Balanced Rudder Design

[h20land]

Hi this is my first time on the site. I have a question about the design of balanced rudders in fast sailing craft.
I'm building a 25 foot sailing skiff. My plan is to build twin balanced inboard rudders (Like an E-scow)so that at least the leeward rudder won't ventilate.(In fact the bearings are half way made and the profiles are cut out.) I'm using what might be termed a NACA 0013.5 section to accomodate the 0.875 inch ss rudder shaft. Are there thoughts / info on the best position to balance the rudder?
Also the foil resulting shows a somewhat convex trailing edge. Does it help to straighten (flatten) this area?

John

[SailDesign]

If you've used a 00 section, and it has a cusp, then it is the wrong shape The 00-series should not have a cusp.
As far as balance goes, some folks go 20/80, and some swear that less is better (say 12/88). A lot depends on how much feel you want versus how little effort you want to out into steering.
Steve

[h20land]

Thanks for your reply, Steve. I'm in the process of refining the leading edge to get the forward area between 12 and 20%. Also I'll check the section to make sure it is actually an 00 section. The program allows me to manipulate the distance aft to the max cord thickness. Perhaps I moved it to far aft. Is a 64 series superior to the 00?
Anyway thanks again!
John

[sorenfdk]

Hi!

Steve might tell you otherwise, but I would advice strongly against the 6X-series of foilsections as they are prone to leading edge separation and tend to stall at a lower angle of attack than the 00-series.
As for the balance, I would make it about 14/86, but if you want to do it right, you should find the center of effort of the rudder and then check the distance to the centerline of the shaft. Larsson & Eliasson recommends a distance of 50 mm (regardless of rudder- and/or boatsize), but we've gone down to 40 mm on a number of our designs (and even 25 mm on one occasion), and they all behaved beautifully on all courses.
Finding the center of effort is easy if you have a rectangular rudder - it's on the 25% chord line at a height that corresponds the geometric center. If the shape is not rectangular, the calculations become a bit more difficult - it will certainly take up too much space to explain it here, som I'm afraid you'll have to work that one out yourself!

Good luck with your project!

Søren Flening

[SailDesign]

Søren says:

"Steve might tell you otherwise,"

Ummmmmm..... Pppppfffffttttttt

I like the 00 series for rudders, but have used the 63-series with success. 'Nuff said.

Steve

[sorenfdk]

Okay-okay! You may just have been lucky, then... (I wanted to place a smiling smilie here, but I just get a "An error occurred"-message! Hope you get the message anyhow).

[SailDesign]

"Okay-okay! You may just have been lucky, then... "

It's only luck when it goes wrong (bad luck, that is) - when it goes right it's called "good judgement". Or that's what they tell me.

Got the message about the smilie.

Steve

[sorenfdk]

Yeah - that's what I mean! You were lucky to posess good judgment!

[h20land]

Originally Posted by sorenfdk

Hi!

Steve might tell you otherwise, but I would advice strongly against the 6X-series of foilsections as they are prone to leading edge separation and tend to stall at a lower angle of attack than the 00-series.
As for the balance, I would make it about 14/86, but if you want to do it right, you should find the center of effort of the rudder and then check the distance to the centerline of the shaft. Larsson & Eliasson recommends a distance of 50 mm (regardless of rudder- and/or boatsize), but we've gone down to 40 mm on a number of our designs (and even 25 mm on one occasion), and they all behaved beautifully on all courses.
Finding the center of effort is easy if you have a rectangular rudder - it's on the 25% chord line at a height that corresponds the geometric center. If the shape is not rectangular, the calculations become a bit more difficult - it will certainly take up too much space to explain it here, som I'm afraid you'll have to work that one out yourself!

Good luck with your project!

Søren Flening

Soren and Steve- thanks for your info. I'll stick with the 00-series for now and I know the range I want to balance the rudder in .I'll use the additional method that Soren sited to check my balance. You've been most helpful.

Thanks
John

[tspeer]

You might take a look at a symmetrical Eppler or Wortmann section. They are more modern than the NACA sections. Compared to the 4-digit sections they'd have more laminar flow, but not be as susceptible to leading edge separation as the NACA 6-series sections.

[danielro]

Do anyone know where can i get the Eppler and Wortmann section data?
Thanks in advance
Daniel

[tspeer]

Most complete collection of section coordinates I know:
http://www.aae.uiuc.edu/m-selig/ads.html

Less complete, but has some data in addition to coordinates:
http://www.nasg.com/afdb/index-e.phtml

[toraikiki]

I know naca 16 series to be best section for hydrofoil. While not for rudders?

[SailDesign]

I know naca 16 series to be best section for hydrofoil. While not for rudders?

Mainly because they are asymmetrical

[toraikiki]

Originally Posted by SailDesign

Mainly because they are asymmetrical

I don't agree. There's also 16-0xx series that's symmetrical one