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dlucier
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Response Posted - 12/11/2006 : 21:56:55

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by aeckhart
 All veeeeeery interesting reading, but.........now I've got a headache.<hr height="1" noshade id="quote"></blockquote id="quote">
Are you eating a balanced diet?

Edited by - on

Arlyn Stewart
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Response Posted - 12/11/2006 : 22:55:03

Dave, I'm wondering if your friend wasn't implying that the boat sailed to the windward mark slightly faster with 3-4 deg of weather helm (positive angle of attack on both keel and rudder) so it wasn't speed as much as efficiency to weather. Any rudder deflection will reduce boat speed, but a slight bit of deflection to yield a positive angle of attack for the keel and rudder will greatly help in providing lift towards the weather mark.

One of the reasons for a balanced rudder on boats like the C25 and C250 is that they require a larger sized rudder than traditional designs having a more favorable beam to width ratio. The reason for the oversize is that control is needed when a puff pushes the boat over. The boat needs a margin of forgiveness and the extra rudder area gives it.

Ideally rudders are designed with a foil and aspect ratio that provides the best lift to drag ratio. Unfortunately such a ratio doesn't produce much lift at slow speeds so a compromise is reached with the foil and length. A high aspect ratio (longer with thinner foil and smaller chord) rudder is also more difficult to make strong.

Steve is very right in theory with his suggestion that a sailboat can be trimmed to have a light helm. The problem however is application. Both the sailplan and the hull experience dynamic balance shifts from puffs. It is these balance shifts that require increased amounts of control, more especially the dynamic balance shift resulting from sudden heeling and hence the need for a forgiveness margin of control.

A balanced rudder also reduces helm torque when backing.

Edited by - on

cat1951
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Response Posted - 12/12/2006 : 08:52:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 Mike: I'm not talking about Catalina's owner's manual... I'm talking about a looseleaf binder published by Catalina Direct, the third-party supplier of many replacements and upgrades for our boats. There's a link to their site on the Links page. They and their handbook are great resources.
<hr height="1" noshade id="quote"></blockquote id="quote">

Thanks Dave, I do use Catalina Direct already and have purchased some parts from them. I guess I need to get the binder. It sounds like a great resource.

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Dave Bristle
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Response Posted - 12/12/2006 : 09:29:38

Sorry, Al... Milby knew what he was doing when he planted this--especially when Arlyn finally took the bait.

Arlyn: When the rudder corrects for weather helm, it's pushes the keel's angle of attack to leeward, which by itself would seem to lift the boat below the desired course. However, the keel and the rudder are canceling each other out, thereby holding the desired course with the rudder turned slightly (and both are creating some additional drag). So if the speed theory is true, it must be from some other effect, such as helping the sails to generate more power as the keel pushes the mast more upright to a degree sufficient to overcome the additional drag (as a canting keel creates more power even though its bulb adds drag and its cant causes some loss of its lift). Several articles out there support this, but say vague things like "slight weather helm creates increased lift", which didn't sufficiently explain the phenomenon to me, so this is what I came up with.

Now I need a short nap.

Edited by - Dave Bristle on 12/12/2006 09:40:37

aeckhart
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Response Posted - 12/12/2006 : 09:38:06

First, thanks Steve.......thanks everyone, for an interesting and informative discussion on the balanced rudder. Although I am not a student of the physics and dynamics of boat and rudder design, I do have a passing interest in the interelationship between rudder, keel, hull, sails, rig, wind, and water. It shoud be of interest to most average sailors because the interelationship is what makes a sailboat go....or not. Even after sailing a C25 for 17 years, and sailing for 21 years, I still learning things.

