Sail camber

I have been reading some of the books published on sail aerodynamics to try to get a better understanding on sail camber. They all say that the "ideal" camber is in the region 10-12%. But when I look at the fast sails around some of them have cambers up to 18%. I would be interested in any theories why this is so. Are the theorists wrong? Or do the sailors just like deep draggy sails? ( this is all in power wind speed conditions 8-12knots).

 

What is the material of the sails you are looking at? How did you measure the camber? Are they used with a bendy mast?

 

For some hints see http://www.wb-sails.fi/Portals/209338/news/98_11_PerfectShape/Main.htm

 

The sails could be mylar laminates or dacron it doesn't really matter. I have measured cambers by a string and tape measure, but I have also digitised shots of fast sails from the web. I am confident of the numbers. The flatest dinghy sails that I have found are on 29ers and 49 ers. which are 10-12% I was wondering if this was the influence of Frank Bethwaitemon on his son the designer. Bethwaites books talk about 10% being ideal apart from when lumpy when it goes up to 12%. Even on the WB sails website they have a masthead shot of " the fastest 470 sail " and it has a camber of ~14% in the head. I have shots of North SailsnSnipe sails with 18% in the head now that is a long way from the "ideal" 10%.

 

Is this related to boat speed? Slow boats generally point lower thus the rigs operate at larger angles of attack, therefore they can tolerate a deeper sail. Where as faster boats pointing higher are more close to wind tunnel experiments and a lower drag flatter profile that can work at smaller angles of attack is preferable. These boats will also probably be overcanvased so they will soon be in depower more and the flatter prfile again will be beneficial. Miko -put me out of my misery!

 

Mylar and Dacron sails should have very different cut and depth on the camber. If you are measuring the same, they are either cut wrong or the measurements are off.

 

I would have thought that the flying shape of the sails should be the same, whether they are made from dacron or mylar. I would accept that because of the greater stretch of dacron the cut may have to be slightly different for both materials to achieve the same shape from the 2 materials. From the wind's point of view it doesn't really care what the sail is made of what is important is the amount of camber and its distribution around the sail ( eg. Head 12%, mid 10%, lower 6%).

 

The stretch of the sail makes the camber change with wind speed. Also, the camber can be changed with mast bend, halyard tension, outhaul, cunningham, etc. The changes are more dramatic in a Dacron sail because the fabric is less stable.

 

the term "ideal camber" seems such a misnomer, what is ideal depends on what your goals are for the sail. Each point of sail, every different wind speed, and even size and shape and configuration of the sail profile, and also if a jib or other foresail is used, will affect what "ideal" is. The size and shape of the mast will also affect sail shape too.

A sail is essentially a variable geometry lift device, you can alter and change is shape as the conditions require.

To get what they call "drive" (same as lift on an aircraft wing) you need to accelerate the mass of the air flow over a surface, curving the airflow is what causes the acceleration. the more curvature (or camber) the more lift is generated. Poor shapes still generate lift, but they also generate a lot of drag, which in most cases on a sail boat will increase the heel angle. Most sailing is not done to maximize lift, but to minimize heel. Most people will reef the sails for example to keep the heel angle manageable. so what is the ideal camber when you want to lower the heel? If you are just trying to get the most drive (or lift) out of the sail, why not just put the largest sail you can fit on your boat? Because it will heel over and be unmanageable. so it is a more complicated question I think than simply "ideal", the question is, ideal for what conditions?

The whole question seems problematic because it just depends on what you are trying to achieve to know what is idea.

12 percent camber makes a good performing airfoil, it balances a number of trade off that are desirable in aircraft. But in a boat your are not trying to keep a vessel suspended in the air, but only make head way with minimum heel angles.

I think however your take on either Mylar or Dacron, the camber you are trying to get should be the same. It just takes different means to get it with different materials.

In light wind you want more camber to get more acceleration of the slower moving air, in high winds you want less camber to minimize heel. Of course in really light wind you want both more area and more curvature; hence the need for a spinnaker. Is there an ideal camber for a spinnaker? I just depends on what you are trying to achieve.

BTW, there is a silly statement made by old sailing salts, in light winds you want more curve in the sails to "catch more air". The sail "catches" the same amount of "air" be it flat or fully curved, the difference is the amount of acceleration you get as the air moves over it. so it may be an easy thing to remember, and kind of works, the reason is false. Remember the old force=mass x acceleration. F=ma. You get more lift with more acceleration, that is curvature on the surface, or camber.

I hope this helps to understand how sails operate.

