Underwater foils --- Center / Effort and Percent / Chord?

I have read that the center of effort of an underwater foil (such as a NACA 0012) is at 25% from the leading edge. Can anyone help me with a few questions?

- Is this simple figure actually accurate? Does it differ with different section shapes?

- Does that mean that a rudder based on this foil would be 100% neutral (ie no weather or lee helm) if the position of the post were exactly at 25%?

- As you reach or approach the stall point does the center of effort (now I assume mostly determined by pressure on the windward side) shift backwards?

Thanks much!

 

For "normal" plan forms around AR 6:1 or so the AC is about 25% of the chord. That does not prevent a foils from having a pitch moment relative to the AC. For low aspect ratio foils it is closer to the leading edge, perhaps 1/6c rather than 1/4c.

Don't confuse load on the tiller or steering gear with weather or lee helm. They are not the same. Weather or Lee helm is the tendency for the boat to turn into or away from the wind. The rudder angle to stop the turn can produce force on the steering opposite the rudder angle or against it.

"There are several important problems to consider when determining the center of pressure for an airfoil. As we change angle of attack, the pressure at every point on the airfoil changes. And, therefore, the location of the center of pressure changes as well. The movement of the center of pressure caused a major problem for early airfoil designers because the amount (and sometimes the direction) of the movement was different for different designs. In general, the pressure variation around the airfoil also imparts a torque, or "twisting force", to the airfoil. If a flying airfoil is not restrained in some way it will flip as it moves through the air. (As a further complication, the center of pressure also moves because of viscosity and compressibility effects on the flow field. But let's save that discussion for another page.)

To resolve some of these design problems, aeronautical engineers prefer to characterize the forces on an airfoil by the aerodynamic force, described above, coupled with an aerodynamic moment to account for the torque. It was found both experimentally and analytically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center of the airfoil. Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis. (For supersonic airfoils, the aerodynamic center is nearer the 1/2 chord location.)

Are you trying to fix something, or just wondering about the neutral point?

 

In response to, "Are you trying to fix something, or just wondering about the neutral point?"...

Well, a little of both. I really like my boat in many ways but it has a problem in that the CE of the sails is too far behind the CE of the underwater surfaces. And this is with the mast & rig tipped all the way forward, such that the genoa clew is a couple feet off the deck. The balance is workable in really light air with the 155 up but as the wind increases, the nose goes down a little, and you go to a 135 or a 120 and the rudder has to have 8-10 degrees or more to go straight, upwind. I don't care so much about the heavy weather helm as much as the awful drag the rudder is inducing. Sometimes it will literally stall the rudder if you try to stop the boat from rounding up, unless you feather to the point of luffing the jib. (I should add that I am reasonably proficient at sailing the groove, normally.)

I don't have the money for a comparable size and equipped newer boat without this issue. One option other than living with it is a keel modification or replacement.--Huge job. Another is to build a rudder with more area and efficiency, and let it carry more of the lift to the point where only 4-5 degrees is needed, but to balance the rudder to the point of keeping the tiller forces reasonable. (I have done a lot of composite construction and can do this.) It's a big job but not as overwhelming as moving, modifying, or replacing the keel. Ultimately I want to be able to tilt the rig back where it belongs, which I also think will help overall performance.

Suggestions? Thoughts?

 

Ok, sure.

8-10 deg is a bit on the high side. 7 deg maximum average rudder angle is figure that used to be quoted in North Sails trim seminars. However 8-10 deg is not necessarily huge drag. The downwash from the keel effects the rudder angle of attack to the local flow. It's huge drag if the foil is close to stall.

Is the boat high effort or just high angle?

As a baseline how big is your rudder now? 8-10% of total lateral area is a ballpark number.

If you are into building a new rudder, I'd extend the current rudder to be almost as deep as the keel. Don't change the chord, just extend it as deep as you can. Sail the boat and see what effect the added area and higher AR give you. If it cures or helps the problem, you have a data point to aid you in a new design. If it moves the CLP aft too much, trim chunks off the bottom until you get what you want. Then evaluate the force needed to hold the rudder at the new trim angle. You can compensate for the steering effort with a different area distribution about the rudder axis on the new rudder.

 

Yes, I was thinking the same thing... Set the old rudder aside (in case the new one is a disaster), and build a larger area (higher aspect ratio) rudder not quite as deep as the keel (reduce grounding risks), set the rudder post at about 24 percent of chord, try to hit a NACA 0012, tip the rig back to its normal rake and see if it works. It would be asking a lot of the rudder in terms of lift, but theoretically seems like it should work. A concern is that if the position of the shaft were off even a little bit the thing could get unworkable forcewise, and there is no easy way (that I have thought of at least, to make the position of the rudder post adjustable in the structure of the rudder blade.

