Foil Design 1970's Keel Boat

[alex_sailor]

Hi Guys,

I am a new poster and sent a message concerning my rudder for a C&C 35 which looks to be re-constructed from the original, but is really messy. Unfortunately, I could not retrieve the answers so here it is again

I have a few questions:
1) Is a NACA 0012 the most likely foil number for this boat? When I measure thickness and chord, that's what it seems like.
2) What is the consensus for the tip? Squared off, rounded off, or elliptical (e.g. Spitfire wing tip) When I look at others, I see all kinds.
3) Should the thickness and chord remain constant through the full length? OR should I taper it?

BTW, a month ago, I posted the same question, but couldn't find the reply which apparently was sent to me

[gonzo]

This is one of thousands on the internet
http://www.google.co.uk/images?q=C%26C+35&rls=com.microsoft:en-GB:{referrer:source}&oe=&redir_esc=&um=1&ie=UTF-8&source=univ&ei=v2RETdS7GsqAhAfSgK2AAg&sa=X&oi=image_result_group&ct=title&resnum=8&ved=0CFYQsAQwBw&biw=1280&bih=685

[alex_sailor]

Thanks Gonzo, but when I clicked on the link, I got lots of images of C&C 35's but no answer to to my questionsl

Alex

[Perm Stress]

Naca 0012 is not the best for rudder of 1970 boat. It is a laminar flow foil, whose main benefit is significantly less drag at 3..5 degrees angle of attack, depending on aspect ratio. For rudders turbulent flow profiles are better, as they tend to have more maximum lift and most importantly, higher stall angles.

[alex_sailor]

Thanks Perm Stress, so this brings up 2 more questions, 1) why would the profile of the hull molding/fairing above the spade rudder fit a 0012 (thickness about 12% of chord)? and 2) what is the appropriate NACA number for the speed range and turning characteristics of the rudder/boat?

[sorenfdk]

Originally Posted by Perm Stress

Naca 0012 is not the best for rudder of 1970 boat. It is a laminar flow foil, whose main benefit is significantly less drag at 3..5 degrees angle of attack, depending on aspect ratio. For rudders turbulent flow profiles are better, as they tend to have more maximum lift and most importantly, higher stall angles.

Perm: For once you are wrong! NACA0012 is NOT a laminar flow foil.

Alex: NACA0012 may not be the 110% perfect choice for a rudder, but I can promise you that it's not far off!

[alex_sailor]

Interesting because, here is what I found since Perm Stress' response:

An underwater AirFoil

The foil shape I decided on is the NACA 0012 foil. It''s a narrow elliptical shape and has undergone thousands of hours of testing by the aero industry. Turns out that its the best foil shape for both lift and the absence of drag for air speeds in the range of MACH 1.0 to 1.2. Also turns out that the fluid dynamics of air speeds in that range are very similar if not identical to underwater fluid dynamics at 4-7 knots. Lucky for us sailors. Its the foil design going into almost all new production boats out there (Hunter, Catalina, Beneteau, etc.)

So, now that has been established, the existing rudder appears to have a slight taper and then at the bottom, it has a rounded off base at the leading edge. My question now is, should I maintain the aspect ratio and taper? How does this get finished off at the bottom? Flat? Elliptical? Does foil no. remain the same throughout?

BTW, from the little I know, the keel appears to have a "laminar flow" foil.

[Eric Sponberg]

Section shape is not really all that important compared to area, balance, and planform shape. The NACA 0012 will work fine, as will literally hundreds of other section shapes. I am not totally familiar with the C&C 35, but for reference, you can see what some really good planform shapes are like on my website.

The rudder for Bagatelle: http://sponbergyachtdesign.com/Bagatelle.htm. Scroll down to the close-up photo of Bagetelle's rudder.

The rudder for Copernicus: http://sponbergyachtdesign.com/CopernicusRud.htm. This was a retro-fit, taking the keel-hung rudder off the boat and installing a spade rudder.

A few years ago, David Vacanti and I wrote a pair of articles on keel and rudder design and construction for Professional Boatbuilder magazine. You can download copies of these articles for free from my website here: http://sponbergyachtdesign.com/Articles.htm. Scroll a little more than halfway down, and you'll see the links to "Keel and Rudders Design" and "Keels and Rudders: Engineering and Construction". This article gives a close-up view of Bagatelle's rudder.

I hope that helps.

Eric

[alex_sailor]

Thanks Eric!

This is really helpful, especially coming from a professional designer! I now even recognize Bagatelle, as she may have been featured in at least an ad in Wooden Boat magazine.

I will let you know how the project develops.

Cheers

Alex

[alex_sailor]

Eric,

I looked at Bagatelle and the plan view looks somewhat similar to the existing spade rudder on my boat except yours has a higher aspect ratio and a bit of an elliptical leading/trailing edge, but the bottom leading edge corner has about the same radius, so that is what I will be lofting to from the existing dimensions. Thanks for all the help.