Without carrying the discussion further and getting into the usefullness of weather helm, which we all know is a good thing in small doses, the bottom line is this, when your rudder breaks, as mine did several years ago, you ask yourself the following questions. Should I repair the rudder or buy a new one? If I buy new one should I buy old technology or the latest technology? The obvious answer, at least for me, is/was to buy the latest technology. In this case the balanced rudder. I have not regretted my decision because it does what it was advertised to do, makes steering a lot easier than with the old rudder. Regardless of design parameters, I bought it because I read about it on this forum, and liked what I read. It was a no brainer.

Edited by - on

Steve Milby
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Response Posted - 12/12/2006 : 10:37:46

Thanks all for a good discussion. You may have guessed that I really enjoy these talks, because they help us crystallize our thinking about complicated subjects, and they're never boring.

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Arlyn Stewart
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Response Posted - 12/12/2006 : 11:17:02

Dave, I'm wondering where this theory came from that the keel has a negative angle of attack when going to weather with a slight bit of weather helm trim?

A slight bit of weather helm for weathering means that the tiller is windward 3-4 deg giving the rudder a slight leeward trim to overcome the windward turn. I'm thinking we are in agreement on this.

Your arguing (I think)that because the rudder is showing a leeward turn trim, boat course is yawed to leeward thus yielding a negative angle of attack to weather. I don't think that is true.

The reason is that that forces on the hull have yawed it to weather with rudder trim countering those forces to hold a steady course. In actuality, this steady course is not the track but somewhere to weather of the track and a positive angle of attack to the track.

The keel generates far more lift than does the rudder. If it were true that the keel were generating negative lift to weather in your scenario, we'd all be very quick to trim the boat for a slight bit of lee helm and suffer the net loss of rudder lift. Fortunately that is not true, both the keel and the rudder are given positive angles of attack with 3-4 degrees of weather helm though the keel will show less than the rudder, perhaps 1-2 degrees.

What are under appreciated is the asymmetrical footprint lifting forces that are to leeward when heeled excessively. The hull form is the largest lifting foil existent of the many foil sections involved in a sailboat. Though the hull form footprint doesn't offer a very efficient lifting foil, its shear size brings it into play.

Bad news weather helm when heeling is mostly the product of that lift when the center of that lift is out of balance to the center of lateral resistance (CLR) because when the boat heels excessive, the boat experiences a great dynamic shift of CLR and boat balance goes to hell in a hand basket.

btw, there are racing hulls designed to generate weathering lift from the hull form. Basically they have long length to width ratios, have a gentle sheer with soft bilges and have greater amounts of rocker (belly) with the result that when heeled, the windward hull footprint is longer than the leeward... thus generating lift to weather instead of leeway as do boats like our C25 and 250. They still suffer the leeway forces of the sail plan but have everything else working to lift the boat to a weather course.

Edited by - Arlyn Stewart on 12/12/2006 11:44:13

Dave Bristle
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Response Posted - 12/12/2006 : 12:45:34

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 A slight bit of weather helm for weathering means that the tiller is windward 3-4 deg giving the rudder a slight leeward trim to overcome the windward turn. I'm thinking we are in agreement on this.
<hr height="1" noshade id="quote"></blockquote id="quote">
So far...
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Your arguing (I think)that because the rudder is showing a leeward turn trim, boat course is yawed to leeward thus yielding a negative angle of attack to weather. I don't think that is true.

The reason is that that forces on the hull have yawed it to weather with rudder trim countering those forces to hold a steady course. In actuality, this steady course is not the track but somewhere to weather of the track and a positive angle of attack to the track.
<hr height="1" noshade id="quote"></blockquote id="quote">
Whatever combination of forces (hull form, sail balance, and overall CE/CLR) is trying to turn the boat to weather, the rudder is counteracting that so the boat is on the desired track. It seems to me the rudder couldn't do so without a corresponding increase of windward pressure on the keel. When the leeward rudder has set a course where the keel's slight angle of attack to leeward balances the hull and rig's forces to weather, that's where the boat achieves a steady track. The rudder can't do that alone by turning to leeward--it's at the aft end of a very long lever, with the keel essentially as the fulcrum. The good news, as I see it, is that this causes the keel to push the boat more upright, increasing its own lifting efficiency and that of the sails.

