Sail camber

Also the angle of incidence changes with angle of heel. At high angle of heel, the wind almost goes vertically on the sail. At the other extreme, like in windsurfers with the mast canted to windward, the air moves horizontally. Therefore, the camber measured at 90 degrees is rather meaningless.

 

Yes, luckily for me the boats I sail are dinghies and they should not be heeling ( not to say that they don't but that is just my bad sailing technique!)

 

I think there are several things going on here when sail makers claim there is an ideal camber. One is the sail makers are trying to simplify their design process for marketing purposes (not so much to make it sound like a mystery, but rather are trying to demonstrate their expertise to a non-technical customer). The other is they are trying to down play the importance of the skill of the crew, i.e. buy our sail and you will sail with the winners.

Much of fluid mechanics is not naturally intuitive, the reason it took thousands of years to design and build a controllable glider or aircraft, yet during the same time sailboats have been plying the world's water ways. Sailor developed terminology that is completely at odds to our modern understanding of fluid mechanics, yet these old terms are still is used to teach sailing. Accordingly using accurate descriptions of the mechanics of generating drive from a moving air mass would confuse most of their customers.

The other is that skilled crews do not do fancy analysis of what camber distribution they are acheiving on the rig. They learn by feel when they achieve the best drive, how they can get that much more by pulling here, tweaking there, etc. You can never overestimate the value of that "human in the loop" feed back on getting everything optimized when under way. I am no expert sailor by any means, though I have been in sailboats since I was young, I am highly educated in fluid mechanics and have worked the field, yet when ever I get on a new sailboat I am completely lost as to what I must do to optimism drive, they all feel different. Intellectually I know what we are trying to achieve, but my relative inexperience with it makes it difficult to get all the tuning correct. It is not until I sail with a skilled and experienced crew in that type of boat do I start to get a feel for how it responds to the different adjustments. Even than I am still often fumbling around to get it all correct.

I once worked with an aerodynamics that had worked with a university wind tunnel program to try and assist the US Olympic ski jumping team to try and develop techniques to extend the length of their jumps. By altering their hand and body potions they can actually change the L/D of their "flight", the better L/D they can achieve, the further down the ramp they would land. It was discovered very shortly that the best jumpers had already learned by feel what gives them their longest jumps. IOW, they could arch their back, spread their arms to the side with their palms down and put themselves in the best place to get the longest glide without having to have wind tunnel data to tell them which worked best. When I worked with the US Olympic cycling team to develop lower drag bicycles for the '84 Olympics, we also found that they riders could feel which body position would lower their drag the best.

So the fact that the sailing crew has so much control over the sail shape to optimize drive must also be a major factor in determine how much camber the sail should have for each condition encountered. Particularly if all are sailing in a one design race. Of course if you sell expensive sails, you will not help your bottom line much if you actually admit this to all of your customers. But that is not to say that it is not a valid marketing advantage to say that your sails have been used by the teams that won the race, it just makes the sale much easier to down play the importance of the crew's skill. After all it is much easier to separate a potential customer and his money by building up his ego: "buy our sails and you can be a winner too!"

Please do not misunderstand me, I am not being cynically, just realistic. Each company has to do what they can to stay in business. Sometimes just making a better product is not enough. And sometimes many poor or mediocre products manage to do good business based on apealling to the ego of their customers.

 

Petros, yes I go along with a lot of what you say. I think that very good sailors (olmypic level) end up with a very good understanding of what is fast and what is not. This is becuase they 1. Spend a lot of time testing sails, 2. They work with the best sailmakers in the world, 3. Have coaches that get them up to speed technically so they dont have to learn from scratch - Oh and lastly, they are very good sailors! However most sailboat racing is not done at this level. But I have found that if you study what the olympic sailors do you can piggy back on their experience. If you take a tuning guide for a 470 off the web you can morpth it it into a tuning guide for what ever you are intertested in once you understand the basic underlying concepts.

On the other hand sailors sometimes get it wrong. There is a story about how the Americans came over to race against the British in ~1860 and beat them badly. The American boat had much flatter sails and was in a different league speed wise. Then the British went and made their sails flatter, but they went to the other extreme and made them too flat and for a period of time the British boats were still slow but this time because their sails were too flat. They just couldnt get it right. What this says to me is that sailors do tend to get into trends and if all the boats copy each other they end up with the same rig set up and they go at the same speed. So I am trying to understand the underlying principles so that I can get a faster rig, not by copying the rest and making minor modifications to that.

 

If you are still trying to understand the underlying principles you must first start with the basics: go get a few text books on aerodynamics and read about lift generation (I do not think there any good ones related to sailing yet). And try and forget everything you think you know about sails and sailboats when studying the books. And than apply the same understanding of lift from the areo textbooks to a sail.

