Windsurfers just don't get it!

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barney831 said..
Apparent wind is measured relative to the direction of travel and includes the leeway.

No one I have ever spoken to, thinks that ... true wind doesn't vary (much) so that is the frame of reference, so is usually measured from that. The other alternative is relative to North (true or magnetic, take your pick).... Be it pilot's, sailors, or joe from hicksville.

Which - in my opinion - is why Roo's apparent-angle-speed thing, is gibberish... ie: who sails at 152deg apparent wind?

A bit off-topic, but: cool plots on the America's cup link. VMGs of 20 knots with 25 knot board speed and 30 knot speeds (almost) dead downwind, that's amazing.

Looking at the graph above, it seems the America's cup boats were optimized for high speed over a very wide wind angle range (75-165 degrees), which raises the question: do all windsurf foil races have a large upwind-downwind component? If so, I wonder what would happen to foil speeds if there were more slalom-type races. Maybe we'd see similar top speed differences as between slalom boards and formula boards?

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Ian K said..

mathew said..

barney831 said..
Apparent wind is measured relative to the direction of travel and includes the leeway.

No one I have ever spoken to, thinks that ... true wind doesn't vary (much) so that is the frame of reference, so is usually measured from that. The other alternative is relative to North (true or magnetic, take your pick).... Be it pilot's, sailors, or joe from hicksville.

Which - in my opinion - is why Roo's apparent-angle-speed thing, is gibberish... ie: who sails at 152deg apparent wind?

You can use any frame of reference you like (as long as you make it clear what you are using) . We usually go with the easiest.

Q. How does a sailor on the water ( wanting to use the direction of true wind as the reference) determine the direction of the true wind?

Just to screw things up a bit more, are you going to measure in the gusts, or lulls? Or set an average wind direction?

Im no expert at speed sailing, but we generally bear off in a gust to slingshot. Gusts generally have a different angle to the direction the wind is travelling in.

Apparent wind is just that, and it's frame of reference is the sailor, or the boat. There is no reference to true wind direction and for the observation it is irrelevant. To me, the way to describe the apparent wind angle has always been 'the deviation angle to one side from the direction of travel'. Ie. In still conditions riding a bike at 30Km/h, the apparent wind is zero degrees and 30Km/h. Ie. right in your face!

However, as Ian says, the measurement of apparent wind can be used to calculate the true wind if you also know your boat speed. Electronic instruments are designed and sold to Yachties just for this purpose.

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sailquik said..
Apparent wind is just that, and it's frame of reference is the sailor, or the boat. There is no reference to true wind direction and for the observation it is irrelevant. To me, the way to describe the apparent wind angle has always been 'the deviation angle to one side from the direction of travel'. Ie. In still conditions riding a bike at 30Km/h, the apparent wind is zero degrees and 30Km/h. Ie. right in your face!

However, as Ian says, the measurement of apparent wind can be used to calculate the true wind if you also know your boat speed. Electronic instruments are designed and sold to Yachties just for this purpose.

Bikes don't sail, they have a power source independent of the wind (like pedals or a rocket motor).

Here is a simple experiment, Andrew. The direction of the true wind can be estimated with reasonable accuracy from windrows of foam on the water or blowing snow on the ice. The direction of the apparent wind can be estimated from a tell-tale (4" of string taped to the nose of your board). The maximum board speed occurs on a broad reach when the apparent wind direction is at right angles to the true wind direction (exactly when on ice, approximately when on water). Even if you understand the science, it still messes with your head to see it when standing on your board going 40 knots. Been there, done that!

Of course bikes do not 'normally' sail, but it is irrelevant to what apparent wind is. Apparent wind is the air speed and direction that is felt by the perceiver. It just happens to ge highly relevant to 'sailing'.

I can very well understand, feel and even visualise apparent wind.

I am most interested in how you came to the 'right angles to the true wind is fastest' position?

The thing to note, well it confused me for a while anyway, is that the hydrodynamic drag angle is not the same as your leeway angle. And the aerodynamic drag angle is not the same as your sheeting angle. And a crucial bit of coming up with the theorem is that the hydrodynamic drag is in the exact opposite direction to the direction of travel. Same with aerodynamic drag, it's in the exact opposite direction to apparent wind. The windage of bits of you and the rig are included in the aerodynamic drag as are the underwater bits of the hull that aren't actually the fin.

And the assumption that angle between apparent wind and direction of travel is constant is dependant on being tuned for the heading you are going. i.e. You select a sail and fin to go max speed off the wind, where the angle between apparent wind and actual wind will be 90 (well it should be if you've tuned correctly) Once you've done this at max speed you might note that the angle between course and apparent wind turns out to be say 33 degrees( just a guess). This is the Beta angle that in the theory is assumed to be constant for all headings.

You will then find that when you're returning upwind to the top of the course your fin is too small, your sail too big and that the angle Beta has increased to 40 degrees (just a guess). In this case Beta is not acting as the constant that it should be!

But if you selected fin and sail to maximise your travel back up the course, ( i.e you selected a sized fin/sail to give the same l/d that you had off the wind) , you should find you get close to that constant Beta of 33 degrees ( just a guess) again.

