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Steve Milby
Past Commodore

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USA
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Response Posted - 09/30/2003 : 01:26:50

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>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.[/quote}

Arlyn, the reason why sandbagging the bow reduces weather helm is because the boat is sailing with its bow down. When the bow is tilted down, the boat is sailing with its mast permanently tilted forward, and it is well-known that tilting the mast forward moves the CE of the Sailplan forward and reduces weather helm.

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

The CLR is intended to be a static value, not a dynamic value. It is intended to take into consideration only factors that are immutable, and not dynamic factors, because dynamic factors create infinitely variable results that would make the formula less reliable and useful.

[quote] But... there are fin keel boats that don't suffer adverse weather helm with greater heel... <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Name a fin keel boat that doesn’t have excessive weather helm when overpowered. I’ve never seen or heard of one. (We are not talking about a boat with a balanced rudder.)

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

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Arlyn Stewart
Master Marine Consultant

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2980 Posts

Response Posted - 09/30/2003 : 01:47:48

I don't follow your argument that righting ability plays a significant part in the CE/CLR relationship.

There are lots of sailboats that will capsize instead of developing weather helm and rounding up. The CE shifting aft theory doesn't fit.

I think I've demonstrated that
<ul><li> asymmetrical foils on boats have significant lift (see my catamaran explanations </li><li>those lifting forces come into play on a wide hulled, hard chined, flat bottom boat with slight rocker when heeling </li><li>those forces should be factored into the CLR equation </li><li> doing so makes the CLR become dynamic </li><li>sandbagging the bow reduces weather helm by altering the asymetrical footprint in favor of more balance </li><li>it explains why some boats don't develop adverse helm when heeled </li><li> it explains why non symmetry hulls with wide aft sections suffer weather helm when heeling</li><li> it explains why wide hulled boats suffer more leeway when heeled than narrow beam boats </li></ul>

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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Arlyn Stewart
Master Marine Consultant

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2980 Posts

Response Posted - 09/30/2003 : 02:07:51

Eric, Yes.. I agree that the more aft center board will produce a greater leverage to reduce yaw and that also, it would produce a more aft CLR to combat Steve's argument that an aft CE shift is the cause.

But, I think that the aft shift to combat yaw would be more than overshadowed by the much greater leverage arm of the bow wave.

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

I think digging the bow deeper is going to form a larger wave and that larger wave is going to have more hull surface to push against. I don't see much resistance to that bow wave on the windward side of the boat... so if there is more water pushing and it has a more surface to push on... and the result is the opposite of what that would suggest... I just don't see that theory as having much merit.

Many argue that weighting the stern will reduce weather helm at it increases CLR... and it very well on even most boats... but not all of them... why not? On the C250 it has the opposite effect. Why? The answer I believe is because the c250 suffers such a drastic asymmetrical foil shape when heeling that the change to that foil with 300lbs on the bow significantly moves the foil center of lift forward, thus reducing the lifts detraction from aft CLR.

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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2980 Posts

Response Posted - 09/30/2003 : 07:39:08

The only thing a balanced rudder has to do with weather helm is its ability to mask it when it exist. Though, an unbalance rudder can actually produce whats called static weather helm which is the effect of the force against the rudder blade when the boat is making leeway but no headway.

Exactly my point... if the CLR is fixed in mind as a static value... then your theory that the CE shifts has to be the answer. At one time the earth was static and didn't rotate too.

As many times as you've encouraged someone to harden their sails...to reduce the foil and the lift of those sails... and lessen their asymmetrical form...

That stability is now part of your explanation does indicate the sensing of hull shape may play a factor.

Re-read what is said about the Hobie 16 lifting foiled hulls.

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

Even their amount of lift is not static...it varies on how deep the leeward hull is driven compared to the windward. I doubt much is static on a sailboat... and to place restrictions that the CLR be so... I don't get it.

Arlyn C-250 W/B #224

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 09/30/2003 07:40:18

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Champipple
Master Marine Consultant

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6855 Posts

Response Posted - 09/30/2003 : 08:12:48

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

I created it in Microsoft Powerpoint. Its probably not the correct application for the job, in fact, I know its not. However it did the job. I would have put the keel on as well, but that was beyond my skills and time frame.

