70's Keel Upgrade to NACA 0010 Profile ?

I've heard that my O'Day 37 (70's) keel design has low drag below an angle of attack of 1 degree, but stalls above that, greatly increasing drag under most sailing conditions. My current keel design is a NACA 0018-35 (max width of 16-18% "C", located 40% aft). O'Day actually made the leading edge "needle like" to further reduce low angle of attack drag.

Should I replace my keel with a NACA 0010 profile (max width of 10% "C", located 35% aft) to greatly increase the angle of attack before stalling, and reduce the drag under most sailing conditions?

 

You'll find a discussion of just this question at http://www.boatdesign.net/forums/showthread.php?t=9136

 

Excellent Thread on NACA Keel Shapes

Do any of the available software packages accurately calculate the stall angle of attacks, drag, etc. for the NACA keel shapes?

 

We have used Profili 2 (Pro version) to optimize several keels and rudders with some success (a couple of world championships, one olympic bronze and a couple of national championships). Of course these championships were not won just because of our effort, but still...
I must admit that I don't know how accurately Profili 2 works, but it's based on Xfoil, which I guess is no. 1 in it's class. Anyway, precision to the n'th degree isn't really necessary.
Besides Profili 2, we also use software from Hanley Innovation and - to a lesser degree - xflr5. We wouldn't live without any of these programs!

 

XFOIL (Profili is a graphical user interface for XFOIL) is the state of the art for calculating the lift and drag. It probably over-estimates the maximum lift somewhat, but the trends due to different design variations are probably OK. It's the only program out there that does a good job of handling laminar separation bubbles. You can compare the computed results with NACA data to satisfy yourself as to its accuracy, then compute the data for Reynolds numbers that match your sailing conditions.

XFOIL will easily generate NACA 4-digit keel shapes. But the real power of XFOIL is its ability to design shapes based on the pressure distribution - in effect you tailor the fluid dynamics and it tells you what shape will meet your specification.

I'd take the coordinates of your existing keel and put them into XFOIL to see what you've got to start with. For example, no foil stalls at 1 degree angle of attack! And the sharp leading edge is likely to lead to a pressure spike that causes premature separation and drag.

Then I'd go to MDES (changes whole foil shape at once) or QDES (changes only the portion you specify) to smooth out the pressure distribution, eliminate pressure peaks, etc. Then go back to GDES to adjust the geometry (perhaps you need to change the thickness, etc.) and finally analyze the new shape in OPER to get the polar. You will probably have to go back and forth a number of times to home in on what you want. When you're computing the polars in OPER, go to the VPAR menu and set Ncrit to something like 3 versus the default value of 9. This seems to make the transition match better with tow tank data in water vs wind tunnel testing in air.

Using XFOIL this way will let you fix your keel with the minimum amount of change. Often only a modest amout of re-profiling can make a dramatic difference. Especially if the original shape was just done by intuition.

 

Did you read Vacanti's article in PBO?
http://www.proboat-digital.com/proboat/200506/?=pg1
It's on page 76 "Keel and rudder design".
Quite interesting on how to use theoretical data in practical design.

 

Keel Modification Software

Thanks for your recommendations!

I am currently evaluating Profili, XFoil and Wings 32.

Based on these and the "Theory of Wing Sections", changing the leading edge of my current "70's design keel", may be a big help.

I'm interested in feedback from anyone who might have already benefited from such an analysis?

From snowy and cold Wilmington, Delaware

Captain Rich

 

Take a look at this thread: http://www.boatdesign.net/forums/showthread.php?t=8743. It has an example of using XFOIL to solve a problem similar to yours.