A bit speculative in that last paragraph(s), that constant Beta of 33 degrees is just a guess, Barney can correct me if I'm wrong.

As for any wing, there will be an optimum AoA for it in any given situation. That was the very thing I was exploring in eariler posts.

Too much AoA and you create too much drag. Too little AoA and you have less lift. (less advantageous L/D in both cases)

I can only go on what I feel, and we can easily be misled by 'feelings'. But I often 'feel' that I get the best L/D if I try to sheet in less when fully powered. The other factor in that I 'feel' is that the resultant lift in in a more favourable vector - i.e.. more in the direction of travel and less pressure on the fin.

One thing I am still trying to get a good handle on is that many modern slalom/speed sails have a LOT of twist! This means that only a part of the foil will be at the optimum AoA. I know that there seem to be good reasons for this (Lower CoE, less drive pusing the nose of the board down, less tip vortex induced drag etc), but it complicates matters a bit. Lift will be less, but the effective L/D seems to be better. I do think it is easy to overdo this twist though, and I think we are seeing that in some situations. (A discussion of Dynamic Twist v's Static Twist would be interesting). One reson why sails with a every large twist may feel good is that they may 'forgive' less than ideal sheeting angles??? Ie. even if you are constantly over sheeting, some part of the sail will still be close to or at a good angle.?

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Ian K said..
The thing to note, well it confused me for a while anyway, is that the hydrodynamic drag angle is not the same as your leeway angle. And the aerodynamic drag angle is not the same as your sheeting angle. And a crucial bit of coming up with the theorem is that the hydrodynamic drag is in the exact opposite direction to the direction of travel. Same with aerodynamic drag, it's in the exact opposite direction to apparent wind. The windage of bits of you and the rig are included in the aerodynamic drag as are the underwater bits of the hull that aren't actually the fin.

And the assumption that angle between apparent wind and direction of travel is constant is dependant on being tuned for the heading you are going. i.e. You select a sail and fin to go max speed off the wind, where the angle between apparent wind and actual wind will be 90 (well it should be if you've tuned correctly) Once you've done this at max speed you might note that the angle between course and apparent wind turns out to be say 33 degrees( just a guess). This is the Beta angle that in the theory is assumed to be constant for all headings.

You will then find that when you're returning upwind to the top of the course your fin is too small, your sail too big and that the angle Beta has increased to 40 degrees (just a guess). In this case Beta is not acting as the constant that it should be!

But if you selected fin and sail to maximise your travel back up the course, ( i.e you selected a sized fin/sail to give the same l/d that you had off the wind) , you should find you get close to that constant Beta of 33 degrees ( just a guess) again.

A bit speculative in that last paragraph(s), that constant Beta of 33 degrees is just a guess, Barney can correct me if I'm wrong.

Beta is a direct measure of performance and the overall L/D of a windsurfer, just like L/D is a direct measure of performance for a high performance sailplane. The L/D of the most efficient sailplanes is greater than 60 and corresponds to an apparent wind angle of less than 1 degree. By way of comparison, the beta on a windsurfer is often greater than 45 degrees corresponding to an overall L/D of less than 1. It is not hyperwind sailing when you need a wind of 50 gusting to 60 knots to get a board speed of 50 knots. It is not efficient sailing at all.

Beta remains the performance metric of choice, however, even when it is not constant for different course headings because it can be estimated on the board without knowing the board speed or wind speed. You don't need a GPS with 3 or 4 decimal place accuracy running at 10 or 20 hz. Perfectly circular lobes on a performance polar require beta to be constant independent of course and is an idealization with specific conditions attached (e.g. constant sail trim, resultant sail force perpendicular to the longitudinal axis, no interface drag). An iceboard on good black ice can comes close to meeting these conditions in light winds and the performance polar has close to circular lobes. I regularly measure beta between 10 and 18 degrees on ice.

If you really want to get technical I suggest that you read all the books and papers referenced in Kenney, 2017, Catalyst #52, AYRS. It is now available from www.ayrs.org for 5 Br. pounds.

thanks SQ for reply..I'm reading all these posts and pulling what I can out of it, so many variables.

Does anyone know if the likes of Dunkerbenck and AA etc work all these things out them selves? or do they rely on the brains of others and then do the muscle work to add value to the science...I'm struggling to imagine Dunks sitting on the beach with a lap top chilling out with a cold one whilst calculating all the mathematics :)

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barney831 said..

Ian K said..
Thanks Barney, you've introduced us a to another perspective on sailing. Might be worth putting a little tell male on the bow of the board after all. Do you think a Beta of 33 is achievable on a windsurfer under ideal conditions? Maybe not? I'd guess a foiler should get under that easily.

I'm not sure what your setup looks like but I think 33 degrees should be possible on the right equipment (better sails, better fins). I got about 40 degrees a couple of years ago but I'm 75 years old now and probably done.

