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Arlyn Stewart
Master Marine Consultant

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USA
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Initially Posted - 09/26/2003 : 07:32:00

http://www.stewartfam.net/arlyn/cause_of_weather_helm.html

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 09/26/2003 : 12:27:34

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
http://www.stewartfam.net/arlyn/cause_of_weather_helm.html

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Would they be tunable? Left-right, flaps out or flaps in. Up-down, deeper shallower. Rotate cw-ccw, Angle to centerline- away from center line. Could they be "gimbaled" and ballasted to effect a self tuning? Is the lift so predictable that the placement of the tab is a matter of computation and tuning would be superfluous? What about a trim tab on the keel or CB? Is the RC technology there to have an underwater actuator and a wireless controller, could a micro processor could allow you to program the tab response to the conditions reported by a gyro. I don't have and have never used an autohelm but is that what is going on there to a degree? Your idea seems elegant and straight forward, usually the best ideas are.

<img src="http://members.cox.net/fhopper/Catalina25/sigbow.jpg" border=0>Frank and Martha in Wichita KS. Lake Cheney

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 09/26/2003 : 16:00:52

Arlyn: That's an interesting read, at least to those of us (or maybe just me) who enjoy trying to understand or figure out how/why things really work.

I suspect that, just as with the physics of how a marconi rig sails upwind, there are several factors involved--one being Bill's (which I call the "wedge theory", which I think your diagram illustrates very well). Another may be your "foil theory"--just a little tougher for us barnyard physicists. I'll add a new one (I just made it up) to the fray--I'll call it the "stick theory"...

This theory, in very early stages of development, says that as the heel angle increases, the headsail, being on an angled, slighly sagging forestay and generally being more full than the mains'l (particularly with Catalina sheeting angles), depowers more than the main, which is on two sticks with various hardware (vang, traveler, cunningham, outhaul, etc.) keeping it more tightly oriented to the wind. The headsail (particularly on a C-25) could still have more power than the main, but in relative terms, it also loses more. Thus, the CE moves aft as the heel angle increases, while the CLR remains roughly the same. That's about it.

Whaddaya think?

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - dave bristle on 09/26/2003 16:03:45

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Arlyn Stewart
Master Marine Consultant

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2980 Posts

Response Posted - 09/27/2003 : 08:56:20

Dave,

As noted, I couldn't reconcile the stacking up and bow pushing windward theory to my observed results, hence the reason to continue to ask why. You may have misinterpreted my drawing as it was not intended to support that theory.

I've edited and added a slight bit of material to illustrate the forces which I believe are the ones that do provide an explanation that fit observed results. That illustration draws upon beach cat design where lack of symmetry is used as a tool for lifting purposes.

Now to your throw in on this, does a C25 running under a 150 genoa without mainsail... experience any weather helm when overpowerd and suffering significant heeling?

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 09/27/2003 : 09:11:49

Frank... my vision of them would be more of a spoiler than a flap as they would extend angular from the low pressure side of the foil rather than as a flap does on the high pressure side. I'm thinking they would be fixed...and their effect would be completely automatic. While some form of gimbaled or counter balanced control is interesting ... the mechanics start getting a lot more complicated than these simple tabs would be. Yes, simple is often the best.

Also, I'll speculate that the C250 2nd generation rudder which is capable of providing adequate control to deal with adverse weather helm is probable twice as large as would be required if no propensity for weather helm came into play. This means that if tabs handled that extra requirement... the drag of the current rudder could be reduced dramatically.

I probably need to say...that I'm not suggesting tabs for a c25... as I don't think it suffers rounding up issues that the C250 has. I posted in the general section because of the theory aspect of the cause of the weather helm.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - arlyn stewart on 09/27/2003 09:15:07

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Steve Milby
Past Commodore

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5902 Posts

Response Posted - 09/27/2003 : 09:40:17

Excessive weather helm is caused by an imbalance between the Center of Effort of the sail plan (CE) and the Center of Lateral Resistance of the keel (CLR). Normally, the CE is located slightly forward of the CLR. If the CE moves aft, in relation to the CLR, weather helm increases.

Because of the curved shape of the sails, each sail produces vectors of force all along the curve that are perpendicular to the surface of the sail. The force vectors in the leading 1/3 of the sail create forward drive. The force vectors in the middle 1/3 of the sail create heeling force. The force vectors in the aft 1/3 of the sail create drag. Normally, the force vectors in the forward 1/3 of the sail are longer and stronger than those in the other 2/3 of the sail. As the wind velocity increases, the draft of each sail (i.e., the pocket of the sail, where its power is generated) moves aft. As a result, more power is being generated on the parts of each sail that create heeling force and drag, and less power is being generated on the part of each sail that creates forward drive. This causes the CE of the whole sail plan to move aft, and this is what causes increased weather helm.