Alex

[Perm Stress]

Originally Posted by sorenfdk

Perm: For once you are wrong! NACA0012 is NOT a laminar flow foil.

Alex: NACA0012 may not be the 110% perfect choice for a rudder, but I can promise you that it's not far off!

I did mistake 0012 for 65-0012 foil. THEY (6-digit series, for example 65-0012) are laminar flow foils and are not perfect for rudders of hull-speed-limited sailboats.
My apologies to all.

[alex_sailor]

Stress,

Thanks for the correction. I just completed the data entry from NAC4GEN into AUTOCAD to create a relative profile for a 0012 which I will check against the existing rudder. Then I will check all the other dimensions and have to decide whether is is nobler to fair and re-glass the existing rudder, or to build a new one from scratch.

The one on my boat handles well in all conditions, except when we are sailing at high winds and downwind under spinnaker, where she gets a bit heavy but not overwhelming, so I am happy with the balance on the spade.

I am trying to achieve 2 goals here. One is to refurbish it especially around the collar where water is ingressing and saturates the foam and the other is to get a cleaner more symmetrical flow.

Being somewhat conservative and noticing that Eric's Bagatelle design looks very similar in shape to mine already, I want to fine tune more than radically alter.

FYI, the C&C's were originally designed in the 1960's by a partnership of George Cuthbertson, and ?George? Cassian. The former was a boat designer for a long time, but the latter was originally one of the designers of Canada's only supersonic aircraft, the Avro Arrow, in the 1950's and when you look the keel of their first designs, you can mirror it and voila, you will see something very close to a supersonic delta wing. Coincidence?

Cheers and thanks to all.

[Perm Stress]

I'll run in to risk to be wrong again.

As far as I know, sailboats of the type and era ALL are not pleasant to steer downwind. Good boats of the type/era are just difficult, demanding and unpleasant; bad boats even dangerous/uncontrollable in certain cases/conditions.
...
There was a good article in Russian magazine "Katera i Yachty" ("sailboats and motorboats" loosely translated) in ~'80 on the subject, with quantitative method for evaluation of sailboat rudder efficiency, depending on length, height of the rig and distance between rudder and keel. Calculations were confirmed by evaluating rudder system efficiency numbers for some widely used (in SSSR) boats, and comparing results with helmsman's scoring for the same boat types. Later I tried to steer some these boats myself and found perfect agreement between proposed calculations and reality.
....
Generally speaking, conclusion of article was there is no substitute for rudder depth and area, with clear trend: the smaller the boat -> the bigger relative rudder area is necessary for the same level of control.

I may try to find and post it in few days.
It is in Russian, but graphs and equations are a universal language.

[Tim B]

This question keeps cropping up in different guises on this forum. But it all boils down to what makes a good rudder. For a cruising boat, use a simple single or double taper planform, and a pretty standard NACA 00xx section. Don't get too hung up on the section shape, the planform is vastly more important until you're chasing the last 1%.

There are a few thoughts on taper and sweep:

Use "inverse" taper (ie. tip chord greater than root chord) to put a lot of area low down. This can be good for transom-hung rudders, but is not common practice now, as our understanding of rudder fluid dynamics improves.

Use single, or multiple taper swept back. This is fine, but will require a slightly aft stock position from a rectangular planform.

Use single, or multiple taper swept forward. This is also fine, it will require the stock slightly forward of standard, and may produce slightly more induced drag. This is a good solution for a transom-hung rudder, as it will make the rudder feel lighter.

Use single, or multiple taper with no sweep. This is as close to an elliptical rudder as you'll get without a lot of effort. It's a perfectly good solution, but you'll lose area quickly if you apply too much taper.

Tip-shape is pretty similar to foil selection really. It will make a difference, but you probably wouldn't notice it. try to aim for a Kucheman tip (in planform), or something of that sort. A second-order spline tangential to the leading edge forward, and flow direction aft is also a good solution. The tip-shape should really only be the last few inches of the foil (your choice), otherwise you'll lose valuable area.

As for down-wind handling, there is no substitute for a big rudder. That's why modern yachts have big aft-ends and big rudders to stay in control downseas. Big rudders also have the benefit that they require smaller control inputs for given lift (obvious really), and hence MAY (subject to skin friction) provide better drag characteristics, and avoid stalled conditions.

Cheers,

Tim B.

[sorenfdk]

Originally Posted by Perm Stress

I did mistake 0012 for 65-0012 foil. THEY (6-digit series, for example 65-0012) are laminar flow foils and are not perfect for rudders of hull-speed-limited sailboats.
My apologies to all.

Don't worry - these things happen to the best of us

Looking forward to the russian paper - I may have something similar laying around somewhere. I'll post it if/when I find it!