On the other hand, if you are running 55 degree weed fins, I'd say probably not. Wing sweep reduces the slope of the lift curve and increases induced drag so unless compensated for by reducing the lift coefficient it is highly unlikely the you would see at beta of 33 degrees or less. I wrote a bit about this in my 2017 paper.

Was that Beta of 40 degrees on a windsurfer or ice Barney?

I don't think anyone is running 55 degree weed fins Barney. Not for serious speed anyhow. Even on the weediest lakes where it is the only way to even sail, they rarely use more than 50 degrees, and in those conditions anything over low 40's is excellent.

Most of the succesful speed fins 45-50 kts + seem to be in the range 25 to 40 degrees, the latter only when catching weed is a problem. Catching one strand of thin weed can totally destroy the performance of a speed fin. It is rare to see a speed fin these days that is not raked at least 25 degrees from the base on the leading edge. In the water it will be more vertical because of board pitch angle.

Well from what we've gathered here Andrew that should be calculated by just applying that diagram from wikipeadia

It's for the case of maximum speed where of course (as we've just found out) apparent wind is at right angles to the true wind. Putting in Beta = 33 degrees you get
Vt/Vy = Sin 33 or Vt/Vy =0.5446 or Vy = 1.836 Vt. So if we could achieve Beta of 33 degrees in a 20 knot breeze we should do 36.7 knots. And in a 40 knot breeze double that board speed! I'd say we're not quite there.

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JonesySail said..
thanks SQ for reply..I'm reading all these posts and pulling what I can out of it, so many variables.

Does anyone know if the likes of Dunkerbenck and AA etc work all these things out them selves? or do they rely on the brains of others and then do the muscle work to add value to the science...I'm struggling to imagine Dunks sitting on the beach with a lap top chilling out with a cold one whilst calculating all the mathematics :)

I would be very surprised if more than a small handful of the top speed sailors would be sitting around doing math and physics.

I recon there are one or two who might though. Like maybe Eric Beale? Mal Wright used to think about such things as well. I can think of a few others with a tech curiosity besides Barney and Ian. I would guess that the top slalom sailors and speed sailors rely on their equipment designers for the tech stuff, and then bring their considerable experience to evaluation and tactics, to see what works best for them in various conditions. There have always been those racers who do a lot of side by side testing with their team mates, and speed sailors who test a lot of equipment and get a lot of feedback from GPS runs. I think the feedback from that to the equipment designers is mainly what drives progress and understanding, rather than pure maths.

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Ian K said..
I agree with that. A good feel for the wind and water will beat maths and physics any day on a windsurfer. Might be different where the sail is buffered by hydraulics and pulleys, and you can order the for'd hand to hold up a tell tale and report back.

But to reverse that calculation you hit the top left reverse button on your mac calculator. You'll get a sin -1 button pop up. Divide 38 by 46 and hit that button. should get Beta = 55.7 degrees.

Very fast but doesn't sound efficient. But then I'm always skeptical of wind measurements. How did you arrive at 38 knots? Might have it been low 30 knots? Still a lot of wind!

Sheeting angle is roughly correlated with Beta. (If you ignore the leeway angle and over and under sheeting of the apparent wind). Were you sheeted in "closing the gap" or had you opened the sail up a bit? Or, at 46 knots!, can't you remember?

I can't find any button like that on my Mac calculator. It is simple like me.

I am very much with you on reports of session wind measurements. I find they are most often just a wild guess and often quite exaggerated.

But I have made a lifetime study (well since the early '80's), of the winds I sail in by actually taking real measurements every time I sail. The 38 knots is an educated estimate based on a lot of actual, on the beach/course obsevations using my Deuta precision anemometer from Sandy Point, to Shark Bay and Luderitz.
I am pretty confident I am with in a couple of knots either way. I have sailed a lot in winds around thirty knots and slightly above, and I can rarely do more than 40 to 42 knots 2 sec in that. 33-34 Knots of wind is usually needed to get me into the 43-44 knots speeds and I know it takes a significant amount more wind to get me past 45. It could possibly be closer to 40 knots but I doubt it (although it certainly was on 29-09-2009!), and I am pretty certain it would not be less than 35 knots.

On the occasions when I was not feeling totally out of control (as I felt on 27-09-2009 ), I am fairly sure my sheeting angle was close to what is shown the photograph I posted from Luderitz earlier in this thread. Probably about 20-25 degrees from course, maybe slightly more, but certainly not 'closing the gap' in the way we do on a slalom board. Its funny how I forget so much these days, and yet I have quite vivid and detailed memories of most of my very best runs.

Edit: I just found my protractor so I went back and checked the drone shot of Dezza at LG. As best I can measure, (and it surprised me), the sheeting angle shown is very close to 30 degrees from the course. Since I think that we sheet in more in lighter winds, I would now estimate my sheeting abgle at top speeds is 25+ degrees

Got it!

Now you have set me off Ian. It just proves again that a little knowledge is a dangerous thing!

The situation I theorised before regarding doing 28 knots in 15 knots of wind, works out to a Beta of 32.4. Hmmmmmm..........

Perhaps it is possible for a windsurfer to get to Beta 33 degrees