Phromsky said..

MHSA said..
Fortunately the aerodynamicists that worked on the americas cup winning solid wings knew the importance of trailing egde twist to enable a water craft to smash top speed.

And I thought this was the speed forum.
In windsurfing not enough downhaul to match the designed luff curve and twist is so slow.

Its often over done in windsurfing. ..If the top cam looks flat and not producing any curve at all, what's the point of putting it there? ..Neither the America's Cup yachts nor any other speed focused sail craft exhibit anything like the amount of washout some windsurfers are using.

There's a pretty strong surface gradient in windsurfing because the sail is right next to the water, so the wind speed (and thus apparent wind direction) changes rapidly up the mast.

The larger sail craft don't have their masts start right on the water where this effect is large...

sheddweller said..

aeroegnr said..

Phromsky said..

MHSA said..
Fortunately the aerodynamicists that worked on the americas cup winning solid wings knew the importance of trailing egde twist to enable a water craft to smash top speed.

And I thought this was the speed forum.
In windsurfing not enough downhaul to match the designed luff curve and twist is so slow.

Its often over done in windsurfing. ..If the top cam looks flat and not producing any curve at all, what's the point of putting it there? ..Neither the America's Cup yachts nor any other speed focused sail craft exhibit anything like the amount of washout some windsurfers are using.

There's a pretty strong surface gradient in windsurfing because the sail is right next to the water, so the wind speed (and thus apparent wind direction) changes rapidly up the mast.

The larger sail craft don't have their masts start right on the water where this effect is large...

Have you measured it?

I haven't but I would think Ezzy has?

Ezzy said..
How does twist stop the upper part of the sail from stalling?The wind angle at the top of the sail is different than at the bottom of the sail. This difference comes from a wind gradient that exists and causes wind strength to be less nearer the surface of the water and stronger higher up. So, the bottom of the sail experiences more apparent wind, which comes from the direction the board is travelling. The top of the sail, on the other hand, "feels" more of the true wind direction. If the top of the sail were set with no twist the flow of air would not stay attached to the surface of the sail and the flow would stop or "stall which would then have detrimental effects over the entire sail. In a sentence, a sail that does not twist is less aerodynamically efficient.

www.ezzy.com/rig-support/ezzy-basic-sail-design-theory/

aeroegnr said..

sheddweller said..

aeroegnr said..

Phromsky said..

MHSA said..
Fortunately the aerodynamicists that worked on the americas cup winning solid wings knew the importance of trailing egde twist to enable a water craft to smash top speed.

And I thought this was the speed forum.
In windsurfing not enough downhaul to match the designed luff curve and twist is so slow.

Its often over done in windsurfing. ..If the top cam looks flat and not producing any curve at all, what's the point of putting it there? ..Neither the America's Cup yachts nor any other speed focused sail craft exhibit anything like the amount of washout some windsurfers are using.

There's a pretty strong surface gradient in windsurfing because the sail is right next to the water, so the wind speed (and thus apparent wind direction) changes rapidly up the mast.

The larger sail craft don't have their masts start right on the water where this effect is large...

Have you measured it?

I haven't but I would think Ezzy has?

Ezzy said..
How does twist stop the upper part of the sail from stalling?The wind angle at the top of the sail is different than at the bottom of the sail. This difference comes from a wind gradient that exists and causes wind strength to be less nearer the surface of the water and stronger higher up. So, the bottom of the sail experiences more apparent wind, which comes from the direction the board is travelling. The top of the sail, on the other hand, "feels" more of the true wind direction. If the top of the sail were set with no twist the flow of air would not stay attached to the surface of the sail and the flow would stop or "stall which would then have detrimental effects over the entire sail. In a sentence, a sail that does not twist is less aerodynamically efficient.

www.ezzy.com/rig-support/ezzy-basic-sail-design-theory/

Well yes, it does occur.
But how much? And what are the variables?
I would argue that topography and water state are going to be hugely impactful of the effect, and so the amount of twist needed to counter this specific phenomenon will be very variable. As such using more or less twist( reducing or increasing the downhaul) is completely valid, which is kinda what the guy was saying before.

Phromsky said..