After creating the drawing, I saved the file as a JPeg and loaded into shutterfly.com as though it were a picture. I then posted the drawing just like you would a picture.

dw

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 09/30/2003 15:45:40

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 09/30/2003 : 09:35:14

What about the prismatic coefficient?
Are boats with a hard chine less susceptible to weather helm? They generate a Keel effect when healed.
Do scows act differently than displacement hulls?
Did the development of submarine hulls to a cylinder result from a fear of bow pressure?
How do 250s survive in SF bay if they round up all the time? Yet they do!
The truth is out there.
<img src=icon_smile_wink.gif border=0 align=middle>

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

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Steve Milby
Past Commodore

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5902 Posts

Response Posted - 09/30/2003 : 14:20:07

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> I don't follow your argument that righting ability plays a significant part in the CE/CLR relationship. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

I didn’t say “righting ability plays a significant part in the CE/CLR relationship,” or anything to that effect. What I said was that the shape of the hull and the amount of a boat’s ballast has a significant effect on the way that a boat behaves when it is overpowered. The CE/CLR relationship is the same, regardless of whether the mast, rig and sails are mounted on a boat hull with a flat bottom and hard chines, or a rounded bottom with soft chines. When the CE moves aft of the CLR, weather helm increases dramatically. The boats righting ability doesn’t change that relationship.

The two types of hulls behave differently when they experience severe weather helm. The flat bottomed boat resists heeling until it reaches a certain angle of heel, and then it suddenly loses stability. (It has high initial stability, but low ultimate stability.) If it is a ballasted boat, it usually rounds up abruptly. If it is an unballasted boat, it might round up, but very often it just capsizes, because it doesn’t have enough ballast weight below the waterline to help it resist capsizing. The round bottomed boat heels fairly easily until it reaches a certain angle of heel, and then it strongly resists heeling any further. (It has low initial stability, but high ultimate stability.) If it rounds up, it does so more gradually than a flat bottomed boat.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> There are lots of sailboats that will capsize instead of developing weather helm and rounding up. The CE shifting aft theory doesn't fit. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Unballasted dinghies are an example of sailboats that will often capsize instead of developing weather helm and rounding up. They do in fact develop weather helm when they are overpowered, but they lose stability so quickly in strong winds that they often capsize before there is time for them to round up. The “CE shifting aft theory” fits perfectly. Incidentally, it isn’t my personal theory. It’s virtually universally accepted by all the authorities who know far more than I do. I almost never discuss my personal theories, but when I do, I usually label them as such. When I see the rest of us nipping at your heels like a pack of dogs, Arlyn, I’m glad I don’t discuss my personal theories. I couldn’t take the consternation we must be causing you.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> At one time the earth was static and didn't rotate too. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Where the heck did that theory come from? You’ve got to stop writing these posts so early in the morning, Arlyn! <img src=icon_smile.gif border=0 align=middle>

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> That stability is now part of your explanation does indicate the sensing of hull shape may play a factor. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

I’m not saying that hull shape doesn’t affect weather helm. A multitude of factors can affect weather helm. I’m just saying that those are factors that you can't alter or control. You can’t do anything about them. What you can control is mast tuning, sail trim, sail balance and helmsmanship. If you want to reduce the effects of weather helm, you have to use those controls. Understanding the CE/CLR relationship helps you understand why your boat's sailplan is out of balance, and it helps you decide what changes you can make to restore balance.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>What about the prismatic coefficient? <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

What the heck is the prismatic coefficient? (Have you and Arlyn been talking to each other early in the morning?) <img src=icon_smile.gif border=0 align=middle>

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> How do 250s survive in SF bay if they round up all the time? Yet they do! <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

They don't round up all the time. They only round up if you let the wind increase without making appropriate adjustments to the sail trim, sail balance and helmsmanship. C-250 sailors learn how to cope with the idiosyncracies of a boat that has a wide, flat bottom and hard chines. There's nothing inherently wrong with that hull design. On the contrary, it is an increasingly popular design. I’ve raced on and against C-250s, and they’re fun, fast boats. They’re just different from the older C-25 design, and you have to learn how to deal with its differences. You can't sail a C-250 the same way you would sail a C-25.

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

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 09/30/2003 : 14:39:34

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>When I see the rest of us nipping at your heels like a pack of dogs, Arlyn, I’m glad I don’t discuss my personal theories.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

We're just havin' a lil' ol' friendly debate now, aren't we?

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>What the heck is the prismatic coefficient?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

[url="http://www.marinetechs.com/prism.htm"]Prismatic Coefficient[/url]

[url="http://www.tedbrewer.com/yachtdesign.html"]Yacht Design 101[/url]

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 09/30/2003 : 15:06:15

Sorry about that prismatic coeffecient remark but I should get something for paying Westlawn all that money for the nautical architect course.
Prismatic coefficient:
This coefficient is calculated by dividing the displacement by the product of the waterline length by the maximum cross sectional area. It is represented by the formula Cp=D / (L x Amax) ( With the caveat of course that the max cross section is aft of the mast. We don't want no cod fish boats here.)

