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Nautiduck
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Initially Posted - 01/12/2010 : 22:08:04

I had a long conversation with a fellow at Catalina today about rudders. The C250 is now made in Florida so they have the molds there. The new balanced rudder for the wing keel model is 71" long and has two "steps" to achieve the balance. I also note that the pintle and Edson steering arm mount areas are slightly depressed so the main rudder may be a bit thicker than the older rudders. The new "dual recessed counter-balanced" rudder for the C250 wing keel is $660 plus shipping. Catalina no longer offers a discount to the owners of older, non-balanced, rudders. Here are a couple of photos.

Compare this Catalina wing keel rudder to the Idasailor high-performance balanced rudder for the C250 wing keel. The Catalina rudder appears to have more surface area and more of it is foreward of the main rudder edge.

Thoughts??

We cannot direct the winds but we can adjust our sails.

Edited by - Nautiduck on 01/13/2010 21:52:03

Arlyn Stewart
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Response Posted - 01/13/2010 : 06:08:44

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Compare this rudder to the Idasailor high-performance balanced rudder for the C250.<hr height="1" noshade id="quote"></blockquote id="quote">

I'm not sure you should be comparing the IDA and the Catalina wing keel rudder as doing so tends to project that some think there is inadequacy of the wing keel rudder.

I don't recall any complaints regarding the wing keel rudder other than cracking between the pintles.

The chatter about using IDA rudders is from water ballast owners who for a long time have complained that the Catalina 3rd generation rudder (at one foot shorter than the wing keel version) is inadequate for more aggressive sailing venues.

While you are comparing Catalina an IDA blade rudders, IDA got involved with Catalina niche rudders with a beaching design and may be trying to expand upon that sometimes with some false hype that all Catalina rudders are inadequate. That is not the case and I've tried hard to say that I've seen no bad reports about the 3rd wing keel rudder, again other than some cracking between pintles.

A few words about beaching or kickup and I guess it is time that the two boat models should be looked at as the two distinctly differing boats they are. When Catalina introduced the 250, they profiled the design in Sail magazine and the design was all about a trailer sailor with bigger boat feel and features but smaller boat ease of trailering and handling of skinny water.

To fill that profile, it was optioned with a beaching rudder. Catalina's design of that beaching rudder had inadequacies that caused them a lot of grief so they withdrew the beaching option... but what they couldn't withdraw was the need for it that had caused its option in the first place.

It is that kind of situation that prompts after market products such as the IDA beaching rudder, to fill niches in an otherwise general market as I don't expect that any wing keel owners would want a beaching rudder and not all of water ballast owners.

Another way of putting this is, if Catalina had not had a beaching rudder option at the time I bought, I'd not now own a C250. It was one of the requirements on my must list. If I were buying a new 250 today, I'd only do so with a credit for the rudder that I'd use against an IDA rudder because skinny water capability is that important to me.

But... this discussion about skinny water needs is outside of the wing keel model realm. I'd never consider a wing keel model because it doesn't fit my needs just like I wouldn't consider a C25 because it didn't fit my needs. I'm certainly not saying that either the wing keel 250 or the C25 are inadequate boats... only so to my needs, each has certainly filled a wider range of others needs very well.

Edited by - Arlyn Stewart on 01/13/2010 06:17:08

Nautiduck
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Response Posted - 01/13/2010 : 06:34:36

Arlyn, it was certainly not my intent to criticize the 250 3rd generation wing keel rudder. My friend has one and it works very well. That is why I want to upgrade from my 2nd gen wing keel rudder to the newer balanced version. The comparison to the Ida rudder is simply because I observed such a difference in design. That being said, I have read positive reviews of the Ida rudder as well. I will be going with the Catalina rudder.

Edited by - on

superbob
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Response Posted - 01/13/2010 : 06:45:04

Randy,

I appreciate the information on the changes made by Catalina to the 250's rudder. Thanks for sharing.

Oh and by the way, a balanced rudder would not make any difference in performance when the skipper is always off balance . . . if you get my drift. Thought a little humor in here to lighten things up was in order.

