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      Abbreviated rules   07/28/2017

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aight, i know it s a bit ate now and youve probably allready discussed this deep into some other thread, but what is with these additions on the stays of the ac50

they look like fairings and i am wondering if they were there for aerodynamic gains and why they werent used throughout the entire length of the shrouds?

cheers

shroud fairing.JPG

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2 minutes ago, MR PLOW 270 said:

aight, i know it s a bit ate now and youve probably allready discussed this deep into some other thread, but what is with these additions on the stays of the ac50

they look like fairings and i am wondering if they were there for aerodynamic gains and why they werent used throughout the entire length of the shrouds?

cheers

shroud fairing.JPG

It was explained by Ken Read during the cup match.

They are to prevent excessive vibration due to resonant frequency. 

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It was to stop the rigging from whistling .. it was so loud on Artemis in the early racing the TV audience could not hear the microphone output. 

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This reminds me of Chris Dickson's efforts with the Tag Heuer boat. (AC 1995)

He trialed foil shaped farings on the running back stays for drag reduction. I think there were maybe 4 or 5 along the length of the backstay that were free to rotate and align to the apparent wind. 

They weren't used in racing (from memory). Not sure why. 

They also weren't used on standing rigging as the rules prohibited fairings on standing rigging. Tag was exploiting a loophole to use on the running back stays as they're not considered standing rigging. 

They may have been prohibited in the design rule for AC35 too. 

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One of the rule requirements in recent Matches has been that the rigging can have a fineness ratio no greater than 3:1.  It's hard to get a fairing that meets that requirement to be stable in rotation about the stay.  With a 3:1 fineness ratio, you're not going to have attached flow at any appreciable angle of attack, especially at the low Reynolds number of a shroud.   That drives you to want to make a rotating fairing.

What you need for stability in a rotating fairing is for the center of gravity of the fairing to be ahead of the rotation axis, and for the aerodynamic center to be aft of the axis.  And you don't want the thickness to be any greater than necessary to enclose the stay, because thickness is the biggest determinant of the drag. 

An alternative approach is to accept that the flow is going to separate shortly aft of the maximum thickness point and minimize the width of the separated wake.  If you view the shroud along the apparent wind vector, what you want to do is to minimize the thickness of the shroud in that view.  Of course, the boat has to sail on both tacks, so for a fixed fairing you need to also view the shroud along the apparent wind vector of the opposite tack.  The intersection of the width in those two views leads to a diamond shaped area in cross section.  You want the shroud to fill that diamond as much as possible so as to pack the most material into the thinnest shape with respect to the apparent wind direction.  Rounding off the corners doesn't sacrifice much cross sectional area and makes for a practical shroud section.  That's why a 2:1 ellipse makes for a reasonable shroud design.  You can do better, but not dramatically better.

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1 hour ago, Basiliscus said:

One of the rule requirements in recent Matches has been that the rigging can have a fineness ratio no greater than 3:1.  It's hard to get a fairing that meets that requirement to be stable in rotation about the stay.  With a 3:1 fineness ratio, you're not going to have attached flow at any appreciable angle of attack, especially at the low Reynolds number of a shroud.   That drives you to want to make a rotating fairing.

What you need for stability in a rotating fairing is for the center of gravity of the fairing to be ahead of the rotation axis, and for the aerodynamic center to be aft of the axis.  And you don't want the thickness to be any greater than necessary to enclose the stay, because thickness is the biggest determinant of the drag. 

An alternative approach is to accept that the flow is going to separate shortly aft of the maximum thickness point and minimize the width of the separated wake.  If you view the shroud along the apparent wind vector, what you want to do is to minimize the thickness of the shroud in that view.  Of course, the boat has to sail on both tacks, so for a fixed fairing you need to also view the shroud along the apparent wind vector of the opposite tack.  The intersection of the width in those two views leads to a diamond shaped area in cross section.  You want the shroud to fill that diamond as much as possible so as to pack the most material into the thinnest shape with respect to the apparent wind direction.  Rounding off the corners doesn't sacrifice much cross sectional area and makes for a practical shroud section.  That's why a 2:1 ellipse makes for a reasonable shroud design.  You can do better, but not dramatically better.

 

Interesting stuff^ 

A couple of things. On a rotating fairing, I understand the need for the aerodynamic center to be aft of the center of the stay, but what is the rationale for the fairing CoG being forward of it? Does it dampen or reduce flutter? 

The diamond compromise for fixed rigging makes good sense. It would be interesting to know the relative drag of the diamond vs round section.

Interestingly, the rotating shroud foils on Tag Heuer had a small tail at the center of the trailing edge (similar to an arrow tail). I guess to assist aerodynamic center aft ward. 

I searched for pictures without luck. 

 

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