Good links JB

<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
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/30/2003 : 15:06:27

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
...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....
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
My point, Steve, was that the position of the total CE on the line between the two sails' CEs is not necessarily at the midpoint of that line. Its position on the line is based on the relative amounts of the efforts of the two sails (like computing a center-of-gravity). If the genny is bigger than the main, the point is forward of the midpoint. If it's smaller than the main, the point is aft of the midpoint. If you reduce the size of the jib, moving its CE forward (as Duane shows), and leave the main alone, the overall CE does not move forward as much as the midpoint between the CEs. If you roll the jib down to a handkerchief, it still has its own CE, but the overall CE has moved back to being very close to the CE of the main, which obviously will increase weather helm. What about sizes in between? The position is weighted by the relative size of the sails, assuming they're comparably trimmed.

OK, back to work... <img src=icon_smile_blackeye.gif border=0 align=middle>

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

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Steve Milby
Past Commodore

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USA
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Response Posted - 09/30/2003 : 15:09:40

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>We're just havin' a lil' ol' friendly debate now, aren't we? <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Of Course. I just forgot to put a smiley face at the end of my "nipping at your heels" remark.

Thanks for the info on the "prismatic coefficient," J.B. Now I know Frank and Arlyn didn't just "dream" it up! <img src=icon_smile.gif border=0 align=middle>

According to Ted Brewer (who is a pretty darn reliable authority on boat design) the prismatic coefficient is a formula that relates to the underwater shape of a boat.

Frank asked the following question:

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>What about the prismatic coefficient? <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

In an earlier post I said:

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>I’m not saying that hull shape doesn’t affect weather helm. A multitude of factors can affect weather helm. I’m just saying that those are factors that you can't alter or control. You can’t do anything about them. What you can control is mast tuning, sail trim, sail balance and helmsmanship. If you want to reduce the effects of weather helm, you have to use those controls. Understanding the CE/CLR relationship helps you understand why your boat's sailplan is out of balance, and it helps you decide what changes you can make to restore balance.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

The CE/CLR relationship is equally applicable to all monohulls that I know of, but different hull types behave in different ways.

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

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Steve Milby
Past Commodore

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Response Posted - 09/30/2003 : 15:41:54

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> My point, Steve, was that the position of the total CE on the line between the two sails' CEs is not necessarily at the midpoint of that line. Its position on the line is based on the relative amounts of the efforts of the two sails (like computing a center-of-gravity). If the genny is bigger than the main, the point is forward of the midpoint. If it's smaller than the main, the point is aft of the midpoint. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Dave, go to the Ted Brewer article, and scroll down to the definition of "Center of Effort." Look at the drawing that illustrates the principle. The Jib is slightly larger than the mainsail, so the midpoint on the line between the CE of the jib and the CE of the mainsail is just forward of the mast. I have seen similar illustrations of the principle applied to 150% jibs, and the diagram works the same.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> If the genny is bigger than the main, the point is forward of the midpoint. If it's smaller than the main, the point is aft of the midpoint.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

That's not so, Dave, and I think I can explain the reason. Bear in mind that a 150% jib partially overlaps the mainsail. When you are flying a 100% headsail, the CE of the Sailplan is slightly forward of the mast. If you take down the 100% jib, and raise a 150% jib, much of the sail area that you are adding is aft of the previous CE of the Sailplan for the 100% jib (which was just forward of the mast). By adding sail area aft of that point, you cause the CE of the Sailplan to move aft, to a location slightly aft of the mast, just as is shown by Duane’s drawing.

Edited by - Steve Milby on 09/30/2003 19:00:52

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Arlyn Stewart
Master Marine Consultant

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Response Posted - 09/30/2003 : 18:57:30

Why do designers have to vary the "lead" so much in their designs depending on the hull form? I have Ted Brewers Understanding Boat Design and in it he suggest that 6 meter boats have a lead of about 6% but beamy center board yachts require 18-20% and more. Why is this? Could it be that boat balance is more affected by the hull form than the sail plan?

He also relates that the traditional lead percentage of around 6% had to be thrown out when hulls became wider. If 6 % lead was capable of handling the aft shift CE of a sloops sailplan... then why do designers now have to go above 20%? Is the CE now shifting farther back for a sailplan than it used to... maybe its the slickness of modern fabrics? Or, is it the hull form that is the reason... Brewer offers a chart to help judge the requirements.... and the variables are...mostly the hull form characteristics.

