calculating movement of the center of effort

Parma

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What is the correct formula for calculating the movement of the center of effort of a sail plan from a 140% overlapping Genoa to a 100, maybe 105% “nonoverlapping” jib?

 

12 metre

Super Anarchist
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English Bay
To find CE of the sail plan

First calculate the area of main and jib which will be Am and Aj and add them to get At

Next locate the CE of the main and jib separately.To find the CE of each, draw 2 lines, each from the middle of one side to the opposite corner.

Next draw a line from one CE to the other, and measure the distance between the two.  Call that L

The distance From CE main to CE total is then calculated as  (Aj x L/At)

It's really just a moment balancing equation and you can do the same thing working from CE jib to CE total by swapping out Aj with Am in the above equation.

Here is a link that may explain it better; https://www.diy-wood-boat.com/sail-balance.html#Center_of_Effort

Keep in mind this only calculates the CE based on sail plan geometry.  The actual change in CE may be different, but it will give you an idea of the change.

 
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Bob Perry

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"Center of effort" devised with the old 2D geometric method is not going to tell you what you want to know. I dont know how you calculate what you are after and what you are after is the "center of pressure" ans how it changes as you change trim. That's a pretty complicated 4 dimensional problem. I haven't used the old method to calculate " C of E" for many years. It was not working. Today I trust my eye and my experience. It's not a 2D problem. I'm sure some clever fellow with a lot of time on his hands could write a program for this. It may have already been done. My bet is that it has. Can't be too complex on a solid wing sail. I'm sure the AC designers are not guessing.

 
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Rasputin22

Rasputin22
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Parma,

    There is an app for that !

    RigEdge is the only CFD program I have ever been able to get even the most basic handle on but it can do the sort of calcs that you seek. Here are some screenshots from a rig included in its library.

image.png

image.png image.png

     I hadn't fired the program up in months and I was surprised that I was able to set up that model and get some results without going back to the tutorials. I guess I retained more of it care and feeding that I suspected. The parameters for a boat and rig can be entered parametrically in a series of rig data that looks like this. Much easier and quicker than how I usually meticulously build a virtual rig and sail model in Rhino3D and then you can't really do much with it. Once you set up a rig and sails model in RigEdge then you can proceed to full analysis pretty quickly. 

    We bought a license for use on the rig of the CG certified 49 passenger daysailing catamaran and it was worth if for just that one job, but getting the engineer to accept the analysis was another thing. Still waiting on some valid results from him on that rig! The important thing is getting some output that will pass muster with the Coast Guard and the classification societies. The people behind RigEdge are pretty sharp and they have full RINA approval and the bulk of their customers are for those huge Wally sloops and the even bigger Italian mega ketches. The ultimate module in the package calculated the G loads on a rig imposed by bashing into head seas which apparently is a major concern when rigs get up near 200 feet!

image.png

 

El Borracho

Verified User
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Pacific Rim
"Center of effort" devised with the old 2D geometric method is not going to tell you what you want to know. I dont know how you calculate...
Exactly...However, fortunately for us sailors, the CE's sum of sails, hull and foils conveniently shows up on the rudder gear when sailing straight and steady. So changing the headsail will show up directly in the rudder. Automagically including changes due to speed, heel, leeway, etc.

 

bgytr

Super Anarchist
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622
If i remember right, Larsson and Eliasson Principles of Yacht Design has a decent discussion on this topic.  Been quite a while since I read it though.

 

Parma

Super Anarchist
2,992
398
here
"Center of effort" devised with the old 2D geometric method is not going to tell you what you want to know. I dont know how you calculate what you are after and what you are after is the "center of pressure" ans how it changes as you change trim. That's a pretty complicated 4 dimensional problem. I haven't used the old method to calculate " C of E" for many years. It was not working. Today I trust my eye and my experience. It's not a 2D problem. I'm sure some clever fellow with a lot of time on his hands could write a program for this. It may have already been done. My bet is that it has. Can't be too complex on a solid wing sail. I'm sure the AC designers are not guessing.
That's what I thought, but using 12 metre's formulas it looks as if the layman's 2 dimensional center of effort of the sail plan only moves aft about 10% of the headsail overlap (where the headsail overlap and "E" mainsail foot length are identical) when re-calculating CE for a 100% jib vs the existing 140%.

And I wonder how much of this amount of movement is likely lost or negated by the M & J sail trim angles.

Basically what I'm looking at is how much the CE moves aft if I put a 100 on with inboard sheeting (waaay inboard of current sheeting angles). Less total sail area, true (573 vs 681) but better pointing.

Hmmm.....

 

12 metre

Super Anarchist
3,940
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English Bay
That's what I thought, but using 12 metre's formulas it looks as if the layman's 2 dimensional center of effort of the sail plan only moves aft about 10% of the headsail overlap (where the headsail overlap and "E" mainsail foot length are identical) when re-calculating CE for a 100% jib vs the existing 140%.

And I wonder how much of this amount of movement is likely lost or negated by the M & J sail trim angles.

Basically what I'm looking at is how much the CE moves aft if I put a 100 on with inboard sheeting (waaay inboard of current sheeting angles). Less total sail area, true (573 vs 681) but better pointing.