Bob Watson
Sparkle Plenty/Catalina 250/WK
Moored in Whiskeytown Lake California

Edited by - on

dlucier
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Response Posted - 01/13/2010 : 08:01:46

Looking at both the IDaSailor rudder and the new Catalina rudder, it appears to me that the new Catalina rudder should have a better feel and performance than the Ida rudder simply because the center of pressure (1/4 of the chord length from the leading edge) is at the pintle pins (pivot point) whereas the center of pressure on the Ida rudder is defintitely aft of the pivot point. Also, the chord length appears to be significantly longer on the Catalina rudder.

Edited by - on

Arlyn Stewart
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Response Posted - 01/13/2010 : 09:38:20

Randy, I didn't see or take it that you were in any way criticizing the Catalina 3rd generation rudder, rather that a comparison had it looking better than the IDA and it does indeed do so for the wing keel.

I quoted a line in your first post for a reason and will do so again.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Compare this rudder to the Idasailor high-performance balanced rudder for the C250.<hr height="1" noshade id="quote"></blockquote id="quote">

What you are comparing to the IDA is the wing keel rudder and I got that but someone unaware that there is a difference between the two Catalina rudders could easily project your comparison and conclude that the IDA rudder is the inferior choice for their water ballast and I doubt that would be true.

Again... the Catalina wing keel rudder is 12" longer than the water ballast rudder so when each is compared, the conclusions stack up different. We've got apples and oranges here and need to keep em straight and I'm not too sure IDA sales hype does that. They may correctly think they have a better rudder offering for water ballast owners and be completely unaware thinking all C250 Catalina rudders are the same.

Edited by - Arlyn Stewart on 01/13/2010 09:47:39

Dave Bristle
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Response Posted - 01/13/2010 : 10:39:51

That new Catalina rudder looks strikingly like the "2nd generation" C-25 balanced rudder that I bought from CD in about 2000 for that exact price. I had to modify my pintles to fit on it--a really tough job. The indents probably solve that problem.

"Balance" is achieved by the surface area ahead of an imaginary line through the pintles. The Ida doesn't appear to have much, if any. For control, you don't really want true "balance" (center of pressure at the pivot line)--that would cause the rudder to flap around while under way. That's why the Catalina blade has substantially more area (well over 3/4) behind the pivot line.

Edited by - Dave Bristle on 01/13/2010 10:42:50

Arlyn Stewart
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Response Posted - 01/13/2010 : 11:53:14

Does any one recall when the C-25 2nd rudder was designed and made available? Was it before or after the release of the 250 in '94?

Edited by - on

dlucier
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Response Posted - 01/13/2010 : 12:39:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 "Balance" is achieved by the surface area ahead of an imaginary line through the pintles. The Ida doesn't appear to have much, if any. For control, you don't really want true "balance" (center of pressure at the pivot line)--that would cause the rudder to flap around while under way.<hr height="1" noshade id="quote"></blockquote id="quote">

Actually, I think you want the center of pressure as close to the pivot point as possible to minimize oscillation (flapping) which I believe is an issue with the original C25 rudder whose center of pressure was well aft of the pivot point.

Edited by - on

piseas
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Response Posted - 01/13/2010 : 15:44:31

Randy, how about a review for the Mainsheet once you have it? I am sure many would be interested.
Steve A

Edited by - on

Arlyn Stewart
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Response Posted - 01/13/2010 : 20:14:31

The center of force on many foils is around 25% aft of leading edge. Planes that use flying control surfaces such as a full flying horizontal stab (no separate elevator attached to a fixed stab) sometimes try to creep close to the 25% point but keep in mind that if going over the balance point, then any play in control linkage would manifest slamming the foils angle of attack between positive and negative never allowing a balanced center.

This is the reason why balance on an aircraft is critical and flight stability is compromised if becoming tail heavy as a neutral balance can never be held.

The same would be true for a rudder if balanced too far aft. And, because hydro forces are far greater than air, the percentage is less for boat rudders. Boat rudders are probably more like 15%, whereas plane control surfaces might be around 20%.

I've somewhat balanced my 2nd generation rudder with only one inch ahead of the pintle center line. Combining that balance point with doubling the steering ratio, the helm can be handled in a stiff breeze by a five year old girl.