And, the force driving that CLR shift forward is a changing foil footprint to an asymmetrical shape.

The much greater numbers are necessary to cope with forces caused by the hull... what are those forces? Why does the wide flat lightly displaced boat with a high SD ratio need such a long lead compared to narrow beams?

I think the answer is because the CLR on those hull forms shifts forward with heel... That a greater lead is needed to keep the shifting CLR from getting too far ahead of the CE.

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 09/30/2003 19:01:04

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Arlyn Stewart
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Response Posted - 09/30/2003 : 21:27:04

An examination of Chapman's points to several differing conclusions from those presented by Brewer.

For example: from the 61st edition of Chapmans

Under Heeling: The part of the hull that is underwater becomes asymmetrical, creating turning forces that have to be counteracted.

Chapmans recognizes the force of this asymmetrical foil. Brewer makes no comment about it...nor does Steve Colgate

Both [CE and CLR] centers are simply the sum of all of the lift and drag forces at work anywhere on the foil. 

Chapmans clears the way to apply all forces to the CLR rather than just the traditional resistance. This is especially in variance with brewer who poses CLR to be a static point that is represented by cutting out a hull form from paper and balancing it on a pin and locating the CLR.

Both centers are constantly changing with boat motion and sail adjustment.

Chapmans give credence to the CLR being as dynamic as the CE.

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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Derek Crawford
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Response Posted - 09/30/2003 : 22:04:17

Do y'all mind if we just go sailing <img src=icon_smile_big.gif border=0 align=middle>
Derek

<img src="http://im1.shutterfly.com/procserv/47b3dc30b3127cce942c608d6f1e0000001010" border=0>
TRFK#2262"This Side Up"

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Steve Milby
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Response Posted - 09/30/2003 : 22:30:29

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> Do y'all mind if we just go sailing <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

You're right, Derek. Thanks to all for an interesting discussion.

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 - 09/30/2003 : 23:21:24

Steve: You don't get away that easily...<img src=icon_smile_evil.gif border=0 align=middle>

If what you say is true, the CE would move forward if you changed from a 100% to an 80% (keeping a full main), further forward if you changed to a 60%, and further forward if you changed to a 20%, and further still if you put a riding sail on the forestay. You know that's not true. CE is like CG--it's weighted (roughly) by the size of the sails that make up the sailplan--not just their positions, just as CG is weighted by the weight of the components that make up a thing--not just their positions. The overall CE will be at the midpoint of Duane's line only if the two sails have the same area. Drawings by Ted Brewer or anyone else that suggest otherwise are purposefully simplified. Therefore, reducing the main will reduce weather helm induced by the CE--that's easy. Reducing the jib may slightly, but just up to a point, and then it will increase it as the jib's "E" becomes much smaller than that of the main, even though the jib's "CE" is moving forward.

You and Derek can go sailing--I've gotta go get a "diagnostic procedure"... I'll just say I'm drinking a lot of clear fluids... <img src=icon_smile_dead.gif border=0 align=middle> Go ahead--have some fun with that, but you'll be over 50 some day.<img src=icon_smile_sad.gif border=0 align=middle>

Arlyn: I'm sold. Hull form, just like keel position, definitely affects overall balance, and heel changes the underwater hull form, so heel affects balance. I can even see that a broader stern, when heeled, would change the angle of attack of the keel to windward as the heel raises the broad stern more than the narrow bow.

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

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Dave Bristle
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Djibouti
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Response Posted - 10/01/2003 : 11:14:15

BTW, the prismatic coefficient is one way of measuring the "fineness" of entry and exit in the water. A coefficient of 1 would be a flat-ended barge with a uniform bottom shape from bow to stern. A very small coeeficient is a sailboat with bow and stern that are both tapered inward and slope upward from the beamiest point. That low coefficient allows a C-25 to slip through the water without generating a significant stern wake (drag) as a typical powerboat of the same length and speed.

Now, will somebody explain NZL's "hula" to me? <img src=icon_smile_blush.gif border=0 align=middle> I still haven't seen a picture or an intelligible (to me) explanation...

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

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dlucier
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Response Posted - 10/01/2003 : 12:33:48

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Now, will somebody explain NZL's "hula" to me?<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

The hull appendage(hula) is basically a false bottom attached only at the centerline that mathematically and theoretically makes the hull longer. This appendage is not counted into the actual, smaller hull dimension which enables them to carry more sail.

