests as well.

But when it is all said and done, all I want to do when I get on the water is have fun. And going as fast as I can is a big part of the fun, no matter what the conditions.

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Adriano said..

sailquik said....."You need to be in the right place, at the right time, on the right gear." I have been known to add "With and attitude of a considered disregard for ones own safety"

That's right. If a sailor is thinking about their personal safety, they are never going to sail fast. This has been the undoing of many potentially good sportspeople.

I think about concentrating forces into drive and visualise forward momentum. Only then, do the forces balance into that forward-sucking feeling we love as speed sailors.

As in all things, there is a balance to be found between risk and reward.
That is why I used the word 'considered'.

The biggest thing is confidence. Real confidence, not false confidence.

Confidence comes largely from experience. That experience needs to involve one exploring the limits of their equipment and skills, especially where the consequences of finding them are relatively minor. As limits are found, understanding of how hard one can push is refined. Mostly, it is the knowledge of when you are approching the limits so you can not worry, and can concentrate on the performance at sub-limits levels.

It is also about recognising what the 'limit' or danger area really is and developing the skills to confidently cope with situation where you are on the limits. It could be having a plan for coping better with loss of control and a crash, or knowing there are things you can do to give a good chance of recovery from loss of control, or having the calm mind, focus and skills to stay on the edge without completely loosing control.

And then, the big one! All of that experience and knowledge come into sharp focus when you really are in the right place at the right time. It also means that you should recognise what equipment you need to be on and be familiar with it. But most of all it means that you can have the confidence to push it as far as it will go. You will be OK to do that because you know what the limits feel like, and that even if it all goes wrong, you have a good chance of coming off OK. And that is what I mean by a 'considered disregard for your own safety'.

There is a limit to how much risk any sane person will take when you reach that point. How people decide that is beyond me to analyse, and it is clear that some people will consider another person to be crazy. But remember that they are looking at it from their own perspective and experience. The person they think is taking a crazy risk may actually think/know they are still well within their area of reasonable risk. All you have to do is watch the replays of Marc Marquez recovering his MotoGP bike from what to most would be certain crashes to get that. Or watch the top World Rally champs calmly drift their cars through a gravel road corner at 130 km/h sideways, that a normal driver, inexperienced in those conditions or vehicles, would consider to be a 40km/h corner.

In the end it may come down to how far that very experienced and confident person is willing to go into unknown territory. And that my friends, is what we call Bravery.

Duly noted Grand Master Yoda.

Ask any of the surfers on this video from Denmark about how their day was on the water, - they will all reply "It was bloody fantastic!!"...