 

One of the things that amazed me when I first got involved with sail designers is they didn't really talk about aerodynamics. There was a lot of discussion about what shapes looked good and which ones didn't, and there was a great deal of CFD done to analyze different shapes. But they never said things like, "We're seeing separation here, so we need to flatten the pressure gradient a bit." Or, "The CFD is showing that we have excess wake velocities in this section that raise the induced drag. We should change the planform shape like this, or alter the twist." The language they spoke didn't address what was happening in the flow and how they could change the sail design to address the issues. CFD was used as a numerical wind tunnel to estimate total forces, but there wasn't much use of the data to understand why they were getting the performance differences between the configurations.

There are several groups that are working on instrumentation to measure the pressures on sails. When you ask them, "What pressure distributions do you want to see for a well trimmed sail?" they have no idea. They seem to have a vague notion that if the data were available, they would be really useful in some way. But they don't know what to do with the data they're trying to obtain. Of course, they don't have the resolution needed to capture important phenomena like leading edge pressure peaks, but even if the pressures were measured with high resolution over the entire surface, and those pressure integrated to get the spanwise load distribution, one still could not look at the load distribution and say whether it was optimal or not.

The lowly telltale is still the only piece of instrumentation that really tells us what is happening with the flow and provides a means of adjusting the trim in real time so as to improve the performance.

 

I think partly is that there is still incomplete control of the sail shape, we are only limited to tugging and pulling at various places along the lower edge as the only means of changing its shape. So even if they knew what kind of pressure gradients they would like, there is not that many ways to affect the shape to get it. It would be nice if there was a better way of controlling twist distribution for example.

Of course with a lot of rigid surface wings being used in some classes it is a different matter, but I am not sure if those kind of "sails" are practical for long distance sailing. No way to reef or stow them, not sure how they would hold up over extended use as sea.

 

An interesting observation. When I worked in automobile aerodynamics several decades ago my observation was that wind tunnel testing was the best way to determine oveall forces and moments, and specific local phenomena such as the velocity at a particular location. Good, appropriate CFD was typically a better way to understand the details of the flow.

We had ready access to full size and scale automotive wind tunnels. Depending on what was to be tested models could be production vehicles, either stock configuration or modified, clay models, wood and metal models, or a combination of those. Once during 8 hours we tested 42 configurations of a full size car model. While CFD capabilities have significantly advanced since that time my impression is in many instances wind tunnel testing is still competitive for forces and moments if a model can be obtained economically.

How much wind tunnel testing of sails is currently done? I assume that making sails for testing should be quick and relatively inexpensive given the available software and numerical cutting tables. Photogrammetry can be used to accurately measure actual sail shape during wind tunnel tests.

 

Petros, books on Sail Aerodynamics - I've got the lot ranging from the first book written in 1928 ( got on e-bay for £5 !) by Dr. manfred Currey, up through Marchaj, Bethwaite's 2 books, to Fossati Aero- Hydrodynamics and the performance of sailing yachts. They are all great in their own way but they still miss the point. They dont explain why the theory doesnt match up with the practice. Marchaj even on one page says that the ideal camber is 10% and then shows the data for the fastest 470 sail of the time ( 1977) with cambers of up to 19% ( these sails were french Cherets and were devastating believe me!), and he fails to explain the discrepancy.
They also all use a set of data showing polar plots of varying camber plates which was done by Effiel in 1910- great work by an amazing engineer ( he did some other stuff I think!) but shurely we have some more up to date data than this.

 

Tom, I agree re your comments on sail designers. They dont operate by knowing what they want in terms of camber and draft and then design the sail to suit. They often start with a sail ( sometimes theirs sometimes not!) and then want to make incremental changes and see if it is better. As they work in this way, I suspect that they don't have an understanding of what makes it fast. OK after working on a shape and improving it they can justify the speed increase by saying " we have flattened off the entry", or " firmed up the leach" thats why its better. I have a good friend who makes some world championship winning sails and he is like this. When I ask him what the ideal camber is for my sail in 10 knots he looks at me as if I am from another planet. When I ask him if I should put more mast bend in he says, give it a go and go out and race with it if you win its good.

 

Wish I had more time to comment - but what if your sailmaker gave you the ideal numbers for your sail in the given conditions, how would you go about getting those numbers into your sail while sailing? Much better if he can tell you how much prebend to have, tension in the shrouds in dock, how much outhaul, cunningham, spreader angles, jib halyard etc. - those you can control yourself. You need the eye of a champion, with at least a decade of experience in the class, to "see" the best shape.

Attached a story about modern sail development.

 

Mikko, I wouldnt have any problem hitting the camber values. Once I have worked out what I need. I know how all the controls affect the sail camber ( mast rake, spreader angle, spreader length, rig tension, lowers tension, cunningham, vang.) I can produce cambers between 7.5% and 20%, in most of the sail. I can set the rig up on land and measure these either with a tape measure and string or by taking shots and digitising them ( using UK Sails Accumeasure software). I also can confirm that I am getting these values when sailing by taking shots from a Gopro mounted on the boom. I will eventually ( when it warms up a bit!) do some 2 boat tuning to confirm the settings, but I want to get as close as I can before I get wet, to speed up the optimisation process.

