Keel : Rudder Length

Concerning Lift.

If the board has a flat profile and the rudder a NACA 0012 profile then is the board only producing lateral resistance and the rudder producing both lateral resistance and Lift?

In that case would all lift be generated by the rudder and how would this affect windward performance?

 

lift

Alixander, both foils are producing lift in the course of developing lateral resistance. The flat profile won't be as efficient in doing so as the properly shaped foil.I'm not 100% sure but the more efficient section on the rudder MAY have an effect similar to increasing it's area as compared to the flat profiled daggerboard.
Were both original foils flat? And by flat do you also mean squared off edges?

 

Alixander Beck: I would argue that holding the rudder at 0 degree is the same as a "free" rudder. either way you slice it, the boat will reveal its helm balance with the new rudder if the foil is held at CL.

Careful Alixander, as far as the air and water are concerned, a geometrical line on the hull is irrelevant to how the boat behaves. Even Lorsail agrees that the rudder will exert a force on the boat when held firm on the centerline:

Lorsail: You set up the conditions as the boat having zero weather helm. So the rudder is on the centerline. Now we know that the boat is developing lateral resistance and it should be clear that both the rudder and daggerboard would be contributing lateral resistance roughly in proportion to their areas,right? Then if you all of a sudden double the rudder area the boat would have to "know" that it had more rudder area -45years of experience or not, right?

Lorsail: The illustration of the boat with the rudder held on the centerline was an attempt to illustrate the fact that both foils develop lateral resistance ...

Since we seem to be agreed that the boat will make some leeway, the CL is definately NOT parallel to the boat's true direction of motion, so even on the CL, the tiller is in fact being actively held to weather. The CL is not equivalent to a free tiller, as you will quickly find out if you ever fall overboard when you're singlehanding. If the boat has a weather helm without the rudder, it will round up when the tiller is released, regardless of how big the rudder is.

The only other possibility I can think of is if the the rudder is fairly heavy, so that its weight actually affects the helm. But I really don't think that's common. Other than that, it sounds like we're using different definitions of the word "balance". I would argue that including a helmsman's force on the tiller to define a quality of the boat, is not necessarily the best place to start. Maybe the best argument for including the helmsman's force is that it's more conservative: It will encourage designs that have even more weatherhelm and therefore will round up even more quickly when the tiller is released. But for a large keelboat or a racer, that might be considered either impractical or unnecessary, as I'm sure Eric knows, and would never make the mistake of ignoring. Let's all follow Eric's example, get our definitions straight, and make sure we know whether the helmsman is on the boat or not!

 

rudder

The key point is that if the rudder area were changed substantially the balance(weather, neutral or lee helm) would CHANGE.It is the understanding that BOTH foils contribute to lateral resistance(achieved via the foils operating at an angle of attack to the water) that is key to understanding the effect of a substantial change in rudder area in the previous examples and to understanding the effect of reconfiguring the foil areas in the Arc 21 case or in any case for that matter.
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Speaking of definitions and clarity: "....so even on the CL the tiller is in fact actively being held to weather". Isn't it mutually exclusive to say that a tiller held on the CL is also "actively being held to weather"?
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edit:If the tiller on a boat with weather helm is held on the centerline of the boat the boat will turn into the wind just as it would if the tiller was let go-only slower....

 

Lorsail: The key point is that if the rudder area were changed substantially the balance(weather, neutral or lee helm) would CHANGE. It is the understanding that BOTH foils contribute to lateral resistance(achieved via the foils operating at an angle of attack to the water) that is key to understanding the effect of a substantial change in rudder area in the previous examples and to understanding the effect of reconfiguring the foil areas in the Arc 21 case or in any case for that matter.

The key point, Doug, is that the whole question depends on how you define the word "balance", and that if you define it as the natural behavior of the boat with the helm free, then the rudder does NOT affect the "balance", only ONE foil (the keel) produces lateral resistance in that case, and the key to understanding the issue is to read other people's posts better instead of just rephrasing your original point.

Skippy: Since we seem to be agreed that the boat will make some leeway, the CL is definately NOT parallel to the boat's true direction of motion, so even on the CL, the tiller is in fact being actively held to weather.