Increased weather helm is not caused by water piling up on the leeward bow and effectively trying to push the bow toward the wind. (I’m not saying that water doesn’t pile up on the leeward bow, as Bill suggests. Perhaps its does.) But, if we assume, for the sake of argument, that water piles up on the leeward bow, it is because the boat is heeling. The boat is heeling because the CE has moved aft, the force vectors that cause heeling are predominant, and the boat is overpowered. If the water piles up on the lee bow, it does so because the boat is overpowered. Thus, when water piles up on the lee bow, it is not the cause of weather helm. It is a consequence of heeling. If you prevent the boat from heeling excessively, water will not pile up on the lee bow. Heeling is caused by an overpowered sail plan in which the CE has moved aft. Thus, the cause of excessive weather helm is an imbalance between the CE and CLR.

Likewise, increased weather helm is not caused by the assymetrical shape of the heeled hull generating a pressure difference between the two sides. Once again, I’m not saying that Arlyn’s theory is incorrect. I don’t know enough about the theory of foils to have a worthwhile opinion. But, what I am sure of is that the assymetrical shape of the hull is the consequence of heeling. It is caused by the fact that the boat is heeling, and when the amount of heel becomes excessive, it is because the sails are overpowering the ability of the keel to resist heeling. The boat is heeling, and it has excessive weather helm because the sail plan is overpowered and the CE has moved aft in relation to the CLR.

If increased wind velocity has caused the CE to move aft, you can move the CE of the jib forward again by depowering (flattening) it. It can be flattened by increasing the tension on the backstay adjuster, and by increasing the tension on the halyard, and by increasing the tension on the jibsheet. You can also move the CE of the mainsail forward by flattening it. It can be flattened by increasing the tension on the backstay adjuster, and by increasing the tension on the halyard or the cunningham or the downhaul, and by increasing tension on the outhaul, and by increasing tension on the boom vang. By trimming both sails in this manner, you can move the CE of the sailplan forward again, thereby reducing heeling forces, drag and weather helm. If you use all the sail adjustments to move the draft of the sails forward, and if the windspeed continues to increase, then you need to start reducing sail area in order to depower and balance the sailplan.

Steve Milby "Captiva Wind" C-25 T/FK #2554

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Dave Bristle
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Djibouti
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Response Posted - 09/27/2003 : 10:42:33

Ah hah--the debate os on! <img src=icon_smile_wink.gif border=0 align=middle> OK, here we go...

Arlyn: I do sail the C-25 occasionally in about 15 knots under a 130 alone. In general, I notice a light lee helm--I'll have to pay closer attention to how it shifts in puffs, but I don't think it's ever shifted to weather, at least up to 18-25 knot puffs.

Steve: While my "stick theory" uses the CE/CLR relationship, it trys to explain what happens when the rig isn't adjusted to change the shape of the sails. Most of us don't have the laminated sails, rig controls, or sheeting angles that allow us to flatten the headsail as much as the main... The consequence of that would seem to be to move the CE forward, but that apparently isn't what happens. The "stick theory" suggests that as heel angle increases, the jib loses its presentation to the wind more than the main does. (When you're tacking or cheating up on the wind, which sail luffs first?) In effect, you have a smaller proportion of sail forward of the CLR when heeling than you do when the boat is upright. If that weren't true, wouldn't the effect of overpowering, since the main is naturally flatter, be to decrease weather helm?

As for Bill's and Arlyn's theories about hull shape, I can't dismiss the effect of the asymetrical shape of the wetted hull under heel--it's going to do SOMETHING. Bill and I see a "wedge" that drives the bow windward. Arlyn sees a foil that lifts the stern to leeward. You see an asymetric shape that doesn't have any effect? Of course that shape is the result of heeling--that's the point. But boats are designed with symetric hulls for a reason--an asymetric shape makes them want to go like a Tennessee Ridge Runner on flat ground.

Gotta go...

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - on

Sea Trac
Master Marine Consultant

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Vanuatu
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Response Posted - 09/27/2003 : 22:42:19

I don't remember where I read this, but it was probably The Annapolis Book of Seamanship. The CE moves aft as wind velocity increases and, therefore, we sheet in to move the CE forward again. If you sheet in too much, as in light air, you move the CE forward to or in front of the luff and stall the sail. If you sheet out too much, you move the CE aft to or behind the leech and luff the sail.

When yacht design, specifically racing, went to fin and centerboard keels, weatherhelm and lee helm increased significantly. The fin or centerboard became a pivot point for the boat, whereas full keel boats have substantially more lateral stability. If you forcefully place your finger on the center of a pencil laying on a desk, thereby replicating the center of gravity represented by the fin or centerboard, it is difficult to pivot the pencil by pushing near the center, but easy by pushing near the end.