John340 said..
I've never had a 4 cam sail destroy itself from bad sail rotation. In my experience with various sail brands, if you rig to the correct downhaul and are prepared to put in the time and effort to adjust the length of the cams either using spacers and even grinding the back of the cam if too long, to suit the bend radius of mast you use, then a 4 cam sail will rotate well.

For John 340. ..The measurements on the sail are often wrong, and in a very significant margin! ..But anyway, I'm normally rigging all my sails within a 2 centimetre margin. My favourite sail size is 7.0 with 4 cams, and depending on the wind I'll normally rig between 30% and 50% washout in the top few panels. And I definitely prefer the feeling of the sail this way.

On the occasions when I've rigged washout all the way into the head the sail has felt worse for it to me - twitchy, less stable and underpowered - And actually if you turn the sail over so that its camber is uppermost and watch what happens to the leading when you rig this amount of downhaul you'll see the curve in the leading edge collapsing centimetre by centimetre which is bound to be killing the bottom end of the sail.

The two sails I'm currently most irritated with (7.0 & 6.8) were bought new by me over the internet just a few weeks apart and from different manufacturers, the second because I was really pissed off with the first, and I actually got unlucky with both of them! ..they were both bad news right out of the bag and rigging first time on my lawn, and ultimately with all spacers removed. ..Buying a brand new sail and then having to grind the back of a cam/s down is too much, in fact its ridiculous!

You are just describing the exact reason you have rotation problems. Forget loose leech for a second, and focus on mast bend vs luff curve.

All that depth forward when you have with sail flipped over isn't good.
It just means you have no where near enough mast bend to match the designed luff curve. Not enough mast bend for the luff curve kills the rotation and as you say, eventually destroys the sails.

The sail wasn't designed to with that much depth at the cams in mind. Maybe you like it like that, and that's fine. But as long as you keep rigging like that you'll keep damaging sails.

You can't really fight the designed luff curve of a cammed sail.

If you want cams with a tighter leech why not try a foil sail or go back to a race board sail?
Rather than under downhauling a race sail made with ****loads of downhaul, bend and loose leech in mind.

It's great you bought the manufactures best specified mast.

But it's wasted and actually the suboptimal mast to choose if you aren't bending it to the manufacturers specification for a given sail.

And bending it properly in a race sails case means a loose leech

Phromsky said..
and the sail doesn't actually come with any instructions at all

If you need written instructions to rig and tune a race sail for optimum performance you are probably better off saving some $$$ and purchasing a freerace 2-3 cam sail and enjoy yourself. Rather than trying to reinvent the wheel with gear that doesn't suit your needs. Reckon you could improve the hour PB if you did

Mr Keen said..

Phromsky said..
and the sail doesn't actually come with any instructions at all

If you need written instructions to rig and tune a race sail for optimum performance you are probably better off saving some $$$ and purchasing a freerace 2-3 cam sail and enjoy yourself. Rather than trying to reinvent the wheel with gear that doesn't suit your needs. Reckon you could improve the hour PB if you did

Has your hour got 40+ gybes in mate? ..as in forced to gybe every 700 metres? ..and I'm 67 and only 66 kilos into the bargain

Just to back up Mathew.
Here's my hour and distance attempt as a 75 year old.

www.gpsteamchallenge.com.au/sailor_session/show?date=2020-01-21&team=2

Admitedly good conditions and 1km runs. Only 34 gybes with two stuffed up

My biggest sail is a 6.6 koncept, rotation is great when set properly.

Phromsky said..
For Mathew, I have stated exactly what I believe to be true in accordance with my experience of 37 years windsurfing. I also have relevant knowledge of aerodynamics on account of being a pilot. And so I stand by every single word I've written above.

..And its been very well known for a very long time that camber induced sails (especially 4 cams) often exhibit poor rotation or worse.

That knowledge of aerodynamics isn't really as helpful as you think unless you have designed and tested aircraft. Pretty much every aircraft ever built is compromised aerodynamically for any number of reasons, you make one thing better then something is worse etc. e.g. a shape with less drag may weigh more or bend more, a better shapes may have undesirable control issues, etc endless permutations.