Lorsail: Speaking of definitions and clarity: "....so even on the CL the tiller is in fact actively being held to weather". Isn't it mutually exclusive to say that a tiller held on the CL is also "actively being held to weather"?

Considering the fact that the CL, which defines the apparent heading, points to windward of the true course, and the tiller is held along the CL, no it is not mutually exclusive to say that the CL-oriented tiller is being held to weather of the true course, which is the direction that affects the rudder's lift and is therefore relevant to the boat's behavior.

 

Here is a picture of the original rudder and the new one (not yet glassed)

 

Doug, with a larger rudder you might not need quite as much angle of attack, but the amount of force required from the rudder remains the same, otherwise you'd be sailing in circles. An irate attitude doesn't justify fuzzy thinking.

Yoke.

 

Gentlemen,

My presence has been requested.

When a boat is in perfect balance, and there is neither lee helm or weather helm, the Center of Effort (CE) of the sailplan is some distance forward of the Center of the Lateral Plane (CLP). If the CE moves forward from this position, the boat will tend to have lee helm, and when left on her own (nothing holding the tiller) the boat will tend to bear away from the wind. Keep in mind that the rudder is still causing some drag, and perhaps sporadic lift, even though no one is holding onto it. Similarly, if the CE moves aft of its perfect balance position, the boat will tend to have weather helm, and left on her own the boat will tend to round up into the wind. The same applies if you consider moving the CLP in relation to the CE. Moving the CLP forward creates a tendency toward weather helm, and moving the CLP aft creates a tendency towards lee helm. Changing the position of CLP can be done by moving areas themselves (daggerboard or rudder, as we are discussing) or by changing the areas of those items.

When sailing, that is, when someone is actively holding onto the tiller and trying to sail a consistent course, if the tiller must be held to leeward of the boat's centerline, she has lee helm. If the tiller must be held to windward of the boat's centerline, she has weather helm. The angle of the tiller to the boat's centerline is the "degree of helm", i.e. so many degrees of lee helm or weather helm. In boat design, one always strives for about 2-3 degrees of weather helm. This gives the most natural feel for sailing and gives positive control for steering. You do not want any more weather helm, like 4 or 5 degrees or more, because then the drag created by the rudder actually slows the boat down. And you never want lee helm because steering just feels unnatural, particularly when you are sitting on the weather side of the boat.

In past generations of yacht design, when sailboats had much larger keel areas in relation to the size of the rudder, designers tended to ignore the rudder area altogether in order to compare different boat designs. Gradually, as keels became smaller, rudders did not necessarily become any bigger, but rudders did become more important to the overall balance. Therefore, rudder area started being accounted for in the evaluation of underwater profile area and balance. These days, we always include rudder area in the calculation of CLP, but we correspondingly have had to change our rules of thumb to assess where best to put the rudder and keel (or daggerboard). In the end, however, the definition remains the same: The degree angle of the tiller off the boat's centerline, to windward or leeward, gives the degree of weather or lee helm. Again, we strive for 2-3 degrees of weather helm in modern designs.

If you have a nicely balanced sailboat with such a slight weather helm, and you double or triple the size of the rudder, the degree of weather helm will become smaller, to the point that it may disappear or switch to lee helm--you will be generating more lift at smaller angles of attack (smaller tiller angle) than what you required before. But by the same token, you will also be creating more drag. And the CLP will move correspondingly aft in relation to the CE, and therefore, there is a tendency toward lee helm.

That is how we assess the problem in sailing yacht design.

Eric

 

Uh oh, one of the Big Guns wades into the water! Thanks for participating Eric.

This was the original exchange:

Alixander: I guess there is no mathematical way to calculate whether the new rudder will produce lee helm without actually getting it on the water?
Yoke: No, you won't get any lee helm from merely inreasing the area of the rudder, the balance of forces doesn't change.
Doug: Bad advice, Yoke. ...