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Dave Bristle
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Response Posted - 01/13/2010 : 21:39:32

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 Actually, I think you want the center of pressure as close to the pivot point as possible to minimize oscillation (flapping) which I believe is an issue with the original C25 rudder whose center of pressure was well aft of the pivot point.<hr height="1" noshade id="quote"></blockquote id="quote">If the center of pressure is at the pivot point, it will be like a car with steering that needs no effort to turn the wheel--none. If you breathe on it or a pebble pushes on a wheel, the car turns, and once it starts turning, it tries to turn sharper. That means keeping the course straight requires constant effort on the part of the driver.

On a sailboat, the extreme is illustrated when backing... With the center of pressure aft of the pivot line, the rudder tries to turn in whatever direction that pressure pushes it--to one side or the other. Going forward, having the center aft of the pivot line makes the rudder try to center itself, which makes steering a course easier and requires at least some force to turn. As in a car, you want some force required to turn--otherwise you have a constant battle on your hands. A "balanced" rudder should not be truly balanced--it should just reduce the effort to turn (or to counteract weather helm) by putting a little of the foil forward of the line.

The "fluttering" on the original C-25 rudder was apparently due in part to its thin, sharp foil shape. The balanced rudder used an NACA foil shape that reduced turbulence around the trailing edge, and therefore flutter. I could feel the difference--with the same pintles in the same gudgeons.

Edited by - Dave Bristle on 01/13/2010 21:50:58

Nautiduck
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Response Posted - 01/13/2010 : 21:50:17

Steve, I plan to do a Mainsheet technical article on the upgrade.

Arlyn, I will update the original post to make it clear that the rudders I am talking about are all wing keel rudders.

Edited by - Nautiduck on 01/13/2010 21:59:07

dlucier
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Response Posted - 01/14/2010 : 10:35:09

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 Actually, I think you want the center of pressure as close to the pivot point as possible to minimize oscillation (flapping) which I believe is an issue with the original C25 rudder whose center of pressure was well aft of the pivot point.<hr height="1" noshade id="quote"></blockquote id="quote">If the center of pressure is at the pivot point, it will be like a car with steering that needs no effort to turn the wheel--none. If you breathe on it or a pebble pushes on a wheel, the car turns, and once it starts turning, it tries to turn sharper. That means keeping the course straight requires constant effort on the part of the driver.

On a sailboat, the extreme is illustrated when backing... With the center of pressure aft of the pivot line, the rudder tries to turn in whatever direction that pressure pushes it--to one side or the other. Going forward, having the center aft of the pivot line makes the rudder try to center itself, which makes steering a course easier and requires at least some force to turn. As in a car, you want some force required to turn--otherwise you have a constant battle on your hands. <hr height="1" noshade id="quote"></blockquote id="quote">

Dave,

On a symmetric foil, when you are just motoring no lift is being generated by the rudder. It is simply acting like a wind vane maintaining balance (forces even on both sides) to the oncoming flow. This means as long as the rudder's pivot point is ahead of the midpoint (or thereabouts), it will center itself if you let go of the tiller and not continue to turn more if you breathe on it or if a pebble pushes on the tiller since the forces acting on the aft part of the rudder will still be greater than the forces acting on the forward portion. Granted, the closer to the midpoint the pivot point lies, the easier it is to turn the rudder from the oncoming flow, but it will still return to center when the tiller is let go. When backing up, the properties of lift, as it pertains to foils, are not really coming into play. It's simply the forces acting on one part of the rudder are vastly greater than the other causing it to turn even more.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle

"A "balanced" rudder should not be truly balanced--it should just reduce the effort to turn (or to counteract weather helm) by putting a little of the foil forward of the line."<hr height="1" noshade id="quote"></blockquote id="quote">

I disagree with this statement. A rudder isn't balanced to "just reduce the effort to turn", it's balanced to center the forces of lift on the rudder post (25% from the leading edge of a foil) and a balanced rudder doesn't "counteract weather helm", it works with it. Say you had excessive weatherhelm, tiller under your chin, no matter where the pivot point on the rudder is, the tiller is still going to be in the same spot under your chin. The forces felt on the tiller might not be exactly the same, but you've done nothing to counteract weather helm.

