Windsurfing slalom/speed sail twist

Pfft. 38 knots, 6.2 Severne, 25 knots SE.

Found your write up Dezza,
www.gpsteamchallenge.com.au/sailor_session/show?date=2018-03-06&team=34

I think the 38kts was later in the day when you'd changed down to the 5.5. Sounds like a fantastic session!!!!

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sailquik said..

But to humor you : Sails from that era and that size had a very small wind range. I would guess they were out of their controlable range at around 15 knots of wind, so probably less than that.

Controllability issues were a function of how they were rigged. If you got rid of the camber inducers and rigged them flat with no twist, they were controllable to much higher wind speeds.

Sorry to change the subject but a friend took some photo's the other day and this photo shows my sail in full twist, working perfectly

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ka43 said..
Pfft. 38 knots, 6.2 Severne, 25 knots SE.

That was later in the day in her 5.5m.

Gary didn't wat to fly the drone after the wind picked up more.

Agree Steve except I think he was on an 8.4??

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ka43 said..
Agree Steve except I think he was on an 8.4??

Arhhh ok. So 115/8.4....

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barney831 said..
When you say 'way out of control' what do you actually mean? You could not control your speed? You could not control the AOA? You could not control your heading? You could not control the catapulting moment? Or something else?

In another post on this page you make reference to 'your modified wind speed'. I am curious about the nature of these modifications and about your tech background?

The main thing was uncontrollable draft movement, excessive back handing, sail,occilations and trying to lift board and sailor out of the water, and yes, some of those things you listed too. (Barely evident at all in modern slalom/speed type sails, especially cammed sails). This leads to board instability, tailwalking, spinout, catapult etc. i.e.. "out of control". My experience is that this was much worse on the larger sizes where there was a big difference between true wind speed and apparent wind speed as soon as one got planing and bard speed quickly approached nearly twice the wind speed. Of course, if you were on a slow , high drag board, you may not have noticed this as much.

Re. Forecast modification: Are you taking the piss

OK. Straight bat play:

The Windguru website interface gives anyone the opportunity to add a 'modified' line to the forecast so as to tune it better for local conditions as observed by the modifier. After quite a few seasons, totalling many months of time spent at LG at the height of the summer windsurfing season, I have created and tweaked my modification to this particular spot forecast based on careful and thorough observations and repeated annemometer measurements of the summer SE winds. Simple as that. It seems to work out extremely well for the wind directions I applied it to at this time of the year. In fact, I would say that you could basically ignore the top two standard lines for the modded directions, as the wind tends to be very steady here from the SE in the common clear sky conditions, and just use my modified forecast as a pretty good one for a large proportion of the time.

A modification can be selected, created and tuned on by any Windguru user in the top left of the 'old' Windguru interface, or left off and ignored as they wish. Note that I have only added modifications to the winds from the SE quadrant. The rest of the compass rose Is unchanged as I dont have any relevant observations on it.

here is where one can select it on or off:


Here is the actual mod values:

the notes still say experimental, but I have not seen the need to modify those values now for a few years.

Now Barney, do you have any valuable insights to add on the topic of windsurfing sail twist?

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barney831 said..
I don't think so, at least nothing that you will believe. You did not answer my question about your tech background but it appears that we do not speak the same language. For example, in an earlier post I said I removed the camber inducers and rigged my sails flat. You replied by saying that you rigged your sails flat with the camber inducers in place.
But then above, when talking about your sails being uncontrollable; you refer to, 'uncontrollable draft movement'. What exactly do you mean? A flat sail has no draft!

Sail Twist: I dont have 'beliefs'. I seek and follow the evidence. That is the whole point of this discussion.

That sail pic you posted was anything but flat. I still dont know what your point of the guessing game was, apart from a bit of fun.

And yes, clearly, your idea of what is a 'flat' sail and mine is different. The language you type in seems to be English though.

Barney,

I was not 'using the forecasts as wind data', only to put visual context on my observations. As explained, I had done some actual measurements that day, one at about the same time as that run in the photo was made. The readings also backed up my estimates later in the day, as did the readings from the Robe AWS, (but that station does not always correlate exactly).

Ahhh,, perfect twist? Now that is the question.

How does one know without measurements of Beta angles etc? Well, all I can say is that there are a lot of sailors racing all the time that are fiddling with the trim and twist of their sails. I lot of where we have come to is simply experimenting and/and copying those who beat us. As we said at the start of the thread, We don't really know exactly why twisted sails work well in some conditions, but we do know they work. Progress though competition. Many of us have good theories, some of which are somewhat supported by good reasoning and observation, but as to the relative applicablility and importance of the various theories, that we dont know.

Tech? I didn't go to a Tech school. I went to a High School. (Only Aussies would get that. Sorry ).

