Weather Helm/Lee Helm -What's The Diff?

Apr 16, 2017
841
Federation NCC-1701 Riverside
While simplistic, the Center of Effort of the sails (CE) is calculated by marking the intersection of two lines, both which leave the two leading edges of the sail and bisect the other side. If a boat has two sails, the combined CE is in the middle of the line (actually proportionally) that connects them. That we can adjust.
This is accurate geometrically, but cant possibly be useful for sailing. If it is, then i would say stop trying to sail sideways.

I am just a noob, but even when i sail downwind my rig never looks like that, especially compared to the hull.

The sail area should be viewed in terms of a projection from the perpective of the apparent wind. I also suspect that a top down view is better.

In the sketch you see that while pointing the sails are angled for best lift. The projection of apparent wind over the leading edges reduces the "sail area" to a sliver, especialky the jib, in addotion the center of effort for each sail moves backwards.

Note also that force is not applied equally to the sail area. The sketch shows shaded area that represent the lift. This average lift for the most part should be the CE for each sail. Note to that the lift pulls in a direction way different than wind blowing say a fence. The lift force is applied well forward of even the center of the projection.

The CE drawings in textbooks are junk. The best balance will come from the nth iteration of prototyping. I bet if you took a mini transat or most super high performance sail boats the CE textbooks are waaaay off on 2D side geometry versus holostic results.
 

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Likes: Will Gilmore
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
This is accurate geometrically, but cant possibly be useful for sailing. If it is, then i would say stop trying to sail sideways.

I am just a noob, but even when i sail downwind my rig never looks like that, especially compared to the hull.

The sail area should be viewed in terms of a projection from the perpective of the apparent wind. I also suspect that a top down view is better.

In the sketch you see that while pointing the sails are angled for best lift. The projection of apparent wind over the leading edges reduces the "sail area" to a sliver, especialky the jib, in addotion the center of effort for each sail moves backwards.

Note also that force is not applied equally to the sail area. The sketch shows shaded area that represent the lift. This average lift for the most part should be the CE for each sail. Note to that the lift pulls in a direction way different than wind blowing say a fence. The lift force is applied well forward of even the center of the projection.

The CE drawings in textbooks are junk. The best balance will come from the nth iteration of prototyping. I bet if you took a mini transat or most super high performance sail boats the CE textbooks are waaaay off on 2D side geometry versus holostic results.
Good deal. I said simplistic for a reason, because it is. but we are talking about a discussing for beginners. Sail pressure absolutely does not evenly distribute over the sail. It often looks like this.

951e99bb0dc6a996b5e61d6a66b2e243.jpg


But the geometric calculation is often close. As least for discussion purposes. And yes, in any event no modern designer would still do that, they model every aspect of the boat. And not just statically, but dynamically.
 
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Likes: jon hansen
Apr 16, 2017
841
Federation NCC-1701 Riverside
The CE drawings in textbooks are junk.
I might also add that the jib is well forward of the hulls rotational axis (port or starboard). If the boat is not heeled then the lift from the jib is pulling the bow into the water. As the boat heels the disapprotionate lifting force and torque of the jib pulls the bow to windward versus to the water below.

Any discussion of weather helm must involve this "rounding up".
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
I might also add that the jib is well forward of the hulls rotational axis (port or starboard). If the boat is not heeled then the lift from the jib is pulling the bow into the water. As the boat heels the disapprotionate lifting force and torque of the jib pulls the bow to windward versus to the water below.

Any discussion of weather helm must involve this "rounding up".
The interaction of CE and CLR is dynamic and constantly changing with changes in wind speed, boat speed, heel angle, etc.
With respect to heel angle, it is much more to do with the CLE moving aft when the boat heels vs changes to the CE.
 
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Apr 16, 2017
841
Federation NCC-1701 Riverside
discussing for beginners.
The description for how wind surfing works seems pretty basic. Lean sail forward or back. The windex on the big boats demonstrates the concept as well.

There is an axis on rotation. The forces however complex, may or may not be equal in all directions to that axis.
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
This theory is kinda complicated for a beginner.

On the little Catalina, with the 135 up, the boat can be trimmed so that the rudder can be locked and it will come up or fall off as the wind shifts. Not that it has anything to do with anything, it was just something fun. With the 155 loaded, the AP takes it's direction from the wind input, and frees the crew to concentrate on trim. Or staying hydrated. As long as you are first to the mark, staying hydrated is important. :)
 
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Likes: Will Gilmore
Apr 16, 2017
841
Federation NCC-1701 Riverside
Old school designers would calculate the CLR by cutting out a profile view of the boat below the waterline and then find the balance point. Where it balances would equal the CLR, assuming only cross-sectional friction. Surface area friction changes that a little and adds much more complexity to the calculations.