Edited by - dlucier on 01/14/2010 10:36:23

Dave Bristle
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Response Posted - 01/14/2010 : 19:16:53

Well, I don't think we're actually disagreeing except on a few fine points or terminology. Regarding weather helm and rudder balance, as you know, "weather helm" is the term for the boat's tendency to turn to windward, or "to weather", based on the resolution of the forces from the center of effort of the sails being aft of the center of lateral resistance of the hull/keel. Left alone, the rudder will let the boat turn up and luff. Turned slightly to leeward (tiller pulled to windward), the rudder counteracts those forces so the boat doesn't turn. That's what I meant by "counteracting weather helm."

The rudder "lift" to counteract weather helm will be the same regardless of the rudder balance. The entire blade surface below the water, when turned, generates "lift" (lateral force on the boat) in one direction or the other. The rudder with no "balance" has all of its surface pulling against the tiller set at any angle other than straight ahead. The "balanced" rudder has some of its surface forward of the pivot line, pushing the other way. The resolution of those two forces at the pivot line results in less pull on the tiller. I've used both rudders on my C-25, and I can attest to the significant difference.

However, the overall "lift" of a sailboat rudder is a function of the size and shape of the blade, and its angle of attack. As you said, the angle to make the boat sail straight is the same with either rudder. That's because rudders with approximately the same dimensions, at the same angle, generate approximately the same lift, whether they are "balanced" or not. If balance increased lift, then the balanced rudder would not need to be pulled to as great an angle as the unbalanced rudder to keep the boat sailing straight. I will assert, empirically and theoretically, that the angle is the same. My rig needed for about five degrees of helm to leeward in moderate conditions with either rudder. The only difference was that the balanced rudder pulled less on the tiller. (It also "fluttered" less--I think because of its NACA foil shape.)

So, whether either of us knows it or not, we sort of agree.

But this debate isn't nearly as fun as the one about why the right amount of weather helm, requiring some effort to hold the rudder at an angle, which implies adding drag, makes a boat sail faster! We can start that one in February... (Just don't tell Arlyn.)

Edited by - Dave Bristle on 01/14/2010 19:18:27

Nautiduck
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Response Posted - 01/14/2010 : 21:10:41

You scientists are giving me a headache!

Dave wrote:

"The "balanced" rudder has some of its surface forward of the pivot line, pushing the other way. The resolution of those two forces at the pivot line results in less pull on the tiller. I've used both rudders on my C-25, and I can attest to the significant difference."

Dave, this is what I am after. My unbalanced blade hold the boat fine but it takes more strength to hold the wheel than I like, especially going to weather in a brisk wind. I am hoping to get a significant improvement with the new balanced rudder.

Edited by - Nautiduck on 01/14/2010 21:11:52

dlucier
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Response Posted - 01/14/2010 : 21:14:02

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 The entire blade surface below the water, when turned, generates "lift" (lateral force on the boat) in one direction or the other. The rudder with no "balance" has all of its surface pulling against the tiller set at any angle other than straight ahead. The "balanced" rudder has some of its surface forward of the pivot line, pushing the other way. The resolution of those two forces at the pivot line results in less pull on the tiller.<hr height="1" noshade id="quote"></blockquote id="quote">

Dave,

Yeah, we probably are saying the same thing. I think we agree that having some of the rudder ahead of the pivot point aids in steering by somewhat neutralizing the forces exerted on a rudder due to the redirection of water during steering. But in addition to the forces from the redirection of water, the other force acting on the rudder, lift, is centered exactly 1/4 of the chord behind the leading edge and perpendicular to that point. The closer the pivot point is to the center of pull, the easier it will be on the helm.

Edited by - on

Arlyn Stewart
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Response Posted - 01/14/2010 : 21:38:05

LOL... lets get the parameters of the debate settled... do you mean faster speed or faster time to a weather mark?

Faster speed would always be the benefit of less drag and therefore a rudder amidships.

