# Maximum rudder compensation %

Discussion in 'Sailboats' started by Guillaume C., Aug 12, 2013.

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### PetrosSenior Member

I am not sure 18 percent of AREA is what you want, but 18 percent of mean aerodynamic chord (MAC). the formula is as follows:

The following formula will give the measurement (chord) of the MAC. It does not give the span wise location of the MAC. Span is not relevant to locate MAC on a straight taper plan form.

rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)

MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))

Also, I would think a straight taper would be much easier to make and it is still very efficient. leave a flat "cut off" tip, it is more efficient than rounding it off, and more durable, less likely to be damaged if struck.

The racers use elliptical plan form rudders with sharp edges at the tips, the foil shape has to be accurately maintained all the way to the end, to get just a slight advantage. They are easily damaged and are very sensitive to profile. That is why if I were doing it I would build a straight taper with a squared off end.

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### Guillaume C.Junior Member

Thanks for that clarification, I was not familiar with the MAC concept. It seems that if the pivot goes through 18% of MAC, then there will be 18% of area in front of that pivot, right? And in a same rudder blade, there are many ways to position the shaft to get this result, then...

Should I make the sweep/shaft so that when the boat is heeled over and the top part of the rudder is less efficient (out of the water/close to waterline), the rudder loose proportionally more compensation in that part of the rudder? Not sure if I am clear here, so basically I propose to have the shaft relatively aft on the root chord than on the tip chord, passing through 18% MAC. (simple trapezoid shape)

If the helm is well balanced in normal condition, I don't want it to have "moment reversal" when a gust pass by. Might be better to have a stronger helm feeling in that case...

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### Eric SponbergSenior Member

Guillaume, see the attachments. You want the rudder stock centerline to be forward of the center of area of the rudder, as well as forward of the quarter chord line (the line that runs down the rudder at the 1/4 chord position behind the leading edge. The 1/4 chord line is by definition at 25% of chord, so that is why we choose 15% - 18% of chord position (not area ratio). This assures that the center of lift on the rudder is always behind the stock centerline, and that will always cause the rudder to seek home to centerline in all conditions if the tiller is let go.

The color attachment is an excerpt from a page of a PowerPoint presentation I gave at IBEX in 2004 on keel and rudder design and engineering. The two B&W pictures are excerpts from the ABS rules on rudder design and engineering that I used in that presentation. These give you some sense of the acceptable geometry of rudders. Note the center of area is marked, also shown are the mean chords located at about half draft of the rudder.

The geometry of the stock will be determined both by rudder shape and where the top end of it is supposed to end up at the upper bearing. You want to keep rudder stock sweep as small as possible. The more stock sweep, the harder it is to steer--more stock sweep means more load on the tiller. Then there is leading edge sweep, and the taller the rudder is in relation to the chord (i.e. the higher the aspect ratio) the less sweep you want. Since you have a draft restriction, simply following the proportions of the rudder that you already have is a good starting point.

For anyone else reading this, my PowerPoint presentation was published as an article in Professional Boatbuilder (PBB) magazine in issue #96, Aug/Sept 2005, "Keels and Rudders: Engineering and Construction." Also, Dave Gerr wrote a series of three articles on steering system fundamentals, which included both sail and power boat rudder design, that appeared in PBB issues #98, #99, #100, in early 2006 (Jan-May). You can buy individual issues from PBB at a modest cost. Link: http://www.proboat.com/

I hope that helps.

Eric

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### DCockeySenior Member

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### Guillaume C.Junior Member

Thanks again and it sure help, as I have access to all issues of PBB I will certainly take a look at those articles.

Concerning the placement of shaft in relation to rudder blade, your third image actually show pretty well what I was trying to describe as possibly problematic: the compensation is proportionally higher in the lowest part of the rudder. If such rudder loose effectiveness close to WL, is it not as risk of being over-compensated? Seem like the opposite effect would be more desirable

In case of Bagatelle's rudder, I am curious as why you designed it with so little compensation considering it is steered by tiller? If you want to put your presentation online, I don't think anyone will compain!

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### Eric SponbergSenior Member

7. Joined: Dec 2001
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### Eric SponbergSenior Member

The third figure is just a generic plan of a rudder taken from the ABS Rules which is meant to elucidate some of the defined dimensions, not necessarily to be an accurate representation of an actual rudder. For example, the figure shows that the middle area of the rudder has the leading edge forward of the upper end and lower end leading edges--the leading edge is bowed to the right. I would not design a rudder that way. But what is important in that figure is that the stock centerline is forward of the center of area, which is that little center mark in the figure. The dimensions that are shown are meant to define the variables for those dimensions so that you know what you are using in the many rudder equations in the ABS rules.

Depending on your rudder profile area and the placement of your stock centerline, you may have some differences along the span of what percentage the stock centerline is on any given section. That's not too critical in overall rudder design. What is more critical is that as a whole, the rudder stock is forward of the 1/4 chord line, but not too far forward. If you can keep the rudder stock at about the 15% to 18% chord position through most of the span, you are doing OK.

The stock on Bagatelle's rudder is at the 18% chord position, right within my design parameters. I think the PBB article (which as David Cockey pointed out is on my website) pretty well covers what I said in the PowerPoint presentation. To post it on my website, I would have to make a video with my speech delivery so that the whole topic would be understandable. I really don't have time to do that, not now anyway. But thanks for the suggestion.

Eric

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### PetrosSenior Member

this would only be true for a straight or straight tapered rudder blade (and had a 25 percent line with no sweep). If it has sweep than you have to locate the MAC 18 percent point mathematically. It can also be located graphically as in the following image again for straight taper plan forms. the pivot axis in the illustration should be perendicular to the root line, where it must pass through 18 percent MAC, which would put it much further aft on the root cord. with a swept rudder lifting out of the water, the effective hinge line would actually move forward relative to the part of the rudder remaining in the water, there would be no chance for control reversal.

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