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Old 14-06-2016, 13:33   #16
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Re: Mainsail/Headsail lift?

All I know is that when Bill Lee said to hell with the ratings and started the fast is fun concept he designed masthead rig boats.
And by the way the leading edge of the Americas cup wings are swept back not vertical like your main sail.

When two sails are raised from the same stick they act together as one large wing. Lift or pull is generated at the front of that wing no matter what the aspect ratios of the different components of that wing.
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Old 14-06-2016, 18:02   #17
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Re: Mainsail/Headsail lift?

Stumble,

Yes, I'm aware that the main angle of attack is adjustable.

You are missing the point that a lower aspect ratio sail will produce a higher ultimate coefficient of lift than will the high aspect sail. Quite a bit higher. From memory, a low aspect sail (AR = 1) will produce a max lift coefficient of about 1.57, while a high aspect (AR=6) will max out at something around 1.1. Assuming my memory is correct on lift coefficients, that means the low aspect sail is producing 45% more lift per square foot.

Off the wind, high aspect ratio is less efficient.

This is well established aerodynamic science.
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Old 14-06-2016, 20:02   #18
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Re: Mainsail/Headsail lift?

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Originally Posted by Sarge17 View Post
On a beam reach for example, if a mainsail has the same square foot area as the headsail, and both are trimmed properly. Which one generates the most lift (pull), and why?
Here's a little "light" reading that should take you a long way towards answering that for yourself:

"The Aero-Hydrodynamics of Sailing" by C.A Marchaj
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Old 14-06-2016, 23:16   #19
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Re: Mainsail/Headsail lift?

Pauls,

Yes the lift is higher, but the drag is much higher as well.

I am not sure why this is so controversial. The reason high aspect sails are better is the same reason that gliders use exceedingly high aspect wings, more lift less drag. And the same reason why race boats have moved to progressively higher aspect spade rudders, higher aspect keels, etc.

Not that there aren't downsides to high aspect wings, they stall much easier and are more expensive to make, require higher loads, etc. but they are far more efficient.
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Old 15-06-2016, 13:53   #20
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Re: Mainsail/Headsail lift?

Hi Stumble,

The devil is in the details. Hi aspect wings are best on a sailplane because the high aspect lift to drag is best at low angles of attack. It is unusual to see an angle of attack while flying over about 18-19 degrees, stalls usually occur around there.

A sailboat is not an airplane. Apparent wind angles in the area we're talking about, ie reaching to running, are much greater. You're right that drag on the low aspect sail is greater. However, lift is also greater. When you combine the two forces of lift and drag the combined force can be called the driving force. It is higher with the lower aspect rig from reaches on down.

It's like everything else on a sailboat: everything is a compromise. What works best for one thing rarely is best for another.

Adelie recommended Marchaj's "Aero Hydrdynamics of Sailling". It's a great work but it is also a college level text. His "Sail Performance" is a joy to read, I would say easier to absorb, and will give you a good serving of genuine science on this subject. Marchaj was an internationally recognized aerodynamicist, national champion sailor, consultant to 12 meter teams, etc. We are lucky to have a book like this available on sailing.
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Old 15-06-2016, 16:46   #21
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Re: Mainsail/Headsail lift?

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Originally Posted by UNCIVILIZED View Post
I'm still in the camp of the jib being the HP leader.
This is old thinking.

Catsailors, (Beachcats) discovered it early then the America's Cup boats made the switch to a larger main and smaller jib.

1986: (lower Aspect Ratio, more jib)


2007: (more Main, higher aspect ratio, less jib)

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Old 15-06-2016, 18:12   #22
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Re: Mainsail/Headsail lift?

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Originally Posted by Pauls View Post
Hi Stumble,

The devil is in the details. Hi aspect wings are best on a sailplane because the high aspect lift to drag is best at low angles of attack. It is unusual to see an angle of attack while flying over about 18-19 degrees, stalls usually occur around there.

A sailboat is not an airplane. Apparent wind angles in the area we're talking about, ie reaching to running, are much greater. You're right that drag on the low aspect sail is greater. However, lift is also greater. When you combine the two forces of lift and drag the combined force can be called the driving force. It is higher with the lower aspect rig from reaches on down.

It's like everything else on a sailboat: everything is a compromise. What works best for one thing rarely is best for another.