I seriously doubt Alixander really cares exactly what the angle of the tiller has to be. It sounds like he's just worried that with the big rudder maybe the boat will suddenly start veering to leeward, which we certainly would not want to happen. I would agree with Yoke that Alix does not have to worry about that, that increasing the size of the rudder will not significantly affect how much force Alix will have to apply to the tiller, which way he will have to push it, or what will happen whenever he releases it. I don't see anything bad about Yoke's advice at all.

Eric: The degree angle of the tiller off the boat's centerline, to windward or leeward, gives the degree of weather or lee helm.

Based on the difference between apparent heading and true course, I would argue that this definition is slightly misleading. With a large enough rudder, the tiller could actually require a force to windward even to hold it a degree or so to leeward of the centerline, since it's still pointing slightly to weather of the boat's line of motion.

Eric: If you have a nicely balanced sailboat with such a slight weather helm, and you double or triple the size of the rudder, the degree of weather helm will become smaller, to the point that it may disappear or switch to lee helm

To the extent that this is true within the narrow, technical context of the definition above, I would argue that it's irrelevant and doesn't warrant discussion. It seems to suggest that a large rudder can actually require a force to leeward on the tiller to keep the same boat on course that required a force to windward with a smaller rudder. I would find that exceedingly hard to believe, and would very much like to see an explanation if it's true.

Eric: When a boat is in perfect balance, and there is neither lee helm or weather helm, ... If the CE moves forward from this position, the boat will tend to have lee helm, and when left on her own (nothing holding the tiller) the boat will tend to bear away from the wind.

Based on the definition above of helm balance, I would have to say that, technically speaking, this is not always true given a large enough rudder. Getting back to the definition again, I would say that it's not an accurate measure of what I would consider "perfect balance". I still agree with Yoke's basic point that there's no such thing as the size of the rudder changing the direction of the force on the tiller required to hold the boat on course.

 

So essentially if I am to cut through all the theoretical banter, what you are saying is that there will be little difference in Helm Balance with this new rudder and hopefully there will be a great decrease in drag and chatter.

That is what I expected, and I have thoroughly enjoyed being a "fly on the wall" to this academic pissing contest. and appreciate your experience and opinions.

Perhaps I could introduce a new topic of which to flex your minds around?

Why did the rudder go 'dead' running down wind ? It was a pretty hairy day and the dinghy was overpowered and pushing hull speed. I caught a wave, the Helm got massively heavy and the rudder produced no turning force which lead into a subsequent capsize. I cannot figure out why this had happened and feel that in some way it relates to not having the Jib up (CE being to far aft = increased weather helm?)

 

Skippy said: "Based on the difference between apparent heading and true course, I would argue that this definition is slightly misleading. With a large enough rudder, the tiller could actually require a force to windward even to hold it a degree or so to leeward of the centerline, since it's still pointing slightly to weather of the boat's line of motion."

The difference between the boat's course and its hull centerline does not really enter into the discussion of balance. That difference is called "leeway" and has little to do with lee helm or weather helm. The vast majority of boats have leeway, only a very few specialized boats are capable of "windway", in which the course line is to windward of the boat's hull centerline.

Skippy: It seems to suggest that a large rudder can actually require a force to leeward on the tiller to keep the same boat on course that required a force to windward with a smaller rudder. I would find that exceedingly hard to believe, and would very much like to see an explanation if it's true.

You have to remember that there is drag associated with lift, and there is form drag--that is, the drag just due to the frontal area and volume of the appendage. Taken to the extreme, form drag in a very large rudder can upset the balance of the whole boat to the point that a large rudder could require some lee helm to keep a boat on course. Because of the size of the rudder, that lee helm angle might be quite small. And remember, in most boats, that course we be to leeward of the boat's centerline.

Alixander:Why did the rudder go 'dead' running down wind ? It was a pretty hairy day and the dinghy was overpowered and pushing hull speed. I caught a wave, the Helm got massively heavy and the rudder produced no turning force which lead into a subsequent capsize. I cannot figure out why this had happened and feel that in some way it relates to not having the Jib up (CE being to far aft = increased weather helm?)