I dont understand the question, the context or the relevance. Its a hell of a broad topic. It also sounds a bit like those leading questions where someone is trying to one up you. If you have a specific relevant question, PM or email me.

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mathew said..

barney831 said..
A flat sail has no draft!

There is this thing called "seam shaping"... you may have heard of it... they use it in the clothing industry to shape flat materials, into 3D shapes. It is used within the sailing industry too. Go look it up.

PS. What is your tech-background ?

The correct term is " broadseaming". I don't recall any windsurfing sails from the '80s or '90s that employed broadseaming - too expensive for cheap sails. There is a very simple test for broadseaming in new sails. If the sail lays perfectly flat on a flat surface it has no broadseaming. Old windsurfing sails, however, seldom lay flat because they are stretched out of shape.

BTW my statement is correct as it stands. A flat sail has no draft!

FWIW I have a PhD in fluid dynamics and 40 years experience in scientific research.

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barney831 said..

I will end my participation in this forum by quoting a kiting friend of mine. Geoff said, " I have spent twenty years trying to explain technical aspects of kites to other kiters - with little success. They think that they know everything, BUT THEY DON'T! "

Barney, it will be sad to see you go now that you have achieved your long term goal of proving that you are smarter than everybody else on the forum. But I appreciate that you have new frontiers to conquer and probably new forums full of unwashed masses that can benefit from your vast knowledge.

I'm also sorry to hear that your friend Geoff has suffered from poor communication skills for the past 20 years. Perhaps his mistake is that his attitude hasn't been sufficiently patronising. Maybe you could help him with that?

Head twist and fullness down low. The more things change the more they stay the same... or something like that in French.

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sailquik said..

OK, have a slash at it!

I have never found a comprehensive treatment of a windsurf sail, but this is a summary of the factors that contribute the need for such extreme twist. (At least the ones I have found that make some sense )

-For aspect ratios NOT seen in Slalom sails or windsurf sails in general, to get an ideal span wise loading you need an elliptical plan form. A rectangle plan form is too lightly loaded at the tip, and a triangular plan form is highly loaded at the tip.

-For low aspect ratios (i.e windsurf sail), this effect is exaggerated

-For swept back foils (i.e. Slalom Sails in use) the effect is even MORE exaggerated.

-For Deep draft foils, the effect is further exaggerated AGAIN.

-Where structurally consideration override the desire to have a ideal loading, it is more advantageous to have a greater loading towards the root, and reduce the loading on the tip. i.e. Use a deeper draft at the root, (higher drag) but mitigate that by reducing the loading at the tip. A wide twisted head does this.

You can find plenty of reference for the above in aerodynamic texts if you know where to look.

This I believe is why the twist is so extreme, and not seen in other applications. (Upright, flat profile, freestyle sails don't need as much twist), and high aspect ratio sails on boats don't need this at all.

So for a slalom sail, there is desire for a higher loading towards the root, but you want to achieve this in the most advantageous way possible. Even if they did make 800cm masts, they don't make 3.6metre tall sailors...

Of course sail twist can also be used a trimming aid. (More load on the tip can push the nose down etc)

My 2 cents.

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racerX said..
I have never found a comprehensive treatment of a windsurf sail, but this is a summary of the factors that contribute the need for such extreme twist. (At least the ones I have found that make some sense )

-For aspect ratios NOT seen in Slalom sails or windsurf sails in general, to get an ideal span wise loading you need an elliptical plan form. A rectangle plan form is too lightly loaded at the tip, and a triangular plan form is highly loaded at the tip.

-For low aspect ratios (i.e windsurf sail), this effect is exaggerated

-For swept back foils (i.e. Slalom Sails in use) the effect is even MORE exaggerated.

-For Deep draft foils, the effect is further exaggerated AGAIN.

-Where structurally consideration override the desire to have a ideal loading, it is more advantageous to have a greater loading towards the root, and reduce the loading on the tip. i.e. Use a deeper draft at the root, (higher drag) but mitigate that by reducing the loading at the tip. A wide twisted head does this.

You can find plenty of reference for the above in aerodynamic texts if you know where to look.

This I believe is why the twist is so extreme, and not seen in other applications. (Upright, flat profile, freestyle sails don't need as much twist), and high aspect ratio sails on boats don't need this at all.

So for a slalom sail, there is desire for a higher loading towards the root, but you want to achieve this in the most advantageous way possible. Even if they did make 800cm masts, they don't make 3.6metre tall sailors...

Of course sail twist can also be used a trimming aid. (More load on the tip can push the nose down etc)

My 2 cents.

Thanks for the comments RacerX.

Can you list some of the specific references you allude to that back up your list of theories please?