Whew! (I'm sure I haven't won anything here...)

And Al: I agree--it's a no-brainer! C-25 sailors who haven't tried the balanced rudder can't appreciate it. Weather helm is just part of the story--every tack, turn, and gust is easier, and the tiller doesn't wiggle in your hand. Even backing up is easier--the tiller doesn't pull as hard. It'll also make things much easier for an auto-pilot. Of course Steve is right--the boat should be properly balanced (as mine was)--but IMHO a balanced rudder is a definite improvement to a balanced boat.

Edited by - Dave Bristle on 12/12/2006 12:49:36

Steve Milby
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Response Posted - 12/12/2006 : 13:26:36

It seems to me that, as a sailboat moves through the water, under sail, it's not only moving forward, but it's also constantly drifting slightly to leeward, all along it's track. If it is allowed to continue drifting to leeward, it will very gradually bear away from the wind. In order to keep it on course, and to keep it sailing as close to the wind as possible, you have to keep constantly turning it slightly to windward. That, I think, explains why you always want a little weather helm.

When the weather helm causes the boat to turn slightly to windward, the keel's angle of attack naturally follows. Instead of moving straight, in the same direction that the water is flowing past the keel, the boat (and consequently the keel) is yawed to windward at a slight angle, across the direction of the water's flow, as the boat moves through the water.

Now, I have three questions: (1) Do you think that is a reasonably accurate explanation of the general theory? (2) If so, how does that fit in with the current discussion between Dave, Arlyn and Al? (I think it has some relationship to it, but my logic got to a sticking point, and I can't get past it.) (3) If my initial statement is incorrect, what am I missing?

I just re-read Arlyn's last post, and, if I understand this complicated discussion correctly, I think it's consistent with his thinking.

Edited by - Steve Milby on 12/12/2006 13:44:59

dlucier
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Response Posted - 12/12/2006 : 16:09:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Milby
 It seems to me that, as a sailboat moves through the water, under sail, it's not only moving forward, but it's also constantly drifting slightly to leeward, all along it's track. If it is allowed to continue drifting to leeward, it will very gradually bear away from the wind. In order to keep it on course, and to keep it sailing as close to the wind as possible, you have to keep constantly turning it slightly to windward.<hr height="1" noshade id="quote"></blockquote id="quote">

Steve,

As a sailboat moves through the water (upwind), the lift being genrated by the keel pulls the boat to windward which requires the helmsman to use the rudder to steer the boat slightly to leeward to compensate and keep the boat on track.

Edited by - on

Arlyn Stewart
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Response Posted - 12/12/2006 : 19:41:51

Steve, yes... I thought your explanation was was both correct and in agreement with my comments.
<blockquote id="quote">quote:<hr height="1" noshade id="quote"> the lift being generated by the keel pulls the boat to windward which requires the helmsman to use the rudder to steer the boat slightly to leeward to compensate and keep the boat on track. <hr height="1" noshade id="quote"></blockquote id="quote">
Don, my exception to that phrase would be the inference that keel lift is the originator of weather helm yaw and I don't think that is true. Keel lift is a result of yaw, the difference between course and track.

Dave, one more time... sketch a boat hull footprint on the water and draw a center line through it. That line is the course of the boat. At a 45 degree angle, draw an arrow towards the boat representing the wind.

Now, draw another line through the boat, which will depict the leeward path of the boat due to windage. That line is the actual track of the boat. Now note angle difference between the two lines. That is the yaw angle of the boat and of course the angle of attack of the keel. Keep in mind that increasing attack angles also increase drag.

Now draw a rudder on the boat deflected for a leeward turn to counter weather helm. It should be seen as pointed out that the rudder will have a couple of more degrees angle of attack than the keel but they both will have a positive windward angle of attack.