Faster time to a weather mark is a product of both speed and weathering to windward. A boat exhibiting a slight bit of weather helm is positioned so hull, keel and rudder have positive angles of attack to the boat track thus producing lift to windward and an altered track favoring windward. The speed reduction caused by the increased drag is then compared to the lift benefit where normally a 3-5 degree rudder angle produces a net gain or positive lift to drag ratio.

Edited by - Arlyn Stewart on 01/14/2010 21:45:35

dlucier
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Response Posted - 01/15/2010 : 09:52:29

Here's my take on the Ida Rudder. Since the leading edge is ever so slightly ahead of the pivot point, it is balanced in the sense that the pivot point is not at the leading edge and this will aid steering as the forces exerted due to the redirection of water are more balanced fore and aft giving the helmsman the power steering he's alway wanted. The problem with the Ida rudder lies in the fact that the pivot point is not at the 1/4 chord point.

Let me explain. You leave the marina with the Ida rudder and that little area ahead of the pivot point is making steering much easier. All is good. You then put up sails and start going to weather in light air. Sails are trimmed properly and you're sailing along at 3kts with the appropriate (and essential) amount of weather helm, say 3-5 degrees, and you feel a slight pressure on the tiller as the center of lift starts pulling on the rudder's 1/4 chord point, which is aft of the Ida's pivot point, to windward.

The wind then starts to pick up a little bit and as boatspeed increases so does the amount of lift pulling on the 1/4 chord point to weather. You still have the optimal 3-5 degrees of correct weather helm, but you are now noticing more effort is required to hold the tiller at the normal 3-5 degrees. All is still good since the wind is just light to moderate.

The wind then picks up yet again, and again both boatspeed and the pull on the 1/4 chord point increases which is now requiring more effort on the tiller to hold that optimal 3-5 degrees. Yeah, you cruising along smartly, but geez this tiller is getting a little heavy.

The wind then finally pipes up to the point where any more and you'll have to start making adjustments to your sail pan. You're at max boatspeed and the center of pull force on the 1/4 chord point is maxed as well. You still have the correct 3-5 degrees of weatherhelm, but now it requires two hands to comfortably hold the tiller...Huhh? If it now requires two hands to hold the tiller, surely there must be something wrong with my sail plan, right? Excessive weatherhelm? Maybe, but I still only have 3-5 degrees on the tiller? One might think they have an unbalanced sailplan causing excessive weatherhelm, but in reality they are simply feeling the normal center of pull force. If the pivot point were at the 1/4 chord point, the only forces felt on the tiller would be the normal pressures due to the flow of water around the rudder.

Along with the pivot point not being at the 1/4 chord point, I believe the Ida rudder has some other performance shortcomings as some members have reported that this rudder performs great in light air and around the docks, but performance drops off (read round ups) as the wind picks up. This leads me to believe the rudder is not generating enough lift to windward in higher winds thereby reducing it's contribution to the CLR as the CE moves aft resulting in round ups.

Edited by - dlucier on 01/15/2010 09:59:47

Dave Bristle
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Response Posted - 01/15/2010 : 10:46:49

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 ...A boat exhibiting a slight bit of weather helm is positioned so hull, keel and rudder have positive angles of attack to the boat track thus producing lift to windward and an altered track favoring windward.
<hr height="1" noshade id="quote"></blockquote id="quote">I knew it!

Now I'm going to have to expand on this to verify that my understanding of the basic principles is in agreement with Arlyn's summary. (It is mid-winter--snow on the ice up here...)

As I understand basic physics, for every action there is an equal and opposite reaction. Stated differently, forces will balance out one way or the other. When a boat exhibits weather helm, the helmsman steers slightly to leeward, causing the rudder to push the stern slightly to windward and the bow to leeward of the actual track. Thus, the rudder is causing drag as well as lift (force against the stern) to windward. The "equal and opposite" force comes from the keel, whose angle of attack is now shifted very slightly to leeward. (It has to.) This alters the forces on the keel such that the pressure on its windward side equals the pressure on the rudder's leeward side. The result is that the boat goes straight on its course, which is very slightly to windward of where the hull is pointed.