As wind velocity continues to increase and the sails have been trimmed to the greatest extent possible, due to the configuration of the rigging, the CEs continue to move aft. The CE of the jib moves closer to the mast (i.e., center of the pencil) and the CE of the main moves closer to the stern (i.e., end of the pencil). At a certain point, the CE of the jib has moved far enough back to either fall off the leech or to at least lose its ability to counterbalance the CE of the main. Once this happens, the boat immediately rounds up due to the amount of lateral force being exerted behind the keel near the stern. The opposite happens (i.e., lee helm or bow blowing off) when the CE of the jib remains forward and the CE of the main is too far forward (i.e., over trimmed) or too far aft (i.e., under trimmed) and loses its ability to counterbalance the CE of the jib.

Racing designs (e.g., Open 60s) counteract these forces by moving the fin keel aft, dropping the keel bulb deeper, canting the keel, placing dual rudders outboard, and by running rigging designs that allow "over" trimming of the sails. The result, of course, is substantial heel angles and knock-downs as opposed to rounding up.

P.s. I just reread the thread and Bill's Tech Tip, and I think we're all basically saying the same thing, only differently. <img src=icon_smile.gif border=0 align=middle>

J.B. Manley
<img src="http://im1.shutterfly.com/procserv/47b3d609b3127cce8d2e212441c60000008010" border=0>
Antares '85 FK/SR #4849
36°29'58" -94°59'59"

Edited by - Antares on 09/27/2003 23:06:21

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Steve Milby
Past Commodore

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USA
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Response Posted - 09/28/2003 : 09:37:17

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> Most of us don't have the laminated sails, rig controls, or sheeting angles that allow us to flatten the headsail as much as the main... The consequence of that would seem to be to move the CE forward, but that apparently isn't what happens. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Dave, my sails are plain dacron, and you can flatten a dacron jib just as much as a laminated jib. The only controls you need to flatten your jib is a jib halyard and a jibsheet. Any other control devices are completely dispensable.

In order to flatten your jib properly, the mast has to be tuned correctly, with enough headstay tension so that the headstay doesn’t sag in a strong wind. When you raise the jib, you have to put enough tension on the jib halyard so that the luff of the jib doesn’t sag. The stronger the wind, the more tension you need on the jib halyard. (If you have a roller furler, I’ll grant you that the halyard can’t be adjusted as it can with hank on sails. That makes it especially important that your rig be correctly tuned and the halyard tension be adjusted correctly. The inability to adjust luff tension for varying wind strengths is one of the reasons why roller furlers aren’t as efficient as hank on sails.) The genoa car has to be adjusted correctly for the amount of wind and the size of the jib. Finally, the jibsheet has to be adjusted correctly for sailing closehauled. This is where many sailors go wrong. Many sailors don’t trim their jibsheet tight enough when they beat to windward. A 150% headsail should be adjusted so that the sail comes within about 6-8 inches from the leeward spreader. If you will look at your sail while beating to windward, I’ll bet you don’t trim it that close to the spreader. (In your reply you ask “When you're tacking or cheating up on the wind, which sail luffs first? That question implies that your jib luffs first. If that’s so, it means your jib is sheeted too full, because that will cause your jib to luff first. If your sails are trimmed correctly, the jib and mainsail will luff at about the same time when you tack.) If you don’t trim the jib flat enough, the sail’s draft is too deep and the draft is too far aft to beat to windward efficiently. If the shape of the jib is too full, it causes the boat to heel more, and the wind pulls your bow off to leeward, especially in the puffs. If your jib is flatter, the boat stands up better in the puffs, and points higher. Many people think that you should always adjust the jib so that it is a little full, to increase its power, but that only applies to light air. In strong wind you have more power than you need, so you need to depower the sail, to reduce heeling and weather helm, and you depower the sail by flattening it. If you are flying the 110% jib, you need to sheet it even closer to the spreader in strong wind. I trim mine as close to the spreader as possible without touching the spreader. (about 1”)

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> I can't dismiss the effect of the asymetrical shape of the wetted hull under heel--it's going to do SOMETHING.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

The asymmetrical shape of the hull does do something. It creates drag, but it is not the cause of weather helm. Excessive heeling and weather helm are caused when the CE moves too far aft.

Got to go. We're expecting a cold, wet race today!

Steve Milby "Captiva Wind" C-25 T/FK #2554

Edited by - on

Arlyn Stewart
Master Marine Consultant

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Response Posted - 09/28/2003 : 12:08:38

Back in my Hobie Cat racing days... I found it always fascinating how the first windward leg was so competitive between the Hobie 18 and the 16. Those frigging 16's were witches to weather... after making the mark... my 18 could quickly walk away.

