Maximum rudder compensation %

Hello, I will be making a new rudder for my 1985 Hunter 34', one of the reason for doing so is to switch from wheel to tiller steering. Concerning the rudder compensation my main concern is to have the maximum acceptable compensation in order to reduce the autopilot energy consumption. I am OK with a somewhat weak feedback when steering by hand.

Profile will be NACA0012, where do I put the shaft?


Current rudder:


Thanks!

 

the dynamic center of the profile is available usually with the foil data from the NACA archives. However almost all of the older 00xx foils have it at about 25 percent of the cord, so the pivot has to be just ahead of that. The later laminar type foils usual have the dynamic center at about 33-35 percent, so the pivot can be further back.

If you are using the older 00XX type foil, I would think putting the hinge line at about 20 percent would be as far back as you want to go (this would be the average chord location for the average of whole plan form of the rudder). the later laminar type foils you can go back to about 25 percent I would guess, however the laminar foils can be twitchy, and if it is not a clean surface it will revert to turbulent flow behavior and might revert back to 25 percent, so I would not advise going that far back.

 

The data and plots in Theory of Wing Sections by Abbott and Von Doenhoff shows the experimentally determined aerodynamic center at subsonic speeds for a variety of airfoil sections including 00xx, 63, 64, 65 and 66 series are between 22% and 28% of chord from the leading edge, and for thinner symmetric sections it is generally close to 25%. This is in agreement with subsonic, thin airfoil theory which has the aerodynamic center at 25%.

However the 25% figure is for a 2D airfoil section. For a rudder which has finite span the location of aerodynamic center will vary depending on the aspect ratio, planform shape, sweep angle and free surface effects.

At transonic speeds with shock waves present on the wing surfaces the aerodynamic center moves aft. At supersonic speeds the aerodynamic center approaches 50% of chord. The figure of 33% for "laminar flow" sections might be due to the use of "laminar flow" sections such as the 64 series in much of the published work on transonic aerodynamics. The further aft location of the aerodynamic center at transonic speeds is due to formation of shock waves on the airfoil surfaces, not the specific section shape.

 

See if you can hook up tiller steering to the existing rudder and see how it works. There may have been provision for this anyway since an emergency steering system is on many boats.

 

this is all true, but I did not consider that his sailboat will be traveling at transonic speeds, so I left this all out of my response. I did not want to confuse him with with a bunch of scientific mumbo jumbo.

his question what how far ahead of the dynamic center should the pivot or hingle line is it good for his boat to lower the tiller forces. I would put it at about 15 to 20 percent of the ave chord with a 00xx type foil. That is all he needs to know.

 

The "bunch of scientific mumbo jumbo" was included as a possible explanation for the erroneous statement "The later laminar type foils usual have the dynamic center at about 33-35 percent, so the pivot can be further back" might have ooccured. Clearly even if a laminar flow section is used for the rudder the aerodynamic center will not be anywhere near 33-35%.

 

Thanks for the information! As I aim for most compensation, do you know if 20% have been used on some boat with success? For example the pivot must stay in front of the dynamic center even when the rudder stall or a passing wave change the fluid dynamics around the profile...

As for trying out the current rudder, the boat is out of the water until next year, and this one is heavy on the wheel anyway, there is little doubt that it would make a good arms exercise with a tiller! (+ those hunters have a strong tendency for weather helm)

While on the subject, I kind of choose NACA0012 without much research, Marchaj mention using 4 digits with 12-15% thickness, is this opinions shared by naval architects?

 

I almost always use a 10% to 15% foil section. I've gone as high as 20%, as on Copernicus (link: http://www.sponbergyachtdesign.com/CopernicusRud.htm). I tend to use a modified LS(1) or GA(W) airfoil, but NACA 00XX series work well and are very common. In my opinion, people waste way too much time worrying about what foil section they have when they should be worrying much more about profile shape and the structure of how the whole thing is built so that it fits and works.

As for the position of the rudder stock, I always place it at about 15% to 18% chord position of the top chord. That gives enough balance ahead of the rudder for easy steering without either overworking the helmsman or autopilot.

I hope that helps.

Eric

 

It sure help! Seem like 18-20% should create the desired effect and is confirmed by practice

So when you mention the profile shape, what general shape would you use, considering this is a shallow (4'3") draft? If I try to make an elliptical tip, I either loose surface or put it up close to the hull where there is more turbulences. So when constrained by draft, is it worth going for an ellipse or is it better to end it abruptly (as done on original)?

Structurally is there classification society rules concerning the scantling of shaft and internal structure? As I am not an engineer I was going to simply use the exact same shaft as current (2.5" OD, 5/32 Wall) but without a weld just below waterline (the other reason for making a new rudder is the history of failure of these weld)

 

I'm with Eric on this, that it is better to KNOW which side of the balance you are going to come down on. While placing the shaft at the moment center will reduce load on the tiller/quadrant, vessel motion/trim and wave profile/orbitals will could cause all sorts of problems if cut too fine, such as moment reversal. Additionaly, for hand steering, having too light of a helm is downright annoying and very uncomfortable in a following seaway.

Also to consider is the taper of the rudder which effects sweep and the overall CP of the foil. The axis of the shaft needs to be placed ahead of the CP where ever it is, not at a fixed position on the root.

 

Right, JEH makes a good point--the stock has to be ahead of the quarter chord line all through the rudder. Most of the load is along the quarter chord line, so the stock is always ahead of that, usually approximately parallel to the quarter chord line.

You have three things to consider--total area, planform shape, and tip draft. If you stick with what you have already for area, planform shape, and draft, you are going to be pretty close. I presume that the rudder works OK now, so why change those parameters?

Eric

 

I used 16% and it works really well, easy on my Raymarine ST1000

 

The profile have to change a bit because I want to use a straight shaft from tiller to about 3/4 of the rudder down (the rudder tip being "sacrificial"), the original has a bend close to the root: (and a questionable weld location!)



While changing the profile, I wonder if it might be interesting to alter the tip design to to reflect the evolution of rudder design in the last 30 years...But with the limited draft of the boat, it might not be possible to really improve on 1983 design...

For the shaft placement, I will calculate the surface areas so that 18% is in front and 82% behind, not just at the root

So any rules concerning scantling? If not I'll go by what has worked so far(without the weld..)

 

Yes, there are rules for rudder stock and rudder skin scantlings, and they are not easy to calculate--lots of algebra and geometry. If you keep the same area and draft of the rudder, and a similar stock position, then you can stick with what you have since it has worked for 30 years now. Cut open the old rudder and measure the thickness of the laminate. Use modern knitted fabrics to the same total thickness with epoxy resin, and you'll be stronger than before, because they probably used woven fabrics and polyester resin originally, and those are weaker materials than a knitted/epoxy mix.

Eric

 

have a look on standard ISO 12215-8. It is dedicated precisely to the scantlings of rudders of boats.