Adelie recommended Marchaj's "Aero Hydrdynamics of Sailling". It's a great work but it is also a college level text. His "Sail Performance" is a joy to read, I would say easier to absorb, and will give you a good serving of genuine science on this subject. Marchaj was an internationally recognized aerodynamicist, national champion sailor, consultant to 12 meter teams, etc. We are lucky to have a book like this available on sailing.
I have Marchaj, it's sitting on the shelf along side my copy of Bethwaite. But Marchaj focused primarily on pinheaded sails w/o battens, which are pretty far removed from square headed mains that are far closer to the ideal elliptical shape. But then he was writing in 82 (I think was the original publication date) when sail design was not as sophisticated.
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Old 15-06-2016, 18:32   #23
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Re: Mainsail/Headsail lift?

Thomm225,

I am hesitant to use AC boats much as an example of what happens when designers have the freedom to do whatever they want. There are too many constraints on design caused by the class rules that have been traditionally been raced under.

Probably the better example is a boat like Comanche whose design brief was 'win line honors and to hell with the rating.' Given these marchin orders VPLP came up with a pretty close to maximum roach main, 7/8ths non-overlapping jib. Weirdly enough she is the first of a new class, with the mast much further back than you would normally expect. The designers talked about this, and the decision was to pull the rig back to the maximum possible, in order to maximize the spinnaker size for downwind.

The thinking being that any loss to windward is more than made up when you hear off and can put up the fricking enormous spinnaker.

Below you can see her sail plan, basically a 100% fractional jib, and a huge square top main, with an absolutely monsterous spinnaker.
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Old 15-06-2016, 19:35   #24
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Re: Mainsail/Headsail lift?

Good you've got Marchaj. Square heads are a more efficient planform than traiangular, for sure. But planform shape meaning the top view of an aircraft wing or the side view of a sail) and aspect ratio are two different subjects.

If you review Marchaj you will find a chart showing lift, drag and driving force for airfoils with ARs of 6, 3 and 1. The performances of the 3 are quite different. It is wind tunnel data.

Marchaj goes into detail about why the low aspect foil has a higher max lift. It is because the tip vortex, wrapping around the top of the sail, acts as a suction device which keeps the flow attached up to much larger apparent wind angles. The lower aspect sail has a longer tip (top) more parallel to the airflow and thus has a longer, more effective tip vortex.

This effect is largely independent of modern improvements in sail making. It's just an effect of A/R.
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Old 15-06-2016, 20:11   #25
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Re: Mainsail/Headsail lift?

Stumble

Notice how on a reach Comanche lowers that superior supper efficient mainsail so they can carry much larger overlapping jibs that are almost the worst possible shape for an airfoil. Their low aspect foils are providing huge amounts of drag for minimal amounts of power. What's up with all that cantenary adding drag and reducing lift?

I bet when the air gets light they those fools run there headsail all the way to the top of the mast.

I wonder why the designer would sacrifice the "most efficient high aspect and powerful sail" by moving it aft in the boat to alow it to carry more low aspect inefficient sail area forward of the mast?

The 7/8 rig does make a nice tradeoff between the ease of sail shape control and the mighty power of the masthead rig.
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Old 15-06-2016, 23:28   #26
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Re: Mainsail/Headsail lift?

Tod,

They reefed for wind speed, took down the jib, and put up a spinnaker.
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Old 15-06-2016, 23:31   #27
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Re: Mainsail/Headsail lift?

Quote:
Originally Posted by Pauls View Post
Good you've got Marchaj. Square heads are a more efficient planform than traiangular, for sure. But planform shape meaning the top view of an aircraft wing or the side view of a sail) and aspect ratio are two different subjects.

If you review Marchaj you will find a chart showing lift, drag and driving force for airfoils with ARs of 6, 3 and 1. The performances of the 3 are quite different. It is wind tunnel data.

Marchaj goes into detail about why the low aspect foil has a higher max lift. It is because the tip vortex, wrapping around the top of the sail, acts as a suction device which keeps the flow attached up to much larger apparent wind angles. The lower aspect sail has a longer tip (top) more parallel to the airflow and thus has a longer, more effective tip vortex.

This effect is largely independent of modern improvements in sail making. It's just an effect of A/R.
I'll take a look at it first chance I get.
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