Short of major keel mods, I had one other idea. It sounds crazy but folks here seem open minded and helpful so I will bring it up... This is to fabricate and install short chord (12 inches or less) high aspect ratio vertical foil parallel to and about 8 inches behind the trailing edge of the keel, attached to a small end plate extension of the keel at the bottom and the hull at the top. It could be fixed, or could have the capacity to rotate, with a central stainless steel rod. Rotating capacity could occur by extending the rod through the hull with an enclosed rudder post arrangement open at the top for a mini-tiller, which would coincidentally be just at the forward end of the cockpit. By placing the rod behind the CE it would auto rotate to a lifting angle which could be controlled from the cockpit. Of course, this is more difficult than a rudder (but less difficult than a new keel.) My reading on tandem keels suggests that this is less efficient than a single longer keel, (in the right place!), but it might be more efficient than plowing along through the water with the boat "sideways" (a little bit too much), being "held in place" with heroic efforts on the end of an extra long tiller canted at a nasty number of degrees to windward.

Any advice is appreciated!

 

Can you tell us what kind of boat youare talking about?

Why are you talking about tipping the rig back if you have too much helm?

If you put the shaft through the 24% chord line on a 0012 rudder you are not going to be happy.

 

It's a Capri 30. I'm talking about tipping the mast back AFTER finding an answer to the problem of the underwater CE being too far forward. The mast is tipped so far forward right now that the clew of the 155 is a couple feet off the deck, if you can envison the triangel tipped forward. Now for the important question..

Why will I be unhappy with that rudder post position and what percentage would you recommend given my hoped for results?

 

I've raced on Capri 30s. They have raced very successfully for many years, sometimes by very good people. One of them won Class A at the MORC Internationals in Marina del Rey in the early 1980s, until they lost their protest after the final day.

I do not know what problem you are having, but it does not take 8 degrees of helm to get the boat to track. The underwater CE is not so out of whack.

You do have to sail that fat beast FLAT. Same with the 25s.

I would set the rig up without much rake, just a little prebend (maybe 1 to 1.5") Have you had your sailmaker or a really great local sailor take a look for you? It might be worth it to pay someone really good to help you set things up right.

A new rudder would be better, but it is not necessary.

I will not tell you what to do with your new rudder. If I was building myself a rudder to a 0012 section I would put the post at the fattest part, 30% chord line.

 

Rudder Stock / NEVER 30%!

According to "The Princibles of Yacht Design" by Eliasson and Larsson: "By locating the rudder stock some way aft of the leading edge we are diminishing the turning lever and hence the torque on the shaft. It is a good rule of thumb not to place the center of the shaft more aft than in a position which gives an area forward of the shaftline not more than 15% of the total projected area of the rudder. Balancing the rudder more could make the rudder 'steer by itself' in some conditions."
Pierre Gutelle:
" Generally, the feel of the helm is not very pleasant with this type of rudder, especially when the part forward of the stock is large, because the center of pressure can shift to a position forward of the stock, and that reverses the action of the tiller. This can be avoided by never placing the stock furter aft than about 20% from the leading edge."
Eric Sponberg did a really great article on rudder design in Professional Boatbuilder; you could contact him thru his website: www.sponbergyachtdesign.com and he can tell you which issue. You might mention this thread and he might just join in...

 

Lordy, Lordy, Lord. Once again showing how little you know. Do I have to bust you up in multiple threads?

Go to a yard with Raceboats in like the Capri 30 around and see where the stock enters the rudder.

You don't want much more than 15 percent or so of the area ahead of the pivot point. That doesn't mean that's the percentage where the stock enters the rudder.

Here's a hint: Rudders have some rake to the leading edge. Capri 30s have quite a bit.

Here's another hint: How do you get the thinnest rudder possible for the size of shaft required?

 

Hey, I appreciate ALL the input, and will continue to gather information before trying anything. Further opinions or addition information is welcome!

 

I suggest you tilt the mast back again before doing any underwater mods.

Your description of the yachts behaviour is not wind pushing the back end around.
It is wind pulling the front around.
In plan view the CE is off center maybe a couple of feet to leeward.
So you have forces drive and drag acting off the center line to start with.
This is the origin of the turning moment.

Drive at the CE is giving you weather helm, not drag at the CE

The next suggestion is to change those head sails earlier.
I think you might go faster with a bit less sail.

And reduce weather helm.

I hope this helps you out

 

What's the condition of your sails?

 

They are reaching they are triradial laminate sails in the later stages of life but still have fairly good shape. The 155 takes a nice shape but has a bit too much stretch such that as the wind fluctuates it needs fairly constant adjustment. When all of this is said and done satisfactorily I plan on investing in a new mainsail and 155, possibly by Quantum or North.

 

A couple of new head sails is a great investment.

When the wind picks up, and the max chord moves aft in your big genoas, sheet tension cant fix it.

What kind of conditions have you been sailing in lately epoxyman?