Q. Why was the 16 so efficient going to weather? And, whats this got to do with weather helm on a Catalina?
A. The Hobie 16 uses asymmetrical hulls. It has no dagger boards or other leeway prevention. Burying the leeward hull causes it to lift to weather... not only overcoming leeway... but more. Trailing one of them to the mark, one would swear that they were pointing no better... yet they gain to windward. The application of the principle of lift of an asymmetrical hull shape is proved in their design. And, it demonstrates the strength of a lifting foil, to counter the leeward forces and actually gain shows the power of this force. Have you ever stood on the tarmac of an airport... and watched them load baggage into a airliner? Then thinking of all the passengers that were onboard? Then adding fuel and the weight of the airplane. Did it seem a little overwhelming that this thing would soon break gravity and fly... how can it be? The power in a lifting foil!

Q. A great many designs can heel greatly and yet not incur rounding up kinds of weather helm... why do some and not others?
A. I think the answer is far more the hull form differences rather than the sailplan. Basically the dynamics of the sailplans are either similar to the two comparisons or they can be adjusted similar... but yet one fights rounding up and the other doesn't.

Q. How can this be?
A. In order for yaw (rotation of the boat and thus weather helm) to be produced, a force and an axis of rotation are needed and they would be out of balance. One design then, is out of balance and the other in balance.

Q. I get it that the keel is the axis of rotation... but what forces?
A. Assessing the forces at play is the hard part. To gain that perspective requires reading the clues and discovering what provides explanations and what doesn't.

Q. How can a boat, sailing in very good balance...suddenly go out of balance with heeling?
A. The traditional argument is that the CE shifts aft and and now with heeling it is over the leeward rail where it exerts a force to rotate the boat. I don't accept that heeling significantly shifts the CE aft when moments before it was in good balance. The explanation has to be elsewhere and the chief change that occurs with heeling is the footprint form.

Q. How would a changing footprint bring on a strong force?
A. If the footprint becomes asymmetrical, it creates a lifting foil. Lifting foils inherently contain a lot of force... especially in a hydro medium.

I am not suggesting in my theory that these forces are singular but rather that the prime reason is not being seen. In fact, it may be because of the multiplicity of forces that at some point it was concluded...enough... whatever is causing it... has more than likely been named... and its gotten too complicated now to know for sure any way... so who cares. I also don't think that the force in my theory comes into play until heel becomes significant... so its the others forces which have been traditionally understood that govern weather helm when sailing on or near the water lines. My argument is that, I doubt those traditional forces become so exaggerated with heel as to cause rounding up.

Certainly, excess heel is caused by an overpowered rig. But, getting overpowered is part of sailing, especially on a high SD ratio design. Its both simplistic and unrealistic to say.. keep it flat. To do so is not to go sailing... and thats unacceptable. What is needed is a heeling buffer zone prior to rounding up forces. To gain that... understanding of what is really happening will help. Heeling doesn't have to cause rounding up.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - on

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 09/28/2003 : 12:49:48

Steve: You're right--I probably don't sheet my genny tightly enough, or trim it when I should often enough as the wind increases. But the question here is why something is happening, and that something is the increasing of weather helm as the heel angle increases. The theory I'm presenting suggests that the differing characteristics of the two sails has something to do with it.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>...If your sails are trimmed correctly, the jib and mainsail will luff at about the same time when you tack...<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

You could trim them that way, but if you harden both to the max on a C-25, the jib has to luff first--the sheeting angles (and the spreaders) dictate that. That's a difference in geometry between the sails. Other differences are that the mast is stiffer than the forestay; the outhaul has more horizontal purchase than the angled jib sheet; and you can bend the mast back to flatten the main, but you can only try to straighten the forestay (never completely) to flatten the headsail.

My theory says that as the heel angle increases, the sum of those differences, particularly the sheeting angles, cause the jib to begin to "disappear" at a greater rate than the main--moving the CE aft.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>The asymmetrical shape of the hull does do something. It creates drag, but it is not the cause of weather helm.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

You call it drag, Arlyn calls it lift, and Bill calls it push... Take your pick--if there's more on one side of the CLR than the other, something's gonna give. Arlyn's lift is aft of the CLR, Bill's push is forward of it. Where's your drag?

Weather helm is caused by a combination of forces--some from the wind, others from the water. The resolution of all of those forces results in the boat sailing upwind. Imbalances in those forces cause weather or lee helm. But they all matter.

Unfortunately, I won't be testing any of this today--it's raining, probably all day. <img src=icon_smile_sad.gif border=0 align=middle>

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - on

Arlyn Stewart
Master Marine Consultant

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2980 Posts

Response Posted - 09/28/2003 : 13:32:08

btw...Steve Colgate in an excellent weather helm analysis offers this pic to illustrate Bill's pushing of the bow... I don't buy it a great deal but his overall article is good... I still think it leaves out the greatest results of adverse heel.