Just for grins, now draw the rudder showing leehelm. It now shows that the rudder having an angle of attack to leeward meaning that its lift is added to leeway causing the track angle to fall off more and thus raising the attack angle of the keel and its drag.

The sketch should also illustrate why it is essential to keep speed up during a weather course. Foil lift is dependent on speed so if speed falls off...lift is lost and leeway increases, which in turn increases the angle of attack thus causing a drag increase and even more speed loss.

btw, also illustrated by this discussion is the argument I made a few years ago about swing keel C-25s. Several swinger owners over the years have observed they thought their boat had a favored weather course and I pointed towards a possible worn keel and axle which likely would allow the keel to not hold true to the boat centerline. As can be seen, the angles of attack are not much and only a shift of one degree would give a two degree favor to one tack.

Edited by - Arlyn Stewart on 12/12/2006 20:07:06

Dave Bristle
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Response Posted - 12/12/2006 : 22:05:05

Hmmmmm... Steve: With weather helm, the boat is pulled to weather by the lift of the keel, so the keel probably isn't moving "across the direction of the flow" any more than it does from normal leeway. What I've been saying is that when the rudder is brought into play to correct for weather helm, it does so by changing the keel's angle of attack. As I see it, if Arlyn is right, once the rudder has set that course, it could then be set neutral, but it can't--it must be held under pressure to a constant leeward position (tiller to windward) to maintain the desired track. The reason is that a combination of the sails (with a CE slightly aft of the CLR) and the asymetric hull form (from heeling) are constantly pushing the bow back to windward.

Where Arlyn and I disagree is apparently what's happening to the keel at that point. I can't figure out how the boat, and therefore the keel won't be yawed very slightly to leeward of the track, to counter the rudder. The boat is not turning to leeward because of the aforementioned windward forces even though the rudder is trying to turn it that way (angle of attack slightly to windward). To me, that has to be because the keel's angle of attack is slightly to leeward, in order to maintain a steady track against the weather helm forces. It's the keel, ultimately, that offsets those forces--I think we agree the hull form is contributing to them. The rudder is holding the keel in the position to do so. (Yes Arlyn, I already drew my own version of a force diagram.) With plenty of speed and decent balance, these angles are miniscule--obviously the keel is huge compared to the rudder and doesn't need much of an angle to counteract it--maybe one degree against four--to hold the boat on its track.

Oh well, maybe I'm wrong... but I'm going over to the Dark Side anyway. I'll rethink the whole issue some day out on the Mystic River in a Beetle Cat. Maybe I'll run some string out behind the boat, set up some weather helm, and see how my track compares to the way the boat is pointing. If the string trails off toward the leeward side of the boat, I was right. If it trails to windward, Arlyn was. Hmmmm... then there's leeway... rats. Bad experiment.

Edited by - Dave Bristle on 12/12/2006 22:18:18

Steve Milby
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Response Posted - 12/13/2006 : 07:21:10

Most traditional explanations of weather helm and "balance" speak in terms of 'lifting surfaces" and aerodynamic principles, and those terms ("gobbledeygook" to most of us)(...except perhaps Arlyn

) are confusing to most of us, and make us yawn.

I can understand it, however, if you tell me that, when a boat is sailing to windward, it not only moves forward through the water, but it also drifts to leeward. If you don't do something to counteract that constant drift to leeward, a sailboat will very gradually bear away from the wind, making a slow turn to leeward.

There are two ways you can stop a sailboat from bearing off to leeward. You can use the rudder to steer the boat to windward, but that's not a good way to do it, because, whenever you turn the rudder, it creates drag and slows down the boat. The other way you can stop a sailboat from bearing off to leeward is to change the balance of the whole sailplan (the Center of Effort) in relation to the balance of all the forces acting on the hull and keel (the Center of Lateral Resistance). If you design the boat and adjust the rig so that the CE is slightly forward of the CLR, the boat will have a slight weather helm, and that weather helm will counteract the boat's tendency to bear off to leeward. This is the better way of counteracting the boat's tendency to bear off to leeward, because it greatly reduces the need to use the rudder to make the boat sail to windward. That explains why it's faster to have a little weather helm. If the boat has a lee helm, then you have to use the rudder to keep it on course.

