Asymetrical centreboards

Hi all,

Looking for some ideas as to where I would go to find info for the right section to build a couple of assy boards for a "off the beach" catamaran.

Or alternatively, if someone has been there before what section (NACA) or other did you use. Might give me a starting point.

Understand the positives and negatives of the assy boards but it comes down to the old adage, "nothing ventured, nothing gained" and looking for that small gain

Thanks

 

NACA 63 serie can make it

 

What small gain, exactly?

 

the 63 series foils make good low drag laminar profiles, but for just recreational boating the OOXX (0008 or 0012) make a good non-laminar foil section and makes a more stall resistant profile, same for rudders too.

If you are trying to get the last bit of speed than the laminar flow foils are a good choice, but the surface finish is critical that it be kept smooth and clean. A laminar foil running "dirty" will have more drag than a non-laminar section such as the 00xx foils. the 00xx series also has a large leading edge radius and will be a little more resistant to damage when beaching and hitting submerged objects.

 

00XX foils are symmetric.
Anyone know how much camber to try so they are Asymmetric like the OP wants?

 

http://aerospace.illinois.edu/m-selig/uiuc_lsat/Low-Speed-Airfoil-Data-V1.pdf
Use the 300K and bigger Reynolds number curves. Consider S7055

http://aerospace.illinois.edu/m-selig/uiuc_lsat/Airfoils-at-Low-Speeds.pdf
Clark-Y is a very forgiving section and may be a good choice for real world use. Some of the higher tech sections are harder to build, have odd stall character, and are thinner.

 

Hi Phil, small gain ... the difference in gaining those few metres to windward, difference between first and the rest

Looked at the Clark-Y but thought it was too "fat" to fit the centrecases when it is scaled. Will look at it again.

Did find a nice 63- series asymmetrical section that might give a "jumping"off point, but still looking at all options

Thanks everyones help is appreciated and still considering everything.

 

You can try S3021 that is thinner.

Don't forget that you can also go with a shorter chord because the cambered section can give higher Cl. That reduces wetted area.

Also by setting some angle of attack for the fin you can reduce leeway and it's drag. Try 5-10 degrees.

NACA 63 series have bad stall hysterisis

 

http://www.boatdesign.net/forums/multihulls/asymmetric-daggers-46018.html

 

thanks John,

will go and have a look, trying to find a start point for these blades.

They are about same size as a F16 blade so you will see my limits in size ...

Thanks again

 

Johnhazel, 10 degrees is too much, imo, very high lift, yes, but also plenty of drag. On my asymmetric foils for Sid, they're set at 3 degrees. Remember you're getting some lift with the asymmetric shape anyway, if set at zero, don't want to be too greedy.

 

Your entire boat has a leeway angle as it goes thru the water.
That means your board has the same angle - to the water.
So the total angle of the board to the water will be that much higher.
3 degrees or less.

If your looking at tables of L/D then check out the TOTAL angle of attack to see what kind of performance you will get.

 

I mentionned serie 63 because C-Cat Hydros team uses a 63 serie asymetric but dont know the % of camber.
You should not hesitate to ask them they are very "open"

May be the asym part is only for the lifting part of the L foil ?

Cheers

EK

 

I am using the the standard aerodynamic AOA where the zero lift angle is the reference. If, for zero lift, your asymmetric foil needs -2 degrees, measured by the angle of the chord line, then your 3 degree AOA translates to 5 degrees when specified as engineers/researchers do it.

The daggerboards we are talking about are like light aircraft wings turned sideways. They have best lift/drag around 10 degrees. Hulls probably have lowest drag at zero leeway. So it makes sense to set the daggerboard at its best AOA when the hull is moving straight and size it to balance the sail force.

If it turns out that the angle seems too great there is a definite possibility that the dagger is too large and is actually making the hull operate at what could be called a negative leeway. It is very likely that this would happen if someone were using daggerboard size based on what was best using symetric foils. A NACA 0009 works well at maybe Cl=0.6. An asymmetric foil that would work well at Cl=0.9 would only need to have 2/3 as much area to provide the same side force. To use a dagger of equal area to one optimized for a symmetric foil would be throwing away the potential reduced drag advantage of the asymmetric fin.

 

I don't think the actual profile will make much of a difference. You are likely to make more gains by optimizing the area of the board to operate the profile at a high lift to drag ratio and maximizing the aspect ratio for low inducted drag.
To achieve this you usually have to reduce the size of the foil so that it produces more lift per area. Disadvantage will be that it stalls at low speeds and high sail force. This is usually only an issue at the start of the race or after a slow tack.
If the foil is smaller it is also easy to increase the aspect ratio by keeping the length and only reducing the chord.
A while a go I used this approach for a light weight 505 crew. Suddenly they were competitive in heavy weather. The regular foil was too big for their relatively small righting moment.