Over my head; CE and CLR - SailNet Community

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  #1  
Old 02-20-2012
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Over my head; CE and CLR

It is winter in the northern hemisphere and I have a bit too much time on my hands; hence this question, or questions.

I'm no boat designer but there are a few of you out there who know a lot more then I do (you know who you are).

I am trying to help a fellow Tartan 27' owner who is converting his sloop rigged T27 to a yawl rig. My friend is old enough to possibly have met Olin Stephens and is apparently is looking for information on calculating the CE (center of effort) and CLR (center of lateral resistance) for the T27.
The quick question is: Is there an easy way to do this?

My instincts tell me this is the realm of the architect (S&S) and that they likely did these calculations for the T27 in the design phase of the boat. I looked on the S&S website but found very little pertaining to the CE or CLR of the T27. Would my best bet be to contact S&S seeking an obscure document that relates to my boat and just pay for it?

I am fairly certain that my friend still has the mast height of a sloop rigged T27 where the yawl rig diagram shows a slightly lower mast height as one might expect: TARTAN 27 YAWL sailboat on sailboatdata.com

That is all I've got for now.
Thanks in advance.
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Old 02-20-2012
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Well, I found this link so perhaps I don't need the refresher course on the subject of CE or CLR: Helm Balance – Center of Effort, Lateral Resistance, Centerboard, Mast Rake - Waves « Jordan Yacht Brokerage

I guess he will have some weather helm with all 3 sails up. He could reef and furl in the jib to approximate the yawl rigs original design. He could also leave off the mizzen and sail it as a sloop. Then there is jib n' jigger.
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Old 02-20-2012
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Rather using pins and the sailplan simply find the centre of the sail by drawing lines from the tack to the centre of the leech, the head to the centre of the foot and the clew to the centre of the luff. Do that for each sail. Then use some math to find the combined CofE using the area of each sail.

You can also find the CLR by easing the dock lines and pushing at various points along the toe rail until the bow and stern move away from the dock together.
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Old 02-20-2012
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Generally, it isn’t as complicated as you might think. The center of effort (CE) is a point within the sail plan through which the resultant of all wind forces is assumed to act. In a sail plan of a vessel each sail is assumed to have its center of effort at its geometric center, as Jack described, and on a drawing of a vessel's sail plan the resultant of these forces is assumed to be the center of effort of the whole sail plan. In practice, the sails are never completely flat as shown on the plan, and the actual center of effort moves with the re-trimming of the sails.

The center of lateral resistance (CLR) is a point assumed to lie at the geometric center of a sailing vessel's underwater profile. On a vessel's design plans this is indicated with the hull floating upright on its designed waterline. In practice, however, with sailing vessels heeled under a press of sail and lifting and pitching over seas, the actual center of lateral resistance is constantly shifting.

Thus, because the metaphysically exact CE and CLR change with the rolling of the boat and trimming of the sails, those points should be considered approximations. When weather helm increases as the wind strength increases, it is primarily because the relationship between the CE and CLR is changing.
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Old 02-20-2012
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If you can get a profile drawing and cut out the underwater profile on a piece of cardboard you can find the approx CLR/balance point by placing your cutout on a ruler's edge.. Can't recall if you should do this with or without rudder included..

But as mentioned it's only an approximation.
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Old 02-21-2012
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Quote:
Originally Posted by Faster View Post
If you can get a profile drawing and cut out the underwater profile on a piece of cardboard you can find the approx CLR/balance point by placing your cutout on a ruler's edge.. Can't recall if you should do this with or without rudder included..

But as mentioned it's only an approximation.
Skenes says to include the rudder - it IS a significant piece of the lateral plane.
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Last edited by SloopJonB; 02-22-2012 at 04:38 PM.
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Old 02-21-2012
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Quote:
Originally Posted by jackdale View Post
Rather using pins and the sailplan simply find the centre of the sail by drawing lines from the tack to the centre of the leech, the head to the centre of the foot and the clew to the centre of the luff. Do that for each sail. Then use some math to find the combined CE using the area of each sail.
The "some math" Jack refers to is as follows;

Use his method to find the centre of each sail. Draw a line between the two centres. Determine what percentage of the total sail area each sail makes up.
Find that point on the line between the two centres.

For example, if the main and jib each are 50% of the total sail area, the CE will be at the centre of the line. If the jib is 60% and the main 40% then the CE will be 40% of the line aft of the jib centre.
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Old 02-22-2012
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I can't really see the numbers on this drawing. If you could, it would probably provide sail sizes.
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Over my head;  CE and CLR-t27yd_sp.jpg  
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Old 02-22-2012
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Quote:
Originally Posted by Barquito View Post
I can't really see the numbers on this drawing. If you could, it would probably provide sail sizes.
The sail areas on on this page

TARTAN 27 YAWL Sailboat SailPlan Data and Sail Quoting System
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Old 02-22-2012
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Since CLR changes as the boat heels, how does one estimate the "dynamic" CLR?

For that matter, CE changes with sail shape.......?

My head's starting to hurt..............
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