<img src="http://www.sailnet.com/images/content/authors/colgate/052600sc_figure4.gif" border=0>

[url="http://www.sailnet.com/collections/learningtosail/index.cfm?articleID=colgat005&coll_cat=Theory&Coll_name=Basic%20Sailing%20Theory"]Steve Colgate article [/url]

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 09/28/2003 : 19:56:09

Arlyn: Steve Colgate doesn't give either of our theories much credit... However, I liked his following chapter even better--the explanation of hull speed. I always wondered where the 1.34 came from.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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Arlyn Stewart
Master Marine Consultant

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Response Posted - 09/28/2003 : 21:03:37

Dave... we aren't making our living sailing...we can afford to be speculative and labeled a nut. <img src=icon_smile_cool.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle><img src=icon_smile_big.gif border=0 align=middle>

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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atgep
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1009 Posts

Response Posted - 09/28/2003 : 23:12:44

As a windsurfer for the past 20 years I can offer a few observations on sail balance and shape.

1. on all the sails I have used the center of effort moves aft as the wind velocity picks up.
2. sails that are too large become uncontrollable. As soon as the sail is sheeted in the c/e is so far aft the sail is unstable.
3. as wind speed approached 25 knots the sail is either powered or it is flapping.
4. above 25 knots the wind speed increases cause an exponential increase in force. ie 30 knots is twice as powerfull as 25.
5. I fly for a living and my airplane has a sweet spot of 18-32% mean air chord for proper flight(lifting air foil) . Above 36% and the wing is stalled.

All this leads me to believe something is being overpowered causing the weather helm. It could be the sails, keel, or a combination.

Or perhaps it is something else..............

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Steve Milby
Past Commodore

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Response Posted - 09/29/2003 : 11:52:32

quote: ...If your sails are trimmed correctly, the jib and mainsail will luff at about the same time when you tack...

You said: "You could trim them that way, but if you harden both to the max on a C-25, the jib has to luff first."

Adjusting the jib to within 6-8 inches of the spreader, and adjusting the mainsail in relation to the jib, is not hardening them to the max. That’s just trimming them in the way they were designed to be trimmed. If you trim the jib closer than 6-8 inches from the spreader, the sails will usually be too flat and lose too much drive. The reason why I mention this is because people often think there are two ways to trim sails; the racers’ way and the cruisers’ way. There’s really only one correct way to trim sails that applies equally to racers and cruisers. That one way drives the boat in the most efficient manner, regardless of whether you’re racing or cruising, it reduces stress on the boat and crew by keeping the boat upright and preventing it from laboring, and it’s safer.

The reason why the jib and mainsail luff at about the same time is because they are trimmed in relation to each other. When you tack while beating to windward, you should trim your jib first (about 6-8 inches from the spreader for a 150% jib). Next, you should trim the mainsail in tight, and then slowly ease it out until the luff of the mainsail just begins to lift slightly. As you can see, the mainsail is trimmed in relation to the jib, so that both the jib and the mainsail are just on the verge of luffing. When you steer the boat slightly closer to windward, the jib and mainsail begin to luff at about the same time.

I wish I knew how to post a drawing, but I don’t, so I’ll ask you to do a simple line drawing of a typical masthead sloop sailboat (like a C-25; Remember that the jib usually sweeps the foredeck, and the mainsail is attached to a boom that is somewhat above the coach roof.). Now, draw a line from each corner of the jib to the mid-point of the opposite side of the sail. The three lines will intersect in the middle of the jib, and that point is the Center of Effort for the jib. Now do the same for the mainsail. The point where the three lines intersect is the Center of Effort for the mainsail. Now, draw a line from the Jib’s CE to the Mainsail’s CE. The midpoint of that line is the Center of Effort for the sailplan. If the jib is a 100% jib, the CE for the sailplan will usually be located slightly forward of the mast. The Center of Lateral Resistance will typically be located about in the middle of the keel, but it will be a little higher, in the area of the boat’s bilge. Normally, the CE for the sailplan is located forward of the CLR.

Imagine that the jib on the boat that you drew is a 150% jib. As you can see, the CE for that sail will be located farther aft, near the mast. The CE for the sailplan will also move aft, closer to the CLR. As the CE for the sailplan moves aft of the CLR, weather helm increases, and you have to pull harder on the tiller to keep the boat from heading up to windward. That explains why weather helm diminishes when you change down from a 150% jib to a 110% jib. (Raising a smaller jib moves the CE forward.)

The CE can move forward and aft, depending on the size of either sail. If you reef the mainsail, that also makes the CE move forward, and that explains why reefing the mainsail reduces weather helm. (Reducing the size of the mainsail moves the CE forward.)

Now, suppose you are flying a 150% jib and full mainsail. As the wind increases in strength, the wind causes the draft of each sail to move aft. When the draft of each sail moves aft, the CE of the sailplan also moves aft, and that causes an increase in weather helm, heeling and drag.