- Will (Dragonfly)
At the dock one can gently push the boat away at different locations. The CLR will be where the boat doesnt rotate away.
 
May 17, 2004
2,099
Other Catalina 30 Tucson, AZ
This theory is kinda complicated for a beginner.

Meriachee: You're right it is complicated for a beginner but I think the beauty of this forum is that the basics were covered and explained in a simple manner that a beginner could understand. So now he gets it and once that's completed the discussion can move on to the next level and get input from the more experienced sailor on this forum. At some point in the discussion the beginner will get lost but with more exposure he'll get it -- just like we all did.
 
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Likes: Will Gilmore
Oct 19, 2017
7,732
O'Day 19 Littleton, NH
At some point in the discussion the beginner will get lost but with more exposure he'll get it -- just like we all did.
:plus:
That's how I raised my kids. I use to get criticized by other parents and friends for the way I talked to my children. "You can't talk to your children that way! They don't understand what you're saying. You have to keep it on their level."
My response, "If I talk down to their level, how will they ever learn to understand me?"
My kids have a better vocabulary than I do now. I struggle to keep up with them:).

- Will (Dragonfly)
 
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Oct 11, 2014
5
Freedom 32 Merritt Island
This is well written. I never understood it so completely, because I was always told I had to understand CE and CLR to understand it. Well, that will never happen! I'm a newbie and this was perfect. I thank you.
 
Oct 19, 2017
7,732
O'Day 19 Littleton, NH
I was always told I had to understand CE and CLR to understand it. Well, that will never happen!
Whoever told you that probably didn't understand it themselves, but don't sell yourself short. If you had an interest in understanding it, I'm sure you would pick it up readily. However, you don't need to know CE or CLR to know how to work with weather or lee helm.

-Will (Dragonfly)
 
May 17, 2004
2,099
Other Catalina 30 Tucson, AZ
This is well written. I never understood it so completely, because I was always told I had to understand CE and CLR to understand it. Well, that will never happen! I'm a newbie and this was perfect. I thank you.
Zengirl: Thank you for your comment. This forum was set up years ago to help newbie's (beginners) get through the learning curve that it took all of us to learn how to sail a silly sailboat. Hopefully there are are lot more like you that have benefited from the simple and easy to understand explanations. Most times we never hear how the explanations were received but we hope it helped the sailor improve and get more out of his sailboat.
 
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Oct 26, 2008
6,043
Catalina 320 Barnegat, NJ
With respect to heel angle, it is much more to do with the CLE moving aft when the boat heels vs changes to the CE.
Bingo, I think hull shape and heel angle have more to do with weather helm than changes to CE.

"The description for how wind surfing works seems pretty basic. Lean sail forward or back."
Yes and no. We're taught that a sub-planing board will turn upwind when you rake the rig back and it will turn downwind when you rake the rig forward. But after you have advanced from those early lessons in very light wind conditions, you learn that you really turn a sub-planing board upwind by pressuring the leeward rail and burying it. If you want to turn the board downwind you pressure the windward rail. There is very little adjustment to the rake of the rig necessary.

A planing board is the opposite. It is sailed with the rig fully raked back just about all the time, but especially when sailing in a straight line at just about any point of direction relative to the wind. You go straight by making sure the board is flat on the water and you increase your speed by raking the rig back so that the foot of the sail is essentially laying on the board. You turn by pressuring the windward rail (to head upwind) or the leeward rail (to go downwind). A planing board acts more like a ski that is carving into the turns.

I liken a sailboat to a sub-planing sailboard. Bury the leeward rail and she wants to turn upwind. I think it has lot more to do with the water pressure differentials on the hull more so than CE on the sails. Isn't that why you induce heel on the leeward side in very light wind? To generate windward lift?

An easy explanation for beginners is that weather helm is desired because it causes a boat to luff when un-controlled, which is infinitely safer than lee helm, which will cause an uncontrolled jibe (very bad :(:() if the helm is not attended.
 
Last edited:
Apr 16, 2017
841
Federation NCC-1701 Riverside
I liken a sailboat to a sub-planing sailboard. Bury the leeward rail and she wants to turn upwind. I think it has lot more to do with the water pressure differentials on the hull more so than CE on the sails. Isn't that why you induce heel on the leeward side in very light wind? To generate windward lift?
I feel i need to go on a religious crusade to dispel this myth.