Most likely, your flat plate rudder stalled. The massively heavy helm was your reaction to hold lift (and drag) at a very high angle of attack because you had just the main up, and therefore, very bad weather helm. To sustain balance, you needed a large force on the rudder, and you could only get that with a high angle of attack. Unfortunately, it was too close to the stall angle. Suddenly, the rudder stalled--the flow separated off the leading edge of the rudder and it lost all lift. With no lift, you lost control. It takes some moments for lift to catch again after a stall, and you probably crashed before lift could be generated again.

This is exactly analagous to a plane stalling and going into a spin. It loses all lift suddenly, and then teh plane starts plummeting nose-first into the ground. If you are up high enough, you might be able to get the lift back on the wings again, but you are going to fall a few thousand feet before you succeed. This is why in stall/spin training in an airplane, you should start at least 3,000 feet above the ground, and if you're a student pilot, you should start 5,000 feet above the ground. Make sure you have a CFI in the right seat to catch you if you screw up! So in your case, you lost lift on the rudder in a stall and did not have time to get lift re-established again.

Eric
(Licensed Private Pilot)

 

Eric: The difference between the boat's course and its hull centerline does not really enter into the discussion of balance. That difference is called "leeway" and has little to do with lee helm or weather helm.

That's exactly my point Eric:

Doug: You set up the conditions as the boat having zero weather helm. So the rudder is on the centerline. Now we know that the boat is developing lateral resistance and it should be clear that both the rudder and daggerboard would be contributing lateral resistance roughly in proportion to their areas,right?

Alixander: I would argue that holding the rudder at 0 degree is the same as a "free" rudder. either way you slice it, the boat will reveal its helm balance with the new rudder if the foil is held at CL.

Skippy: Careful Alixander, as far as the air and water are concerned, a geometrical line on the hull is irrelevant to how the boat behaves. Even Lorsail agrees that the rudder will exert a force on the boat when held firm on the centerline...

The boat's course relative to the orientation of the foils is very relevant to the lift they produce. The centerline, on the other hand, is just a geometrical feature of the boat. It does not affect the behavior of an unrestrained rudder, and is related to the keel only because the keel is connected to the hull.

But your definition of helm balance is stated clearly in terms of the centerline:

Eric:
When sailing, that is, when someone is actively holding onto the tiller and trying to sail a consistent course, if the tiller must be held to leeward of the boat's centerline, she has lee helm. If the tiller must be held to windward of the boat's centerline, she has weather helm. The angle of the tiller to the boat's centerline is the "degree of helm", ...
...
In the end, however, the definition remains the same: The degree angle of the tiller off the boat's centerline, to windward or leeward, gives the degree of weather or lee helm.

And you use this definition to predict the boat's behavior in the water:

Eric: When a boat is in perfect balance, and there is neither lee helm or weather helm, ... If the CE moves forward from this position, the boat will tend to have lee helm, and when left on her own (nothing holding the tiller) the boat will tend to bear away from the wind.

I just don't see how that's consistent.

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Skippy: It seems to suggest that a large rudder can actually require a force to leeward on the tiller to keep the same boat on course that required a force to windward with a smaller rudder. I would find that exceedingly hard to believe, and would very much like to see an explanation if it's true.

Eric: You have to remember that there is drag associated with lift, and there is form drag--that is, the drag just due to the frontal area and volume of the appendage. Taken to the extreme, form drag in a very large rudder can upset the balance of the whole boat to the point that a large rudder could require some lee helm to keep a boat on course. Because of the size of the rudder, that lee helm angle might be quite small. And remember, in most boats, that course we be to leeward of the boat's centerline.

Okay, that's fine. Rudder drag combined with leeway generates some amount of lee helm. But the drag will be much smaller than the lift the rudder is capable of generating at normal operating angles. So including rudder area in CLP with a 100% weighting still seems like a gross overestimate of drag effects, especially since the larger rudder will spend more time in its low-drag bucket if it has one. That's the main point of contention, how can you treat an adjustable rudder in the same way as a fixed keel? Even 50% sounds like a lot based on drag alone.

 

Mr. Beck. Boat is at a fast speed and the water passing along side the hull is becoming turbulant. Same or even more so on the bottom section. All the jumbled water converges in front of the rudder. Add a wave which creates even more turbulance. The rudder is operating in a swirling mass. It has " stalled ". The shape and area of rudder are useless till the waters around it are less turbulant.----------Old power boats suffer from the same effect when speeds are raised and small, simple,slab of wood rudder is left on the boat.