What does this mean? I suspect it means the rig has slightly more power than it would if the bow was pointed precisely toward the boat's course. To put it another way, if the boat sailed with a balanced helm (no rudder force), to get the same power, it would have to sail a slightly lower course. Also, the windward pressure on the keel might add to its forces counteracting heel, reducing heel and thereby further improving the efficiency of the sails. In sum, the true course is higher, and the improved power offsets the drag created by steering to leeward.

So, Arlyn, if I'm making any sense, are are we saying the same thing, or the opposite?

Edited by - on

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Response Posted - 01/15/2010 : 11:10:40

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 Here's my take on the Ida Rudder...<hr height="1" noshade id="quote"></blockquote id="quote">Yup, putting a straight-edge on my screen shows virtually no surface forward of the line through the pintles. I don't think my "balanced" rudder (the one Catalina started using in 1988) had the pivot 1/4 back, however. I would be concerned that if it were that balanced, there would be no tendency to center itself, like a car's steering wheel does. I would expect the helm to feel very "squirmy" with virtually no "feel". I suspect that's why the Catalina version is only partially balanced--to reduce effort but not to eliminate it.

Back to the Ida... I vaguely recall reports that its greater depth helped reduce round-ups that are common with the C-250, largely due to its broader stern that tends to lift more of the blade out of the water when heeled. Compared to the C-25, the C-250's shape also contributes to weather helm, since the broader stern creates a sort of "wedge effect" when heeled.

My concern about the Ida is its solid HDLP blade (like Starboard), which according to some reports has snapped under stress. The largest stress on a rudder blade is generally created by heavy fore-aft pitching in seas when the boat is heeled. I would probably trust the Ida on an inland lake, but maybe not in Long Island Sound's large, short chop. I have not heard of a failure of the balanced Catalina C-25 rudder--only the original unbalanced one that has a wood core in its head. Rot causes it to break at the lower pintle. The balanced version is foam-cored fiberglass.

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dlucier
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Response Posted - 01/15/2010 : 12:55:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 The "equal and opposite" force comes from the keel, whose angle of attack is now shifted very slightly to leeward. (It has to.) This alters the forces on the keel such that the pressure on its windward side equals the pressure on the rudder's leeward side. The result is that the boat goes straight on its course, which is very slightly to windward of where the hull is pointed.<hr height="1" noshade id="quote"></blockquote id="quote">

Dave,

Something sounds amiss with your statement. For one, the forces (pressure/lift) are not equal between the keel and rudder. Because of the distance the rudder is back from the keel (lever arm), it needs much less force (lift) than the keel. Also, both keel and rudder generate lift (pressure) on the same side. With a wee bit of weatherhelm, the rudder has a slightly larger angle of attack to the oncoming water, but both generate lift on the same side.

As to the second part of the statement, I'm not sure if this is what you mean but, as a boat sails to weather, the track is to leeward of the course (where the hull is pointing).

Edited by - dlucier on 01/15/2010 12:56:45

Nautiduck
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Response Posted - 01/15/2010 : 13:41:17

I hope you guys can reach some sort of consensus on the aerodynamic principles before my new rudder arrives.

I just ordered it! Takes two weeks to build (built to order) and another two to get it from Florida to Oregon. Still plenty of time to get it on the boat for a late March launch.

I have to say the fellow at Catalina (Warren Paddy) was very helpful.

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Response Posted - 01/15/2010 : 14:35:13

Don wrote; <blockquote id="quote">quote:<hr height="1" noshade id="quote"> Dave, Something sounds amiss with your statement. For one, the forces (pressure/lift) are not equal between the keel and rudder. Because of the distance the rudder is back from the keel (lever arm), it needs much less force (lift) than the keel. Also, both keel and rudder generate lift (pressure) on the same side. With a wee bit of weatherhelm, the rudder has a slightly larger angle of attack to the oncoming water, but both generate lift on the same side. As to the second part of the statement, I'm not sure if this is what you mean but, as a boat sails to weather, the track is to leeward of the course (where the hull is pointing). <hr height="1" noshade id="quote"></blockquote id="quote">

But wait! We forgot enter the rig rake in to this!!