The Steve Colgate illustration shows a heeling boat with a bow wave piling water on the leeward bow. But you can’t alter the shape of your bow or your hull to make that wave go away. All you can do is change the trim of your sails, to cause the boat to sail more upright. If you retrim your sails, the boat will stop heeling excessively, and that pile of water will go away. That’s why it is important to understand that the underlying cause of all these problems (excessive heeling, weather helm, sail draft moving aft, water piling up on the leeward bow, etc.) is the interaction of the wind velocity with your sails. If you understand that, then you know that the cure for all those problems can be found in restoring balance to the sail plan. Once you understand that principle, then the next question is, How do you restore balance to the sailplan? You do that by depowering (flattening the shape of) both sails (thereby moving the CE of each sail and of the sailplan forward). If you have depowered the sails as much as possible and weather helm is still excessive, then you move the CE forward by decreasing the size of the jib, or the mainsail, or both. You also restore balance to the sailplan by putting your crew weight on the windward rail, to help counteract the force of the wind. You also do it by using good helmsmanship (feathering the boat to windward in the puffs, to reduce the pressure on the sails and help keep the boat upright).

Steve Milby "Captiva Wind" C-25 T/FK #2554

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Champipple
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Response Posted - 09/29/2003 : 14:50:35

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>In general, I notice a light lee helm<hr height=1 noshade id=quote></BLOCKQUOTE id=quote> Dave - Didn't General Lee Helms run unopposed for a US Senate Seat in 2000?

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
The reason why I mention this is because people often think there are two ways to trim sails; the racers’ way and the cruisers’ way. There’s really only one correct way to trim sails that applies equally to racers and cruisers. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

That statement assumes Cruisers actually trim sails Steve? I know if I am cruising and not racing, the sails get trimmed to what I deem appropriate. That of course is in direct relation to the congregation, the conversation, the libation and the destination. I am not the greatest artist, but see if this diagram is similar to what you were trying to have us draw Steve.

<img src="http://im1.shutterfly.com/procserv/47b3d708b3127cce8f69d87955180000001610" border=0>

D. Wolff - "The Flying Wasp" #401 sr/sk
Chief Measurer C-25/250 National Assn.
<img src="http://www.flags.com/images/layout/category_images/t_3964.jpg" border=0>

Edited by - Duane Wolff on 09/29/2003 15:13:44

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Response Posted - 09/29/2003 : 14:58:22

Good drawings, Duane! It would help if you could draw in the hull and keel, and then mark the approximate location of the CLR.

How did you post that drawing?

Steve Milby "Captiva Wind" C-25 T/FK #2554

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Response Posted - 09/29/2003 : 16:35:07

Thanks, Steve and Duane... This is bringing up some points that I hadn't thought of, and one that I don't quite get (yet): When you reduce the size of the jib, certainly the CE for the jib moves forward. However, each sail's contribution to the CE is also based on its total effort (size). Since the smaller jib has a smaller total effort, I'm wondering if the combined CE is changed very much. If it is, then reducing the jib will (paradoxically) reduce weather helm--if it is not, then the only contribution the smaller jib will make is reducing heel, which according to Arlyn and Bill (for different reasons) will also reduce weather helm.

If my suspicion has any merit, it would suggest that reducing the main will affect the CE and thereby the helm more than reducing the jib, which is more intuitive anyway. Something else to test with my roller 130...

I hope you guys (and our silent readers) don't mind my questioning and sometimes even challenging these theories... I find the physics of sailboats to be fascinating--something many good sailers I know don't understand. I know one racer who can talk about "lift" until he's confused most people around him, but upon further questioning, he really doesn't know what that means, or the real reason a sailboat can sail toward the wind--he just knows that "lift" is good. Another veteran sailor tried valiantly to convince me that water ballast has no righting moment until part of the water tank rises above the level of the water outside the hull. (I'm sure Arlyn hopes that isn't true!) All we need now is to throw CB and CG into the alphabet soup!

Oops--gotta go.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 09/29/2003 18:05:13

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Arlyn Stewart
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Response Posted - 09/29/2003 : 21:09:52

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>The asymmetrical shape of the hull does do something. It creates drag, but it is not the cause of weather helm. Excessive heeling and weather helm are caused when the CE moves too far aft. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Whether the CE moves aft... or the CLR moves forward... What is the difference? Does the "lead" shorten either way? Does the boat go out of balance either way?

When a boat heels... are the changes that take place more significant in the sailplan or the hull footprint? Are pressure forces of air greater or lesser than hydronamic forces?

Why doesn't a narrow beam, fore and aft symmetrical hull develop weather helm when it heels with overpowered sails?

The cause of excessive heeling is basic stuff... no one doubts that excess rag, or it poorly trimmed, or too high a SD are the causes for heeling. The question is... why when overpowered do some boats heel without causing excessive weather helm and others suffer it?