 

Yes, the boat and its foils produce lift and induced drag because of the leeway angle (angle between the boat's centerline and its course line).

When you are on the boat holding a course, your frame of reference is always the boat's centerline, not its course line. You judge the amount of weather helm or lee helm in relation to the boat's centerline.

Let's say that you are on the boat sailing and have very bad lee helm. Your boat is a shallow draft design like the old Cal 21 which has a keel/centerboard and a narrow, deep rudder. You have just the jib up, and not the main. The center of effort of the sail is way forward, and the bow of the boat persists in falling off the wind. (You can probably tell I am speaking from personal experience!). In order to hold the bow of the boat into the wind, you have to put the tiller way over to the lee side, and the angle of attack on the rudder will be opposite to what you normally want. The boat still has a course to leeward. But the balance of forces on the sails, keel and rudder demand that the rudder be pushed to leeward to hold course.

All we have established in our definition of balance--weather helm and lee helm--is the position of the tiller in relation to the boat's centerline and what is necessary to hold a stable course. Trying to determine the direction of the boat's course, the amount of leeway angle, and the boat's overall performance are completely separate problems that require different analysis.

Skippy: Rudder drag combined with leeway generates some amount of lee helm.

That is not where lee helm comes from. Lee helm or weather helm comes from the balance required between the overall turning moment of the sails in relation to the overall turning moment in the opposite direction from the underwater profile area. Some portion of that underwater area (the rudder) can be used to alter the balance of forces to cause the boat to assume a certain steady and consistent course. That course, in the vast majority of boats, will always be to leeward. The leeway angle is actually the sum of the total aerodynamic drag angle and the hydrodynamic drag angle. (Read C.A. Marchaj's "Aero-Hydrodynamics of Sailing" for a complete description.)

Skippy: But the drag will be much smaller than the lift the rudder is capable of generating at normal operating angles.

You are describing only the induced drag, that is, the drag that is inescapably and by definition the result of creating lift. In most keel and rudder analysis, we temporarily ignore the effects of form drag and friction drag, they being generally small and in proportion to the profile areas and volumes of the appendages. But if you all of a sudden change a normally sized rudder to a very big rudder 2 or 3 times larger, your form and friction drag go way up to the point that they cannot be ignored, and they have a very big influence on balance, even before you start talking about lift and induced drag.

Skippy: So including rudder area in CLP with a 100% weighting still seems like a gross overestimate of drag effects, especially since the larger rudder will spend more time in its low-drag bucket if it has one. That's the main point of contention, how can you treat an adjustable rudder in the same way as a fixed keel? Even 50% sounds like a lot based on drag alone.

In many modern boats, the hull itself under the water has shrunk away to almost nothing, and the only areas left under water are the rudder and keel. So in modern boats, we tend to treat the rudder at 100% of area in order to determine the overall profile area. But then of course, our definitions as to the proper amount of lead, or the positions of the CE and the CLP have to change. We cannot use the old definitions that applied to conventionally designed boats where the hull itself contributes a lot to overall drag, and the rudder is small in proportion to the rest of the boat. Back then, we either ignored the rudder or we treated it at 50% of its area.

In modern boats, we have to treat the rudder at 100% of its area just to be smart, but at the same time, we must change our definition of proper balance. Where should the CE go in relation to the CLP? The effects are going to be the same: CE well forward will create lee helm, and CE well aft will create weather helm. But how much is the lead, and is the lead positive or negative (CE ahead or behind, respectively, of the CLP). Many designers have found (myself included) that in such shallow-hulled boats, the CE should be positioned over the leading edge of the keel. Comparing the Centers, the lead is still positive. This produces acceptable balance with a slight weather helm.

Eric

 

Eric: When you are on the boat holding a course, your frame of reference is always the boat's centerline, not its course line. You judge the amount of weather helm or lee helm in relation to the boat's centerline.

Eric, people including myself may from time to time find it convenient to discuss specific problems in other reference frames as the situation demands. I hope that's not too inconvenient for you. I will always make it very clear whenever I do so.