I like how you brought up skiing. When you bury the rails with skis and snowboards you turn into the buried side. You dont bury tbe the left side of the board to whip a right hand turn.

A sailboat is a pinwheel that due to bouyancy directly opposite of gravity is not allowed to spin in circles. A boat that cant spin goes foward or heels more.

With increasing heel, the spinning force increases since the bow is not being forced into the water, opposite the sail lift.
 
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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
I think it has lot more to do with the water pressure differentials on the hull more so than CE on the sails. Isn't that why you induce heel on the leeward side in very light wind? To generate windward lift?
No, you do this to:
Decrease the wetted surface area of the hull, and
Allow gravity to help the sails hold their best airfoil foil shape
 
Sep 30, 2013
3,538
1988 Catalina 22 North Florida
When you bury the rails with skis and snowboards you turn into the buried side. You dont bury tbe the left side of the board to whip a right hand turn.
But a keelboat is the opposite.

Set a course, lash the tiller, and then put all the crew on, say, the port side. The boat will immediately head to starboard. It's completely counter intuitive, and I'm way too ignorint to explain it. But that's how it is.
 
Oct 19, 2017
7,732
O'Day 19 Littleton, NH
Set a course, lash the tiller, and then put all the crew on, say, the port side. The boat will immediately head to starboard. It's completely counter intuitive, and I'm way too ignorint to explain it. But that's how it is.
I first noticed this phenomenon in my kayak. When paddling in a straight line, if I let my hip dip to one side, just the slightest, the boat will start a turn toward the opposite side. I equate this to changing from a symmetrical to an asymmetrical waterline. The curved chine presses into the water, the relatively straight keel-line shifts off-center. You get something that starts to look like a half-moon pushing through the water. It would be even more pronounced with a keel because the keel swings outboard and represents significant drag. If heeling under sail, this puts the keel to windward. the boat will naturally want to sail a circle around the keel. The design texts say the CE should be ahead of the CLR. The fact, that has been pointed out previously, that these values are inconsistent with actual forces under sail and dynamic, means the designers can only get a close approximation with some idea of behavior for sail and hull configurations as the boat moves through the water. Moving the CE ahead of the CLR by a small amount helps balance these forces in the general case, but not in the specific case.

-Will (Dragonfly)
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
A few years ago I was talking to a designer at the Farr Yachtt Design about the first 36..7. He was talking about the improvements they made between the 367 and the 4 year older 40.7. I think I know a bit about this topic, but the discussion made my head spin.

The point I took out of it is that the deeper levels, this topic is beyond the ken of mortal men and sailors. The levels of dynamic modeling and CFD that supports new hulls at all ranges or speeds, heel, and sea state can’t be simplified into simple sentences or diagrams.

Older designers DID use these simple models and diagrams to get it close. Then they made it good looking and that normally made it fast. And it was at the time. But if you want real speed and real performance, then supercomputers come into play.
 
Apr 16, 2017
841
Federation NCC-1701 Riverside
The point I took out of it is that the deeper levels, this topic is beyond the ken of mortal men and sailors.
Thats why I'm here. To help you sail faster, sail safer, for fun and profit.

It would be amazing to spend a day at an R&D facility for hull and sail design. Im stuck with lead sinkers tied to the top of the mast while i lay down on the dock pushing the RC boat around.

People say learn how to sail on a small boat. Go smaller. If you have basic sail questions, get a cheap RC sailboat. It will sail terrible. When you pick it up and walk it to the dock or waters edge it will want to fly like your hand out the car window. Youll feel the lift up, and then at a razor edge difference of angle of attack it will fall like a rock. Like a God, youll feel the balance points and twist on your wrist. Youll try to balance the whole thing on you finger like a basketball, only to find out the balance point isnt even on the boat. Amazing stuff.

With flat sails you can make new sizes and shapes in minutes.

Want to see what happens when the keel is moved forward and the bulb wieght is moved bacwards? Make a new keel!

Wondering why the RC is impossible to turn downwind in a gust? Try different headsail sizes. You can quickly find out that yes, you can round up and not turn downwind with just a headsail.

Theres no rail meat on RC's so when the wind dies and you can turn downwind agian, you need to decide, heavier bulb keel, or smaller main. Too much heel with no speed gain? Smaller main. Loss of control in gusts? Put a smaller headsail on.

Having the skills for computer design would be epic, but hands on is a blast and there is so much to learn.

This knowledge transfers directly to bigger boats.

youll have to learn about anchoring and keeping the head working somewhere else.