Arlyn C-250 W/B #224
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[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - arlyn stewart on 09/29/2003 21:13:16

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Response Posted - 09/29/2003 : 21:53:51

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
The question is... why when overpowered do some boats heel without causing excessive weather helm and others suffer it?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

IMHO, it is the lack of a full keel (i.e., aft CLR) to counterbalance the downward and leeward pressure of the main toward the stern, once the CE of the jib has become ineffective (i.e., moved aft near the CLR) in counterbalancing those same forces.

The result is the weatherhelm AND BOW POPPING UP sensation experienced when rounding up.

J.B. Manley
Antares '85 FK/SR #4849
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Response Posted - 09/29/2003 : 23:01:57

J.B.

I liked what you offered about the full keel... with little abiltiy for sailplan forces to yaw. The problem is... most sailors want more of a hot rod, docile and without all that stinking wetted surface and drag.

But... there are fin keel boats that don't suffer adverse weather helm with greater heel... so the question is still unanswered. But your close... because your down in the water where the cause is. Here is another throw...

The center of lateral resistance is the sum of all those forces which effect the hull. They are comprised of the resistance to leeway of the keel, the rudder and the hull. They produce a point at which the boat will rotate (unless as J.B. points out, its a full keel and wont rotate) if the sailplan is out of balance. What has not been considered adequately is the need to factor into the lateral forces those forces of the lifting foil of the hull footprint. If the aft section of hull is lifting to leeward... that effect calculated into the lateral resistance will dynamically move the CLR forward, narrowing the "lead" and causing the boat to become unbalanced and yaw. What this means is that a boat may be sailing in good balance, but when heeled it's the CLR that shifts forward rather than the traditionally thought CE shifting aft that causes the boat to go out of balance and round up with adverse weather helm. This explains why a narrow beam boat with a symmetrical fore/aft section can become overpowered and heel excessively but yet not incur weather helm, because it's more about the CLR shifting during heeling than the CE and the described boat doesn't produce much of an asymmetrical foil when heeling so there is little CLR shift.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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eric.werkowitz
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Response Posted - 09/29/2003 : 23:31:52

All right, Arlyn, I've resisted demonstrating my ignorance so far, but I am an engineer (sort of) SO HERE GOES. It seems to me that the aft-beamy Catalinas' hull shape should drive it's nose down when it heels. As it heels, the wider aft portion should actually be deeper in the water resulting in stronger forces pushing up on it than the bow section. These stronger forces aft should cause a yaw to weather.

I prefer the asymetrical bow wave theory, but I can't explain the observed differences with more symetrical hulls.

Eric Werkowitz
C25 #4969

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Arlyn Stewart
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Response Posted - 09/30/2003 : 00:01:54

Eric,

My problem with the bow wave theory mostly results from two things.
<ul><li>If the center board is moved aft... the bow wave ought to have more effect... because the CLR or pivot point is now further aft giving the bow pressure more leverage... however, the opposite effect happens when moving the center board aft...weather helm is greatly reduced. </li><li>It has been done often enough to become a truth... that adding 300 lbs of sandbags to the bow of the C250 greatly reduces weather helm... this would be contrary to an anticipated effect of creating a larger bow wave with more bow surface to push against. </li></ul>

In a nutshell...what I am saying is...that the traditional concept of CLR needs to be modified. That the formula for CLR should not be [CLR = the sum of the latteral resistance of the keel, rudder and hull] but rather it should be [CLR = the sum of the latteral resistance of the keel, rudder and hull -(minus) the lift of the asymmetrical footprint] This of course, makes the CLR dynamic during heeling. But it will only punch significant numbers into the equation for a hull form that presents an asymmetrical footprint. Boats with symmetry wont have a detractor from the lateral resistance numbers. Well, actually they do...but the numbers are in balance to the CLR and have no yaw effect... however, they do have leeway effect.

Arlyn C-250 W/B #224
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N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - arlyn stewart on 09/30/2003 00:13:24

Edited by - arlyn stewart on 09/30/2003 00:19:06

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eric.werkowitz
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Response Posted - 09/30/2003 : 00:36:46

Arlyn,

I'll have to argue both of your points in the last post. Moving the center board aft should also induce a lee yaw. The center board produces lift in the direction opposite to the sideslip of the hull (to weather). As the center board moves aft, the lift vector moves as well inducing a yaw. This should reduce weather helm as you say.

On the second point about the effect of added weight forward, won't this just work to counteract some of the force of the lifting bow wave? In fact, they should balance. The net effect is a shift forward of the center of gravity of the boat.

Eric Werkowitz
C25 #4969

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Response Posted - 09/30/2003 : 00:38:58

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> I find the physics of sailboats to be fascinating--<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Same here, Dave, but I worry that I'll bore you guys stiff talking about it. <img src=icon_smile_dead.gif border=0 align=middle>

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> Since the smaller jib has a smaller total effort, I'm wondering if the combined CE is changed very much. If it is, then reducing the jib will (paradoxically) reduce weather helm--<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Reducing weather helm is all about moving the CE of the Sailplan forward. Any way you can move it forward will reduce weather helm.