This is why I like these discussions. I used to have to re-read and re-learn all the technical terms every time I wanted to talk about the subject. Finally, I'm starting to understand the process without all the gobbledeygook.

Edited by - on

dlucier
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Response Posted - 12/13/2006 : 07:23:57

Edited by - on

dlucier
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Response Posted - 12/13/2006 : 07:43:41

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Milby
 ...The other way you can stop a sailboat from bearing off to leeward is to change the balance of the whole sailplan (the Center of Effort) in relation to the balance of all the forces acting on the hull and keel (the Center of Lateral Resistance). If you design the boat and adjust the rig so that the CE is slightly forward of the CLR, the boat will have a slight weather helm, and that weather helm will counteract the boat's tendency to bear off to leeward. <hr height="1" noshade id="quote"></blockquote id="quote">

Steve,

If the CE is forward of the CLR, wouldn't the boat pivot to leeward?

Edited by - on

Steve Milby
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Response Posted - 12/13/2006 : 08:36:49

<blockquote id="quote">quote:<hr height="1" noshade id="quote">If the CE is forward of the CLR, wouldn't the boat pivot to leeward?<hr height="1" noshade id="quote"></blockquote id="quote">That's a good question. It doesn't seem to make sense, does it! But, in "Fundamentals of Sailing, Cruising & Racing" (pg. 91), Steve Colgate says: "...naval architects design a boat with the CE slightly forward of the CLR (a distance called 'lead') in order to balance the boat.

As I understand it, a sailboat has to be designed with a "lead." Sailboats don't ordinarily sail bolt upright. They usually heel a little to leeward, and as they heel to leeward, the whole sailing rig leans out over the water on one side of the boat. That creates an imbalance that causes the boat to want to turn to windward. Secondly, the shape of the sailboat's hull is curved, and, when the boat is heeled, that curved shape on the leeward side of the hull forces the bow to windward.

Edited by - on

wmeinert@kconline.com
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Response Posted - 12/13/2006 : 09:30:16

OK Guys'
After two days of reading this stuff, what I want to know is if the keel helps generate lift? Then why is a major weather helm required up wind while racing? Second I get completly lost when I'm thinking about fin vs Swing vs Wing. Iherent to each boat there is no magical answer? Up wind swings seem to point higher, down wind they don't as well as fins. With the swing up there is greater weather helm than when it is down. I have used a balanced rudder on my swing with less desirable results than a standard. Does the added water lenght (3inches) add speed or take it away.

Chew on that for a while!

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Dave Bristle
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Response Posted - 12/13/2006 : 09:50:48

An epiphany in the shower this morning! I figured it out! Then I went back and read Arlyn's posts, and he's right (mostly)--I was wrong (sort of) in that I didn't take into account the windward yaw of a keel even with totally balanced helm (not weather, not lee) that's another way of describing leeway. I still believe the rudder ends up doing what I said--it changes the keel's angle of attack. That angle, with the balance that is causing weather helm, must be changed slightly to counteract the weather helm. The change is slightly to leeward, but the keel is still yawed enough to windward to provide its lift--just less so. (The difference, as diagrammed, is leeway.) Thus, less drag on the keel yields better boat speed which increases the keel efficiency which allows a higher track to windward... So getting back to my rigger/racer's assertion, slight weather helm makes a sailboat faster--not slower. Works for me!

To anybody who's still following this tread, I have no apologies...

These are the meanderings of some guys who can't sail right now, are fascinated by sailing, and have a need to understand how it really works (or explain it to those who want to know). There's a lot of writing out there that never really explains it to me, mostly because most writers and many good sailors don't truly understand it--they just regurgitate things they've heard or read. So I'm left to try to figure it out, with more help from Arlyn than from Steve Colgate and his diagrams.

