CE, CLR, and Lead

[Lone Star]

I’m working my way through Teale’s book on boat design and have a question. The book says that CE should lead CLR by about 12% of the waterline length for a centerboard boat. This would normally result in lee helm however there is some sort of effect that occurs when the boat is underway that causes the CLR to shift forward of the CE resulting in the desired outcome of having a slight weather helm. What’s going on to make that happen?

[John E Hardiman]

It's due to a moment shift and it is a speed-angle of leeway-angle of heel effect. Do you rreeaallyy want the absolute correct answer?

[Lone Star]

Being an engineer I have an affectation that causes me to want things to make sense. Yeah, I know that this isn’t always possible. I made it through 2 semesters of electromagnetic wave propagation in college but I still think that stuff is voodoo and black magic. And after 11 years of marriage I’m still lost there as well. I was hoping to do a little better with trying to understand sailboat design.

[JimConlin]

Many things happen when a sailboat heels. Among them:
Particularly if the hull is fat, its immersed shape becomes asymmetrical and it wants to head up.
Particularly if the rig is tall, the center of effort of the rig moves to leeward and tries to turn the boat to windward.

[garyspear]

fields and waves are voodo, but then so are the differentials in boat design. I too would like to read a more complete explanation.

[mmd]

Not trying to curtail discussion on the topic here, but for those deeply interested in the subject I'd suggest that you read Marchaj's Aero-Hydrodynamics of Sail. His Sailing Theory and Practice is pretty good, too.

[John E Hardiman]

To state it simply, the water forces acting on the hull is the integral over the surface on the pressure caused by both the static immersion and the dynamic flow. In general a secondary force (the drag force generated by energy removal from the wind) is required to impose both driving force and heel on the vessel. The vessel will free rotate about its center of mass (not CLR), until energy loss in the wind is minimized. The task of the designer is to assure that a minimum tertiary force (the rudder) is required to ensure a quasi-stable condition where the majority of the energy loss from the air is directed into energy increase to the water via viscous methods (i.e. not pressure drag) to the maximum extent possible consistent with wind direction and desired course over ground. This is accomplished by using the hull shape pressure distribution in the athwartships direction to offset the energy due to rise of the CG under the effect of the driving force. Simultaneously, the designer must minimize the pressure increase over the fore part of the hull and the pressure decrease over the aft part of the hull in order to prevent the hull from rotating up into the wind. As the longitudinal change in pressure distribution cannot be prevented in real world hull, there is tendency for the center of lateral pressure to move forward as hull speed increases on most hulls. This change in lateral pressure forward causes an increase in weather helm.

[John E Hardiman]

MMD, I was typing while you posted.

I started out suggesting AHDoS, but I pulled it off the shelf and a cursory review did not have a good example of moment shift stand out. I think Hoerner Fluid Dynamic Lift where he talks about directional characteristics of bodies is a better choice.

[JimD]

IMHO JimConlin's explanation is probably all you need to know and is understandable, too. These other guys are just show offs. Imagine you are above the boat looking straight down on it. A boat sitting trim in the water has a port side and a starboard side sitting equally in the water. The shape is symetrical. Now lean the hull over to starboard as though it were heeling under way. Looking from above you'll see that the shape of the boat in the water is not symetrical. There is a lot more of the right side of the boat in the water than the left side. The wetted surface is no longer symetrical. More of the 'front' of the boat is in the water. Since the clr is the center of the area in the water it moves forward as the boat heels. 'course I dunno much more about it that what I read in Teales myself ;)

[John E Hardiman]

IMHO These other guys are just show offs.

No, just bored and not wanting to deal with the madness at the office.

[mmd]

"These other guys are just show offs." - JimD

Just pointing an interested party in the direction of information. Showing off would be to direct the reader to the learned tome that I myself authored; however, I am unfortunately not bright enough, experienced enough, educated enough, nor audacious enough to have created such a milestone in my career. <wink>

[JimD]

Looking forward to the publication of your learned tome, mmd :D

[John B]

But Jim.... what about Harrison Butler?......

[bainbridgeisland]

Ugh- guys, especially you John (I can talk to him this way cause I sit next to him at work), you are missing the most important reason why 12% lead is about right for most boats.

The center of underwater profile is not really the center of lateral resistance. Sailboat designers just use it as a convenient point to make the job easy.

Also the center of area of the sail plan is not the center of lift or drag.

So the whole idea of "lead" is based on 'center of effort' that isn't and 'center of lateral resistance' that isn't.

The average sailboat just seems to balance when 12% lead is used with the center of area of the sail plan and the center of area of the underbody profile. If you wanted to actually calculate the real center of effort and real center of lateral resistance, you could follow Johns' lead. Just remember though that you would have to recalculate the static condition of balance for each combination of angle of heel, angle of trim, sail adjustment, sheet angle and wave encounter. So it is not really practical.

[ion barnes]

This is the way my dad showed me a long time ago. Put the boat in the water with the board down and rudder tied off. Fasten a line to the gunwl across from the leading edge of the board and pull gently. If the boat comes across leading by the stern, move the line aft and try again till it does not lead either way. Once you get out on the water under true conditions, you can begin to fine tune with mast rake etc.

[alkorn]

Ted Brewer wrote a Good Old Boat article with some pretty extensive rules of thumb for lead. It's available at:
http://www.boatus.com/goodoldboat/Helm_balance.htm