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A bit of a long shot... Foil shapes


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Hi all,

I'm trying to locate an article on foil shapes from Australian Sailing magazine, February 1998, about foil sections, by a guy called Neil Pollock.

 

Following an awesome little regatta last weekend, I find myself in need of another rudder, this time complete and near indestructible. Evidently I needed much more glass near the stock.

 

Pollock's article was a then-new formula for calculating shapes for flat sided foils, which had proven to be better than the NACA sections.

 

If anyone has a copy, I'd really like to steal it.

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Does this help even if it is not the article you are looking for?

http://www.boat-links.com/foils.html

 

Added 11 h later:

 

Why do you need flat-sided foils? Class rule?

 

Beating my own drum (yes, about dagger boards but still true for rudder blades): http://hem.bredband.net/b262106/pages/daggers/index.html

 

/Martin

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Sorry for the delayed response, big days!

 

Moving on...

 

Pollock did a huge amount of practical testing and found what seemed to be the best section for stall resistance, drag etc had flat sides, which went against all previous research.

 

From memory, his initial idea was for 470 foils, which require flat sides with limited shaping. Again from memory, he then went on to high performance boats and found the flat sides outperformed the NACA 00xx sections.

 

He also coined Pollock's law...

If practice conflicts with theory, theory is wrong.

 

Not an actual law, but still something I tend to utilise.

 

Moving on further...

 

I recently built a super light rudderblade for my Noelex 22, about 25-30-45% le/flat/te which balanced and gripped well but due to the limitations of the rudder stock, and me not having time or sense, was way underbuilt and snapped off in a prestart.

 

Since I'm starting again, I thought I should try to find the formula's and put some real effort in, as well as some carbon.

 

If I can't find the article, I'll try to replicate the broken one, but stronger.

 

Then I'll build a stock to match.

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The finer details of foil design are extremely complex, you need to get on the aerofoil nerd forums and learn Xfoil. When I did my keel I used http://airfoiltools.com/airfoil/naca4digit and had a play around. Once you get close to what you want then I got a freecad and printed them out full size in pdf format on the office copier and glued them onto ply.

 

What i learned is that there are some basic principles, thin is fast downwind but stalls, thick gives more lift upwind but more drag... Front 1/3 is most important, as is symmetry, high aspect ratios are generally more efficient but you are limited by draft/rules etc. For rudders the accepted norm is NACA 0012 for pretty much all non planing garden variety boats as it gives a good balance between lift/drag/stall

 

Flat sided foils are not necessarily better or worse, chuck them in xfoil and see if it works for your specifications. Because rudders operate at an angle then you are making areas of pressure differential so on a flat sided board its making a virtual aerofoil shape. Where a flat (and thin) rudder will definitely be faster is downwind, think knife through water but in all foil design there is tradeoffs, no magic design that does it all :) 

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