Eric: Let's say that you are on the boat sailing and have very bad lee helm. Your boat is a shallow draft design like the old Cal 21 ... the balance of forces on the sails, keel and rudder demand that the rudder be pushed to leeward to hold course.

That's a nice story Eric. I'm not sure what it has to do with the topic at hand, but yes, it is an example of lee helm.

Eric: All we have established in our definition of balance--weather helm and lee helm--is the position of the tiller in relation to the boat's centerline and what is necessary to hold a stable course. Trying to determine the direction of the boat's course, the amount of leeway angle, and the boat's overall performance are completely separate problems that require different analysis.

Eric, I am well aware that defining a technical term and analyzing a boat's dynamics are two different things. Which again is exactly my point. You took your definition one step farther, applying it to the boat's motion, even though it's defined only in terms of the centerline:

Eric: When a boat is in perfect balance, and there is neither lee helm or weather helm, ... If the CE moves forward from this position, the boat will tend to have lee helm, and when left on her own (nothing holding the tiller) the boat will tend to bear away from the wind.

And while we're on the topic of definitions,

Eric: The difference between the boat's course and its hull centerline does not really enter into the discussion of balance. That difference is called "leeway" and has little to do with lee helm or weather helm. The vast majority of boats have leeway

I am also familiar with the meaning of the term "leeway" and the fact that it is quite common. I suspect many other people on this forum know about it as well.

Skippy: Rudder drag combined with leeway generates some amount of lee helm.

Eric: That is not where lee helm comes from.

Then why did you just say in your previous post that rudder drag DOES tend to increase lee helm???

Eric: Taken to the extreme, form drag in a very large rudder can upset the balance of the whole boat to the point that a large rudder could require some lee helm to keep a boat on course.

Leeway should only increase that effect, since the stern will be somewhat farther leeward than it would be with no leeway. So I don't understand those comments at all.

Eric: Lee helm or weather helm comes from the balance required between the overall turning moment of the sails in relation to the overall turning moment in the opposite direction from the underwater profile area. ...

In that case, you still have not explained why rudder area should be part of CLP in predicting the boat's behavior with a free helm.

Skippy: But the drag will be much smaller than the lift the rudder is capable of generating at normal operating angles.

Eric: You are describing only the induced drag, that is, the drag that is inescapably and by definition the result of creating lift. In most keel and rudder analysis, we temporarily ignore the effects of form drag and friction drag, they being generally small and in proportion to the profile areas and volumes of the appendages. But if you all of a sudden change a normally sized rudder to a very big rudder 2 or 3 times larger, your form and friction drag go way up to the point that they cannot be ignored, and they have a very big influence on balance, even before you start talking about lift and induced drag.

No Eric, I am not describing only induced drag. I am referring to the form drag of an unrestrained rudder. Regardless of whether it does or does not induce lee helm, which your posts seem to be unclear on, my point is that I would like to know why it warrants including rudder area in CLP.

Skippy: So including rudder area in CLP with a 100% weighting still seems like a gross overestimate of drag effects, especially since the larger rudder will spend more time in its low-drag bucket if it has one. That's the main point of contention, how can you treat an adjustable rudder in the same way as a fixed keel? Even 50% sounds like a lot based on drag alone.

Eric:
In many modern boats, the hull itself under the water has shrunk away to almost nothing, and the only areas left under water are the rudder and keel. So in modern boats, we tend to treat the rudder at 100% of area in order to determine the overall profile area. ... Back then, we either ignored the rudder or we treated it at 50% of its area.

In modern boats, we have to treat the rudder at 100% of its area just to be smart, ...

With all due respect Eric, do you really think I or most people on this forum need to be lectured on what is or is not "smart"? You say that since the lateral area of the hull is smaller, the rudder area must be weighted more heavily. That doesn't make sense to me. When the rudder area is small, giving it the same 100% weight shouldn't make much difference anyway, and I don't see why the basic dynamics of the rudder itself would be significantly affected by the details of the hull shape. What I do see is the phrase "treat the rudder at 100%" mentioned twice with little or no explanation that makes any sense to me.