Peace! Over and out...

Edited by - on

Dave Bristle
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Response Posted - 12/13/2006 : 10:01:00

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by wmeinert@kconline.com
 ...why is a major weather helm required up wind while racing? <hr height="1" noshade id="quote"></blockquote id="quote">
See if my post above helps. (But scratch the word "major".)
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Second I get completly lost when I'm thinking about fin vs Swing vs Wing. Iherent to each boat there is no magical answer? Up wind swings seem to point higher, down wind they don't as well as fins. With the swing up there is greater weather helm than when it is down.<hr height="1" noshade id="quote"></blockquote id="quote">
The swing keel is just a more efficient foil than the other two--particularly because of its depth compared to its width (aspect ratio). Look at sailplanes and reconnaissance planes--the wings are very long and skinny for maximum efficiency and therefore lift. And racing sailboats have tall masts and short booms for the same reason--maximum lift from the sails. But downwind, a deeper keel creates nothing but more drag--virtually no lift is required unless you're sailing off the wind to gain speed.

Are you sure pulling the swinger up doesn't cause lee helm? If it increases weather helm, I'm at a loss just off hand. (I might need another shower for that one.) Arlyn??

Edited by - Dave Bristle on 12/13/2006 10:10:29

Arlyn Stewart
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Response Posted - 12/13/2006 : 10:10:09

Dave, it seems to me that you've tied weather helm to keel angle of attack.

Forgetting weather helm for a moment, if there were neutral or lee helm there would still be keel angle of attack to weather.

The windward course is what it is, the direction the boat is pointing. The track is course minus leeway. The angle between these is the boat yaw.

It is an entirely separate dynamic than weather or lee helm.

Edited by - on

Dave Bristle
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Response Posted - 12/13/2006 : 10:21:34

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Arlyn Stewart
 Dave, it seems to me that you've tied weather helm to keel angle of attack.

Forgetting weather helm for a moment, if there were neutral or lee helm there would still be keel angle of attack to weather.

The windward course is what it is, the direction the boat is pointing. The track is course minus leeway. The angle between these is the boat yaw.

It is an entirely separate dynamic than weather or lee helm.
<hr height="1" noshade id="quote"></blockquote id="quote">
So, you're not going to let me off the hook...

I'm saying they can't be separated--they all tie together. The rudder doesn't create or eliminate the windward keel angle--it just changes it slightly to counter weather helm. The rudder cannot do that by itself, especially by shifting its angle of attack to windward. It has to be adjusting the keel very slightly or nothing good comes of it. Anything the rudder does affects the keel--they work together (and partly in opposition) to turn the boat or hold it on course against the forces from the rig. That I'll stand by.

Edited by - on

Arlyn Stewart
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Response Posted - 12/13/2006 : 10:30:21

Dave, the argument that because there is rudder deflection to counter helm forces with the result of boat yaw from its course I think is incorrect.

There would be yaw if the helm force were not corrected by an equal opposing force of rudder trim.

The boat course has no yaw as long as helm forces have a rudder correction.

I still disagree that weather helm makes a boat sail faster. Any increased drag will make it slower. What is faster with some weather helm is the fetching of the windward mark and that is because the rudder then starts lifting to windward and reducing leeway.

Edited by - on

Steve Milby
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Response Posted - 12/13/2006 : 10:50:58

<blockquote id="quote">quote:<hr height="1" noshade id="quote">...why is a major weather helm required up wind while racing?<hr height="1" noshade id="quote"></blockquote id="quote">Bill, when we're racing, we aren't necessarily sailing the boat in the smartest, most seamanlike manner possible. When the wind pipes up, the smart, seamanlike thing to do is to reduce sail area, and keep the boat on her feet and in good control. But we racers don't do that. We carry as much sail area as we possibly can under the conditions, even though the boat might be heeling a little too much, and even though it might not be pointing as well as it should, and even though it might be on the ragged edge of being out of control.