 

Mikko, here is a story to illustrate why I am sometimes sceptical of the "eye a champion" when it comes to sail development. When the 470 class was introduced as an olympic class in 1972 for the games in 1976, most of the top small boat sailors in the UK got into the class. Also all the UK sailmakers started to develop sails. The british thought that they had the best small boat sailors in the world but they did not medal at the Kingston games in 1976. At the Nationals in 1977 a mid fleet british 470 sailor put up a suit of unused sails that he had got from Cherets in France, and he was suddenly winning races and went on to win the week. These sails were then copied by all the UK lofts and the sail cambers in Marchaj's book page 283 are from these sails.

My point is that all the best UK 470 sailors thought that prior to seeing the Cherets they had fast sails. If they had seen the Cherets on land they wouldn't have liked them at all because they looked completely different to the sail profiles that they were used to. But when they saw how fast they were they soon learned to love the "new shape" .
So OK sailors can have a lot of positive input into the sail development process but they are not always right, and I would like to base my optimisation on science then get the sailor ( me in this case), to fine tune the set up. I am just struggling to find the correct scientific basis for this right now!

 

Here is my take on why there is such a discrepancy between the theory and the practice in sail cambers.

Why do sailors use deep sails?

Sailors talk about the power of the sail, by this they mean how the boat responds to an increase in wind -this is apparent by increased load on the sheet and helm and a heel of the boat to leeward. In light and marginal conditions this allows the crew to fully hike out or extend themselves on the trapeze. It also allows the helmsman to sheet the main hard and close down the leach. This he knows is the key to speed and pointing in these conditions. So if a helmsman has a " lack of power" his crew will be last onto the trapeze and he will feel underpowered compared to the other boats, sailors do not like this feeling at all and will do everything they can to power up the rig to avoid it.

What sailors do not appreciate is this powerful rig that they like in marginal conditions is not always the fastest. The greater aerodynamic force that this deep sail produces is not the same as having greater drive form the sails ( drive is the component of the aerodynamic force in the direction that the boat is going, the other component is the heeling force that is acting at right angles to the course). In many cases the flatter sail will produce more drive ( and much less heeling force) than the fuller sail. With the deeper sail the sailor feels a gust and the boat heels and the majority of this extra "power" is going into heeling the boat and very little is adding to the forward force driving the boat. It is very difficult for the sailor to tell when the drive force is increasing or not as it is such a small component of the aerodynamic force. Sometimes they may say that the boat felt "stuck" or " tripping over itself" but most of the time the sailor is unaware of these effects. If this happens the normal reaction is to free the sheets a bit and drive the boat off, which will get the boat going but at the expense of losing a lot of height to windward.

The flatter profile will generate much less aerodynamic force so the sheet loads will be much less, the heeling forces will also be much less, but the drive will be more, so the boat will go faster. The crew will be later onto the trapeze, which is not a problem in itself, but also they will be later having to ease the sheets in a strong gust, as the heeling force is greatly reduced. The boat will be sailing faster throughout, and the flatter profile will allow the sail to operate at smaller angles of attack so the boat will point higher. So this is a case of "Higher and Faster " to windward, every sailors dream!

On the downside, ( there is always a downside!) this profile will be more difficult to trim and sail with as it will have a narrower "groove" as it will give the helmsman and trimmer less feedback when it is wrong. It will also generate less drive when it is eased eg. If the boat has to be driven off through a bad set of waves.

 

Seen it - been there... funny really: I was in 1976 in the games, in 470. The Kingston regatta was special, unusual conditions and no favourites did well. Phil Crebbin was there from GBR, he was one of the favourites definitely, but finished 6th or so. The French world champion was 9th.

As to Bertrand Cheret - he was my mentor and tutor, helping me to get the sailmaking business started. WB-Sails was very close to becoming part of Cheret Voiles, but Bertrand advised me not to - the french sailmaker was in financial difficulties and went bust only years later. I was there when the photogrammetry tests were done, in a field outside La Rochelle. I don't have the Marchaj book, but no-one would sail with sails of that shape... testing sails on shore is problematic, since the wind is much more turbulent than on the water to begin with, and the steady speed wind from the boat's motion, which smooths out turbulence, is lacking. For the test, the wind was way too light, and the wind angle too big, to try to keep the sails filled in the gusty wind. There was not enough pressure for the sails to assume the shape they had on the water. When used, the sails look very different but in the lightest zephyrs.

I don't think you have the facts quite correct in your anecdote... As I recall, Crebbin was sailing with Musto sails, already copied from the Cherets before the Olympics. Cheret was simply ahead in sail development in 1976, as the 470 had been very popular in France all 70s', and Philippe Follenfant was working in the loft, making the 470 sails. He was 4 times 2nd in the worlds in the 70s', but never won. We sailed in the Olympics with the Cheret main, but our own jib & spinnaker. Huebner won with german sails, the only major regatta he ever won.

Don't have time for more now,