Edited by - DaveR on 01/15/2010 14:35:47

Arlyn Stewart
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Response Posted - 01/15/2010 : 15:11:57

Dave... sorry, we are not on the same page. I don't think a boat seeking a weatherly mark is going to have a course heading below its track. I think both the hull and of course keel with it as well as the rudder will have positive angles of attack to weather if it is displaying proper 3-5 degrees of weather helm.

Remember your point, the rudder drag is an opposite reaction to the yaw force to weather and the bow angle of attack is going to be slightly to weather. It would be a lot more and round up if the rudder lift didn't hold it in check... and that is paid in full by drag... but the lift to drag ratio is positive in favor of the lift gained by the positive angles of attack of both hull and rudder.

Don, while 1/4 of a foil is a good rough estimate of the center of lift of a foil, it ain't written in stone and can't be because for one reason, the lift center is dynamic and affected by flow speed as well as intrinsics of the foil. If my memory serves me correct, the faster the fluid dynamic, a slight shift forward occurs in the center of lift.

I doubt very much a boat rudder could be found that was balanced at 25% because as Dave very properly pointed out, ya gotta stay away from instability a pebbles distance beyond balance.

The Ida rudder is probably the better of the two rudders when comparing performance with the Catalina 3rd water ballast model because the IDA is a high aspect ratio rudder (uses a foil that generates greater lift by adding length compared to one that generates greater lift by thickening the foil and lengthening the chord). The Catalina 3rd is not a high aspect ratio rudder and it is well understood that high aspect ratio produces a better lift to drag ratio. And as Dave points out, they stay immersed better when heeled. Why then is high aspect ratio not always the best choice? Because of strength demands, long and thin requires special strength considerations.

I think you can take it to the bank that the Catalina 3rd generation rudder has warranty issues as primary factors in its design.

Careful now... I'm still not being critical of the Catalina 3rd generation for the wing keel. At one foot longer, it has the depth to provide boat control... what more is required of a rudder? Cost, it is cheaper than the IDA. Sturdiness? It is more sturdy than the IDA.

Once again, the only problem with the Catalina 3rd is on the water ballast (short) version, it doesn't provide adequate control for aggressive sailing. And, before any one jumps to conclusions, it is not because of the water ballast... it is because the too short rudder is not immersed enough when heeling.

And finally and this is additional reading beyond the argument. Boat balance is the product of CE and CLR relationship as almost all beyond novice sailors come to appreciate. Fortunately, CLR no longer is defined as just center of lateral resistance but is modernly defined as 'center of all forces having to do with the underwater surfaces of the boat'. Unless this view is adopted, one can never hope to figure out what is really happening with the C250 when it heels and why it affects the helm and specifically causes severe weather helm.

Edited by - on

dlucier
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Virgin Islands (United Kingdom)
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Response Posted - 01/15/2010 : 21:58:08

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 Don, while 1/4 of a foil is a good rough estimate of the center of lift of a foil, it ain't written in stone...<hr height="1" noshade id="quote"></blockquote id="quote">

Arlyn,

I don't make up the news, I just report it.

From Wiki...

"(1) on a symmetric airfoil, the center of pressure lies exactly one quarter of the chord behind the leading edge."

"The center of pressure on a symmetric airfoil typically lies close to 25% of the chord length behind the leading edge of the airfoil. (This is called the "quarter-chord point".) For a symmetric airfoil, as angle of attack and lift coefficient change, the center of pressure does not move. It remains around the quarter-chord point for all angles of attack and lift coefficients."

And from Steve Colgate, sailing author, boat designer, racer, and founder of the Offshore Sailing School...

"Separated or "spade" rudders are standard now. The rudder is placed near the stern of the boat where it has the greatest leverage for steering. It is a lifting surface in itself, and since it isn't attached to the keel and is meeting fairly non-turbulent water, such a rudder is very efficient. These rudders usually are "balanced" so that the rudder post enters the rudder about one-fourth of the way back rather than being attached along the leading edge. Hopefully then, the center of the pull to windward will be right at the post..."

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