As a result, a racing sailboat is often overpowered much of the time. When a sailboat is overpowered, it heels more than it should, and, the more a sailboat heels, the more weather helm you'll feel.

Any person with an ounce of common sense might ask the obvious question, "Why do racers sail overpowered, if it's not the smartest, most seamanlike way to sail?" The seamanlike way to sail is to reduce the angle of heel, because it's safer and more comfortable and less tiring for the crew. They don't have to hang onto the high side of the boat with all their strength. A good seaman will also sail the boat in a way that reduces the stress on the sails and rig, and prevents the hull from pounding. None of those things matter to a racer. His concern is to sail the boat around the course in less time than his opponents. Except for the most basic concerns for safety, the general principles of good seamanship fall by the wayside when you're racing. Also, you have to remember that, as you're sailing around a race course, the wind is constantly gusting and lulling. When it's gusting, and you're sailing to windward, the boat might be overpowered with the amount of sail area you're carrying. But, in the lulls or downwind, the boat might be underpowered with the same amount of sail area. In fact, when you're sailing downwind, you can usually carry as much sail area as possible, and more. A racer doesn't want to be the first to reduce sail area as the wind rises, because you don't want to be out there with a "smaller engine" (your sails) than your opponents. It takes a lot of self-confidence to be the first to reduce sail area. So, when you're racing, you fly the sails that you think will give you the best overrall average speed and pointing ability, all the way around the course, on all points of sail, and during the puffs and the lulls.

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
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Response Posted - 12/13/2006 : 11:29:25

Bill, I'll take a stab at the rudder comparison.

The original rudder may have been a high aspect ratio rudder (long, not to thick and not too wide). They have the best lift to drag ratio but there are other foil shapes that can produce more lift (control) but at the cost of much greater amounts of drag.

A high aspect ratio rudder needs a good bit of speed to produce lift as well so its low speed control would be limited. High aspect ratio rudders are thinner and more fragile and more difficult to construct with needed strength.

The replacement balanced rudder was likely a thicker foil and wider chord to produce more control but having more drag. Drag is a very nasty word for a racer who wants as little of it as possible and hence likely why you preferred the original rudder.

As Steve pointed out very well in his original post, a racer also wants to know when weather helm is serious because it means excess drag... so they prefer an unbalanced rudder that quickly informs the skipper that the boat is out of balance by manifesting high degrees of tiller torque.

Edited by - on

Dave Bristle
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Djibouti
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Response Posted - 12/13/2006 : 11:35:16

Well Arlyn, you brought me around partly--to agreeing the the keel is pointing to windward of the track one way or the other... But you aren't getting me the rest of the way--I can't separate the change in forces on the rudder from a corresponding change in forces on the keel. In my mind, the forces don't resolve. I believe that, in correcting for weather helm, the rudder, because of the lever arm that is the hull, will change the angle of attack of the keel, and that those two changes--not just one by itself--are what keeps the boat from turning to windward. The change at the keel is very small--it's several times the size of the rudder--but there has to be a change. I'm not totally sure of the consequences, but I can see how they could be positive in terms of speed and lift. BTW, if all drag is bad, then the fastest boat would have no keel. Absurd, yes, but some things that add drag (like a longer keel, or perhaps a tiller a few degrees to windward) can make a boat faster to windward.

At this point, I think I'll just agree to disagree--I don't want this to become the "war" I had with a physics professor years ago when he asserted absolutely that water ballast could not create a righting moment (countering heel) until the water in the tank was raised above the level of the water outside of the boat! He would not be swayed (or heeled?) by any logic (or basic physics), and got ugly about it. (Was I, after all, a physics professor??) This discussion is a lot more complicated, and I suspect we're both right. On that we can disagree, too.

Edited by - Dave Bristle on 12/13/2006 11:40:11

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