Hmmm.....
Most, if not all the time, when you reduce the jib LP, CE will move forward, not aft.

When you reduce LP, Aj and At will decrease, but keep in mind that  also have to relocate CE jib which tends to  increase L.  That is, L is not a constant  The end result is that you will usually find that the calculated Distance from CE main to CE total will tend to increase rather than decrease

This makes sense because changing down to a smaller LP headsail tends to reduce helm rather than increasing it.

 

SloopJonB

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This is all very interesting but unless one is designing a boat and is trying to calculate the position of the rig I fail to see the purpose of it.

As the C/E moves during sail changes you have to account for it through the helm and sail trim, not through theoretical calculations.

 

12 metre

Super Anarchist
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English Bay
This is all very interesting but unless one is designing a boat and is trying to calculate the position of the rig I fail to see the purpose of it.

As the C/E moves during sail changes you have to account for it through the helm and sail trim, not through theoretical calculations.
Agree that it would have helped the discussion  if the OP should have indicated the reason why he wants to know. because if it is the latter of your statements, then yes it is a bit of a moot discussion.

Nevertheless, the OP wanted to be provided with the calculation for locating the CE of the sail plan and the traditional equation has been provided.  What the OP chooses to do with that knowledge is up to him. 

 
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Parma

Super Anarchist
2,992
398
here
This is all very interesting but unless one is designing a boat and is trying to calculate the position of the rig I fail to see the purpose of it.

As the C/E moves during sail changes you have to account for it through the helm and sail trim, not through theoretical calculations.
I just like to include all possible variables in my decision making process', sometimes important considerations are revealed thereby

 
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MaxHugen

Super Anarchist
Parma,

    There is an app for that !

    RigEdge is the only CFD program I have ever been able to get even the most basic handle on but it can do the sort of calcs that you seek. Here are some screenshots from a rig included in its library.

View attachment 328159

View attachment 328161 View attachment 328160

     I hadn't fired the program up in months and I was surprised that I was able to set up that model and get some results without going back to the tutorials. I guess I retained more of it care and feeding that I suspected. The parameters for a boat and rig can be entered parametrically in a series of rig data that looks like this. Much easier and quicker than how I usually meticulously build a virtual rig and sail model in Rhino3D and then you can't really do much with it. Once you set up a rig and sails model in RigEdge then you can proceed to full analysis pretty quickly. 

    We bought a license for use on the rig of the CG certified 49 passenger daysailing catamaran and it was worth if for just that one job, but getting the engineer to accept the analysis was another thing. Still waiting on some valid results from him on that rig! The important thing is getting some output that will pass muster with the Coast Guard and the classification societies. The people behind RigEdge are pretty sharp and they have full RINA approval and the bulk of their customers are for those huge Wally sloops and the even bigger Italian mega ketches. The ultimate module in the package calculated the G loads on a rig imposed by bashing into head seas which apparently is a major concern when rigs get up near 200 feet!

View attachment 328162
Any chance that I could cajole you into helping me figure out the CEs for the sails on the AC75?

I (and others) are trying analyse performance factors for these boats in the "America's Cup" section of SA.  I have reasonable assessments of the various sails they use: 2 mainsails, a Code Zero, and 3 jibs.

The people at RigEdge did let me download the demo version of AzureProject (I think that's the successor of RigEdge) but it was too much for me to get my head around.

Without some reasonable estimates of CEs, I'm struggling to calculate the sail forces properly, especially as these boats need to use a lot of twist in the main to balance the boat, especially for the righting moments.

If that's a bit too much to ask, I quite understand, but as you've used RigEdge, I thought there would be no harm in asking!

image.png

 

MaxHugen

Super Anarchist
On a plane wing, the centre of effort is about 33% of the chord back from the leading edge.

For a guesstimate, I would use this value, I think it will be closer than the geometric centre.
Thanks.  Actually, using XFLR5 and a reasonably decent profile of this "twin skin" mainsail, I have a good idea where the CE is located along the chord. It varies from 35-45% from LE, depending on the camber & windspeed settings.

The more difficult part is it's vertical height, especially so due to the twist of the sail, again depending on apparent wind speed. And this is super critical when calculating the forces on this boat, seeing as it's basically balancing on two "points" - the main foil, and the rudder foil.

Just small differences make a big impact on the balance of this particular design! Quite a challenge. :p

 

Leeroy Jenkins

Super Anarchist
1,560
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Vancouver
Yes.  I don't know if they've gone through First Narrows or if they have to lower them some or all of the way to get through - hugging the north side

 

JimC

Not actually an anarchist.
8,171
1,064
South East England
Centre of effort is something the cruising boys on boat design.net get enormously excited about, but as racing sailors we are used to raking  our rigs all over the place without the slightest concern.

In practice what happens is that the centre of lift from the foils is somewhere between rudder and keel, and if the rig ce changes the helm automatically makes a minute change to rudder angle which compensates.

In addition, in your particular case the extra rag is on/off roughly in the area of the rig CE, so the chances of it making much difference  to trim are small.

 

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