 

Thanks- yes I realise that the sail controls will affect the camber for example by app
Lication of the cunningham and vang I can reduce the camber of the head of the sail dramatically from full at 12 % to flat at 7.5%. I have now a good handle on how all the rig controls affect the camber of the sail in the various areas. I alos understand that there is no ideal sail shape. I am able to produce a realtively low camber profile 10% for very light 0-6 knots, a "full" profile for power conditions 8-13 knots and flat profiles down to 7.5% for winds in excess of this. This is not an issue - what is however is what should the power camber be -10% ( Bethwaite /Marchaj), 14% ( Mikko's fast 470 sail) or 18% ( north sails Snipe profile). I suspect that this may be boat speed dependant ( i explained how this might be in an earlier post) but I am just guessing on this. Shurely someone knows. I am not a sailmaker, I am a scientist who is a sailor who wants to go faster!

 

Where I am coming from on this is that each type of boat may require a different sail camber in 10 knots of wind. However I am suspicious of this, as it well be mumbo-jumbo from the sailmakers to mystify their art and keep us from hetting a real handle on what is going on. When the aerodynamists put a cambered bit of metal in a wind tunnel and measure lift and drag vs. camber it is not a 470, or a Snipe, or a 505 it is a bit of metal. The data that they produce should be applicable to any sail on any boat. They say that 10% camber is optimal for 10 knots of wind, but we are seeing fast sails with 14 and 18% camber in reality. So what is going on? This is not a small discrepancy its is nearly double!! Either the theorists are wrong - they have oversimplified the model and their conclusions are invalid, or the sailors are wrong- they just like deep sails, they are easy to trim, have a wide "groove" and have a lot of feedback ( ie high sheet loads), or maybe they are both wrong?

 

Did you read the ingress of Quest for the perfect shape: "Have you ever wondered that while text books describe in detail how sail shape can be adjusted through various trim lines - cunningham, outhaul, boom vang, backstay, runners etc. - they never tell you what the best shape is - what shape to strive after with all those trim lines? The reason for this is quite simple: There is no such thing as the best sail shape - there are countless different "best" shapes, depending on the wind, waves, boat type & size, even weather & air temperature.

Furthermore, for a given sail the shape varies from the foot to the top, and the fast shape of a mainsail is different from that of a jib which is different from that of a #1 genoa, not to mention a spinnaker."

It all depends on the boat: Some boats want to sail higher (for max VMG), some lower, some sail faster (smaller apparent wind angle for the same TWA), others slower.

For a sloop, it's not the camber of one sail only - see "think one": "Camber" starts from the luff of the jib and ends at the leech of the main, and is constantly adjusted through the mainsail sheeting angle.

Last but not least, camber is only one shape defining property of the sail - twist from foot to head is another, and as a matter of fact more important than camber. Performance is much more sensitive to changes in twist than camber, although both depend on each other: Usually, if you change the twist of a sail you also change the camber.

In the attach, 49er sails shape analysis from VSPARS. While the main may be only 7% deep here, the jib varies from 8,5% to nearly 15%. In a little less wind, the main would be much fuller, though.

 

A sail is a wing that generates lift to puch the boat. camber affects how much lift is generated by a wing of a certain area with a certainl wind velocity. For most dinghy classses the wing area is fixed, so changing the lift is done by changing the camber and the angle of attack. The boats you are talking about also have vastly different resistance curves, which means that vastly different amounts of lift are required to push the boat. There is also a huge impact due to waves. That is why where it is allowed boats will have several sets of sails.
Boiled down, the question you are asking does not have a single "right answer". More camber will produce more lift, but have more areo drag. Sometimes that might be necessary, other times not.

 

OK some boats have higher resistance hull forms than other, but for all boats shurley you need to maximise the drive from the sails. By drive I mean the component of the aerodynamic force which is parallel to the axis of the boat ( the other force perpendiculra to this is the heeling force). Now if you want to sail your boat that has a high hull resistance at an angle not so close to the wind to increase the drive and gain speed but lose height that is fine, as then you would have a different optimal camber profile ( deeper) as it is working at a higher angle of attack. I can buy that so as I previously said it could be hat slow boats ( with high hull resistance may go faster with deeper sails as they are not pointing so high.

 

Mikko, thanks for the numbers on the 49er. It is as I had thought a camber of 7%. I would surprised if they sailed with any more than 12% camber on this sail. I still don't really get why deep sails ( cambers greater than 12% ) work at all, ( unless you want to reach around the course!). I understand your points on considering the rig as a whole and not 2 sails. Also the twist issue is important, but I am trying to get a basic set up and I can get any camber from 7.5% to 22% and for now Im going for 13.5% camber at the hounds for 10 knots and reducing it down to 10% when it gets very light and down to 7.5% when it gets windy. I'll give hat a go and see how it goes!