The same with sails, 4 cams slalom races sails are designed to be used in slalom boards on a slalom course, not even on a foil. Foil race sails typically have a higher aspect ratio and less twist for a reason! Its a good example, because a foil race sail, can handle a stiffer mast setup with a tighter leach, for better performance when apparent wind speed is high. A low aspect ratio can give better acceleration in combination with a railed slalom board, and the twist can assist in giving the required softness so the nose sit where it needs to be.

You mentioned Bjorn at Luderitz, he is using sails cut especially for Luderitz style sailing, not traditional slalom sails suitable for open waters.

The cams are designed to be used in a very limited range of downhaul settings, as a compromise, some allow more tuning than others in respect of downhaul. I look to tune no more than .5cm in any direction with downhaul, and make other adjustments instead.

outhaul,
mast stiffness (shorter mast, softer top, softer mast)
choice of clew,
mast foot position.
boom height
harness line length etc.
fin size,
footstraps

Perhaps you should look at raceboard sails, with an adjustable downhaul, or maybe you should try a foil sail instead, or make/modify your own.

IMHO it all comes down to weight, we are already using exotic materials, if weight wasn't an issue other trimming adjustments might be possible without compromising the primary performance criteria.

Phromsky said..
For Mathew, I have stated exactly what I believe to be true in accordance with my experience of 37 years windsurfing. I also have relevant knowledge of aerodynamics on account of being a pilot. And so I stand by every single word I've written above.

..And its been very well known for a very long time that camber induced sails (especially 4 cams) often exhibit poor rotation or worse.

Facts are facts, your belief is not required. Stating an opinion, is not a stating a fact. You need to provide actual evidence to support your claim.

sheddweller said..

Phromsky said..
For Mathew, I have stated exactly what I believe to be true in accordance with my experience of 37 years windsurfing. I also have relevant knowledge of aerodynamics on account of being a pilot. And so I stand by every single word I've written above.

..And its been very well known for a very long time that camber induced sails (especially 4 cams) often exhibit poor rotation or worse.

Hahahaha! Good one!
I know a few taxi drivers, no way do they know anything much about the design of cars, and bus drivers and train drivers.

Well, i'm quite sure a pilot by necessity must learn things about aerodynamics that the rest of us don't, but the aerodynamics that apply to a plane wing don't necessarily transfer across very well to a race sail. Theres more than a few key differences.

Phromsky said..
The way I figure it is this :

....

The thing is, flying and sailing/windsurfing are very similar indeed.

To me, windsurfing is vertically banked flight at the interface of two fluid mediums, one of my wings flies in air, whilst the other flies in water, and my board seals the interface.

I agree, pretty much every aircraft could be improved aerodynamically very easily, certainly any light aircraft, by simply changing the aspect ratio, and other tricks. Why isn't this done in the first place? Because the shape whatever it is a comprise to reach the required strength, rigidity (resistance to flutter), weight, stability and control, speed and probably a few other things I can't thing of right now...

A race sail, and they do vary is a compromise to achieve its design goal. Recently they have changed to be more foil friendly along with other features, as world cup level slalom no longer seems to exist If the goal was tunebilty or ability to pump, or the most grunt for smallest area, they would be designed differently. Instead the goal seems to be biggest sail that can sensibly controlled by an athlete, simple really.

sheddweller said..

Phromsky said..
The way I figure it is this :

If I could afford to trash a light aircraft. As in saw off the starboard wing at the wing root (which would likely be around 7 to 8 square metres) and then attach a windsurf boom and a board to it. Meanwhile also teach a Silverback Gorilla to windsurf in a straight line. ..Then I could send him off sailing down that strip in Namibia with my aircraft wing and he just might beat Albeau.

No, he would not. Not even close.

Aircraft wings are a lot more efficient than windsurf sails. A wing off almost any light training aircraft could easily be used to land sail at speeds far in excess of 60 knots. ..The limitation on water would be fin/foil cavitation, as it is for other sailing craft.

just rig your 4 cammed race sail like its a 4 cammed race sail. its not hard.

your cams will rotate, youll go fast.

This is the classic topic where the OP posted because he wanted to share his view on race sail cam rotation but is not interested in learning why this view is unreasonable. The best thing is to ignore him and he will go away.