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Why a sail that pulls does not necessarily have power
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ittiandro



Joined: 22 Nov 2009
Posts: 251

PostPosted: Mon Aug 26, 2019 8:35 pm    Post subject: Why a sail that pulls does not necessarily have power Reply with quote

I often hear that a sail that pulls, however so strongly, does not mean that it has power on it, at least not a power proportionate to the strength of the pull.. It can pull a lot on your arms, but with little traction forward.

Not knowing the physics of the sail beyond a superficial grasp, I find this statement difficult to understand, even somewhat contradictory..

I always thought that when the wind pulls the sail away from you, even to the point that you need a harness to counterbalance this pull, it is because it is fully powered in proportion to wind speed , which is exactly why the sail pulls away...

But I own my ignorance and Id like somebody to explain it to me

Thanks

Ittiandro
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windward1



Joined: 18 Jun 2000
Posts: 1155

PostPosted: Mon Aug 26, 2019 10:10 pm    Post subject: Reply with quote

You should ask the people that you often hear that from. However, it may mean that the sail can be badly shaped. If it has pull, it has power, but where is that power directed? So if the sails shape does not put the center of effort of force in the appropriate place, the sail may pull like hell, but all in a sideways direction rather than with any forward component. Take an extreme example where the sail is so badly shaped it is as if all of the battens had been put in backwards. The deepest "belly" of the sail is towards the clew and leech. That sail would sail terribly. All the power or pull would seem mostly in the downwind direction and not with much, if any, forward component.
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ittiandro



Joined: 22 Nov 2009
Posts: 251

PostPosted: Tue Aug 27, 2019 9:47 am    Post subject: Reply with quote

windward1 wrote:
You should ask the people that you often hear that from. However, it may mean that the sail can be badly shaped. If it has pull, it has power, but where is that power directed? So if the sails shape does not put the center of effort of force in the appropriate place, the sail may pull like hell, but all in a sideways direction rather than with any forward component. Take an extreme example where the sail is so badly shaped it is as if all of the battens had been put in backwards. The deepest "belly" of the sail is towards the clew and leech. That sail would sail terribly. All the power or pull would seem mostly in the downwind direction and not with much, if any, forward component.


Thanks for your explanation

Is this , then, why the boom is designed not in the symmetrical shape of the arc of a circle, but ellyptically, with a bulge towards the mast, tapering down towards the leech , to constrain perhaps the sail's shape towards the leech? If this is not the reason of the boom's shape, it certainly seems to correspond to the ideal shape of the sail , located towards the mast for forward traction.

From what I understand from your explanation, if the shape didn't have this forward component, the power would be directed laterally or even backward. It makes sense. I cannot understand, however, why the shape of a spinnaker on a sailboat is completely center oriented, almost symmetrical like a baloon...But maybe the sail on a sailboat is another ball game,,,,

Ittiandro
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isobars



Joined: 12 Dec 1999
Posts: 19264

PostPosted: Tue Aug 27, 2019 10:01 am    Post subject: Reply with quote

A spinnaker is designed and intended to run pretty much before the wind ... i.e., downwind. It acts more like a parachute than a foil.
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windward1



Joined: 18 Jun 2000
Posts: 1155

PostPosted: Tue Aug 27, 2019 11:30 am    Post subject: Reply with quote

The boom is not there to shape the sail. The boom is that shape because the sail is that shape. And a good sail is that shape because the sail maker cut the sail pattern so when stitched to be a proper airfoil shape and designed the battens to help provide that shape.
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windward1



Joined: 18 Jun 2000
Posts: 1155

PostPosted: Tue Aug 27, 2019 11:30 am    Post subject: Reply with quote

Duplicate erased.
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ctuna



Joined: 27 Jun 1995
Posts: 881
Location: Santa Cruz Ca

PostPosted: Tue Aug 27, 2019 1:37 pm    Post subject: baggers and bed sheets Reply with quote

A deep sail generates lots of power but a flat sail gives lot's of control.
If you look at the early sails they were rotating bags with no stablization
like battens and shaped seams. Result big power but no control .
Trends change
Early on battens and cambers where used to stabilize sails such that
they did not deflate in a lull and pull your arms out in a Gust.
All kinds of design changes where made eventually seam shaping
made cambers unnecessary in Rec , Wave and Freestyle Sails.
Then there is aspect Ratio sails are trending toward short and wide
from tall and narrow.

A sail is suppose to act as a Wing
Wings have different lift to drag ratio's

Rigging is and Art and matching the mast to the Sail is essential
to get maximum performance.
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dllee



Joined: 03 Jul 2009
Posts: 4593
Location: East Bay

PostPosted: Wed Aug 28, 2019 2:02 pm    Post subject: Reply with quote

Takes skill to make a powerful sail drive you forward.
Just hold on to get a bigger sail to drive you forward.
Skill can take years to attain.
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U2U2U2



Joined: 06 Jul 2001
Posts: 5065
Location: Shipsterns Bluff, Tasmania. Colorado

PostPosted: Wed Aug 28, 2019 4:40 pm    Post subject: Reply with quote

So happy to see a post thats has no reference to foils.

Isobars post about parachute and foils not those foil, may actually make sense.

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4Boards....May the fours be with you

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MalibuGuru



Joined: 11 Nov 1993
Posts: 8577

PostPosted: Wed Aug 28, 2019 5:03 pm    Post subject: Reply with quote

Power is produced by a high and low pressure differential. I enjoy using Neil pryde sails because after trying a vast array of sails, they have the widest range of any of them.
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