Look at Duane's two right-hand drawings. The top-right drawing shows that, when the boat is flying a 100% jib, the CE of the Sailplan is located slightly forward of the mast. The lower-right drawing shows that, when the boat is flying a 150% jib, the CE of the Sailplan is located slightly aft of the mast. Therefore, if you reduce the size of the jib from a big one to a little one, the CE of the Sailplan moves forward, and weather helm is reduced.

Likewise, if you reduce the size of the mainsail by reefing it, you will also move the CE of the Sailplan forward, and reduce weather helm. Therefore, reducing the size of either the mainsail or the jib will move the CE of the Sailplan forward, and reduce weather helm.

But, the mainsail is more responsible for weather helm and for heeling than the jib. Because the mainsail extends aft almost to the transom, a large part of the mainsail is located aft of the Center of Lateral Resistance. Therefore, when you reduce the size of the mainsail, the part that is aft of the CLR is furled, and that part of the mainsail is responsible for the greatest part of the weather helm. That explains why, when it is time to reduce sail area, the first sail you should reduce in size is the mainsail. You are getting rid of the part of the mainsail that is producing the most weather helm, drag and heeling moment.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Whether the CE moves aft... or the CLR moves forward... What is the difference? Does the "lead" shorten either way? Does the boat go out of balance either way?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

With a fixed keel boat, the CLR can’t move, either forward or aft. With a centerboard boat, the location of the CLR is generally based on the board being all the way down. The CLR of the centerboard boat can be moved aft a little, by raising the centerboard, but it can’t be moved forward.

The “lead” (pronounced “leed”) is the distance between the CE of the Sailplan and the CLR. As you know, it is generally recommended that all sailboat rigs be tuned so that they have a slight amount of weather helm. The “lead” represents the normal amount of weather helm that a boat should have. The lead is usually increased or decreased by adjusting the rake of the mast slightly forward or aft.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Why doesn't a narrow beam, fore and aft symmetrical hull develop weather helm when it heels with overpowered sails?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

It does develop weather helm. (I’m assuming you are talking about a typical, sloop-rigged, ballasted, monohull sailboat with some type of keel.) In an earlier post in this thread, Arlyn, you were observing that a boat with a broad, flat bottom and hard chines rounds up more abruptly than a boat with a narrower beam, rounded bottom and soft chines. A boat with a broad, flat bottom and hard chines is said to have high initial stability, but low ultimate stability. A boat with a narrower beam, rounded bottom and soft chines has low initial stability, but high ultimate stability.

A boat with high initial stability is very resistant to heeling until it reaches a certain point, at which time it suddenly loses its stability, and either rounds up abruptly or capsizes (i.e. it has low ultimate stability). Dinghies usually have flat bottoms and hard chines, and they strongly resist heeling initially, but their ultimate stability is low, and when they reach the limit of stability, they capsize instead of rounding up, because they don’t have ballast to help them resist capsizing. Ballasted boats of this design will usually round up, instead of capsizing.

A boat with low initial stability will begin to heel easily in light air, but, when it reaches a certain point, it has a strong tendency to maintain that same angle of heel, and to not heel further. (i.e. it has high ultimate stability) The C-22 and C250 are examples of boats that have high initial stability and low ultimate stability. Catamarans also are in that category. The C-25 and C-27 are examples of boats that have low initial stability and high ultimate stability.

Both types of hull will generally develop weather helm when overpowered, if the CE of the sailplan moves aft of the CLR, but, at a certain angle of heel the hard chined boat loses its stability very suddenly, causing it to round up abruptly, while the soft chined boat maintains its stability at greater angles of heel, and rounds up much more gently, if at all. (When my C-25 is well-tuned and the sails are well-balanced, it will gradually continue to heel over in overpowering winds until the rudder almost pulls clear of the water, and then it will just wallow on its side, without rounding up.) (Although, sometimes a little water will pour over the gunwale into the cockpit.) <img src=icon_smile_shock.gif border=0 align=middle>

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>The question is... why when overpowered do some boats heel without causing excessive weather helm and others suffer it?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Any overpowered sailboat will have excessive weather helm if the Center of Effort of the Sailplan moves aft of the CLR. The only time that a sailboat will heel without developing strong weather helm is if the mainsail and jib have been drastically reduced in size. For example, if the boat is flying a storm jib and storm trysail. A storm trysail is so small that all, or almost all of it is located forward of the CLR. Thus, the Center of Effort of the Sailplan is forward of the CLR. Excessive weather helm only occurs when the CE is aft of the CLR.

Steve Milby "Captiva Wind" C-25 T/FK #2554

Edited by - Steve Milby on 09/30/2003 01:25:16

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