<img src="http://www.catalina25-250.org/c25sm.gif" border=0>Don Lucier<img src="http://www.catalina25-250.org/c25sm.gif" border=0>
<img src="http://im1.shutterfly.com/procserv/47b2d904b3127cce9f7cd9ffdf1d0000003010" border=0>
North Star SR/FK

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Steve Milby
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Response Posted - 10/01/2003 : 13:55:30

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> Steve: You don't get away that easily... <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Dave, I could get away from you guys if I wanted to. Remember, when Reagan didn’t want to answer a question shouted at him by a reporter, he acted like he didn’t hear it. I could do the same thing to you guys. Oops! My computer screen just went blank! Must have been a grid failure…

Okay, now, take a heart pill and sit down. The words I am about to say are catching in my throat.

You’re right. Ok, Ok, You’re right!

I plotted the position of the Center of Effort of the Sailplan incorrectly. Wallace Ross, in [u]Sail Power[u], says you should plot the center of effort for each sail as I described, but then he goes on to say, “The two centers are then weighted according to the proportionate size of the sails, and one total center of effort is found along the line connecting the two separate centers. For example, if the foretriangle is two times the area of the main, the total center of area would be two-thirds of the distance between the foretriangle center and main center, forward of the main center.”

What fooled me is that, for sails between 100% AND 150%, the CE of the Sailplan is located close to the mid-point on the line between the two, and when you plot it out in small scale, it’s hard to see the difference, especially when you are looking at it with old eyes. It only becomes obvious when you plot it out with smaller headsails.

Ah! Now I feel better, having confessed my sin. It doesn’t change my main point, which is that the way to reduce weather helm is to keep the CE of the Sailplan forward of the CLR.

I don’t disagree with Arlyn’s premise that hull shape affects the behavior of the boat, but those decisions are made by the designer when the concept of the boat is still on the drawing board. After the boat is built, you have to work with the boat that the designer gave you. Boats don’t heel all by themselves, creating an asymmetrical underwater shape. Wind makes them heel. The tools that you have to use to control wind-induced weather helm and heeling (regardless of whether they are influenced by hull shape, or by water piling up under the leeward bow, or otherwise) are the rig adjustments and sail trimming devices, and changing the sizes of the sails. Keeping the boat as upright as possible minimizes any adverse effects produced by the shape of the hull and stern.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote> I’ve gotta go get a “diagnostic procedure”…I’ll just say I’m drinking a lot of clear fluids…Go ahead—have some fun with that, but you’ll be over 50 some day.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Good luck with your procedure. I hope a little Metamucil is all you need to cure what ails you. I would love to be your age again, but I can’t remember why.

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

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 10/01/2003 : 14:05:43

I wonder what the effect would be if you were to glass on a hollow, fiberglass keel extension.

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 10/01/2003 : 14:25:50

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
Wallace Ross, in [u]Sail Power[u],Steve Milby "Captiva Wind" C-25 T/FK #2554
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
Sail Power was one of my favorite books. I had a library of approximately 30 volumes when I sold my boat. Our city had just rebuilt an historic boat house and Bill Koch had just donated one of the trial horses. I donated my books and they disappeared. So much for generosity.

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

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 10/01/2003 : 16:36:55

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 10/01/2003 : 16:40:19

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
I wonder what the effect would be if you were to glass on a hollow, fiberglass keel extension.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

By that, I mean a "fake" full keel to provide additional lateral stability and to allow the CLR to move back without falling behind the real keel. I think that's what I mean. <img src=icon_smile_tongue.gif border=0 align=middle>

J.B. Manley
Antares '85 FK/SR #4849
Grand Lake O' the Cherokees
36°29'58" -94°59'59"

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 10/01/2003 : 18:43:05

E37 Weather Helm fix

Date: November 24, 1999
From: Mike Miller capnmike_s@yahoo.com

After moving the genoa tack forward onto the bowsprit and adding a larger staysail, I found the weather helm, while reduced, was still too strong...After checking out the hull design, I taped a triangular piece of construction foam in the "cutaway" aft of the keel, after first sanding off the bottom paint, and glassed it in with several layers of biaxial cloth, thus effectively lengthening the keel some 18". (I didn't cut into the hull, figuring if this didn't work I could just cut the addition off with a chisel and sander) ...

Voila! No more excessive weather helm! She probably turns a tad slower, although I haven't noticed much difference, but most of the really strong weather-helm is gone! (total project cost about $35)

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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