Center of Effort vs. Center of Lateral Plane

I’m confused, for years I have believed that the distance the Center of the sail area is ahead of the Center of Lateral Plane is the positive lead, (giving weather helm). However, I was just reading a book that states that the Center of Effort should be behind of the Center of lateral Plane. Should the rudder area be included in the lateral Plane? What ammount of lead is accepted for Sloops, Yawls, Ketches, cat ketches?

 

Have you read Aero-Hydrodynamics of Sailing by Marchaj? I mention this because "lead" rules of thumb are intimately tied to hull type and sail aspect ratio and the way these centers are determined. AHoS does a fair job of explaining how all the sailing forces balance around the CG and how the 3-D aero-hydrodynamic forces relate to each other. It is important to remember that there is a "true" Center of Effort/Resistance where the driving force/resistance are resolved into a single force vector and a "calculated" Center of Effort/Resistance which are 2-D points determined from the plans that can be related to each other. Additionally, hull form and sail aspect ratio change these relationships, so a drag keel sloop with a low aspect main and sternpost hung rudder will have a lead significantly different than a high aspect fin keel sloop with a spade rudder.

 

Most accept that a separate rudder blade isn't used in the CLP calculation, but typically 1/2" the rudder area, for a skeg or keel hung rudder is included.

This is the black magic of the art, though acceptable percentages are: 15% - 17% sloops, 13% - 15% yawls, 12% - 14% ketches and schooners 10% - 12%.

As mentioned there's an awful lot that can affect the lead percentage, so rig and hull for specifics will be necessary for a "fine" tune" of these percentages, not to mention a good does of experience based intuition.

 

Never behind , always positive by similar amounts to PAR's post above.

Also consider that that heel angle is particularly significant.
A stiff boat needs less lead ( Leed not led ) than one that sails on its ear to windward. The center of effort moves horizontally away from the mid-line and becomes a lever arm turning the boat into the wind.

 

I know of several designs that do carry negative lead Mike and are successful. This wouldn't be the case for a modern design (canoe body, Bermudian sloop). Interestingly enough all of the negative lead boats (at least according to memory) have full length, built down keels and keel hung rudders. As a rule, we're all in agreement I suspect.

 

If you use the percentages that Par posted you will be in the ballpark,you can then work with mast rake ,sail cut etc to fine tune the balance and not likely have to move the mast which would be a pita. Bruce,what was the book that you were reading?
Steve.

 

The boat in question is a cross between the B&B EC22 and the Blue Lightning http://www.bluelightning.co.uk/ts/bluelightning.shtm
After reading your posts and going back to Kinny and Marchaj I feel better. Small sail boats are new to me and i can see that they don't play by quite the same rules as the older, bigger, traditional sail boats i worked on. (Hell they weren't traditional boats when I worked on them.)
Thanks for setting me straight.
Bruce

 

The EC-22 will be on the low side of the equation and I suspect Bluelighting will be on the high side of the average lead placement.

 

Yes multimasted vessels particularly have a myriad of option with the actual sails hoisted and can change the center of effort considerably to suit the course. Even shifting the COE by 25% of the LWL with ease.

While heeling 10 degrees moves the CLR around 6% LWL for a full keel and for a fin keel around 3%.

Rudder angle effect is impressive, only 3 degrees of rudder angle on nearly all underbody shapes with a reasonable rudder area moves the CLR by close to 10% LWL.

But you can see the sail options on say a schooner a brig or or even a wishbone ketch can easily balance the boat as the wind picks up pretty much regardless of the hydrodynamic CLR position .

A good paper on this is Prof. K Nomoto "Balance of helm of sailing yachts, a ship hydrodynamics approach"

 

Parallel discussion on WoodenBoat Forum http://forum.woodenboat.com/showthread.php?126681-Center-of-Effort-vs.-Center-of-Lateral-Plane

 

"CoE" as the geometric center of the sail area is generally not the location the the net aerodynamic forces act through.

"CLR" as the geometric center of the some or all of the underwater profile is generally not the location the the net aerodynamic forces act through.

But the lead methods when used with knowledge from similar vessels are a reasonable empirical method.

 

I've read both forwards and aft...

It is that you design CoE to be forwards of CLR upright, but when sailing (heeling), CoE is actually behind CLR ie, causing the hull to round-up in a blow? ie. Heeding moves the CoE aft?

As the hull heels it becomes more asymmetrical and also the CoE becomes out-board of the canoe body and creates a turning moment?

If you think of it, CoE NEEDS to be aft of CLR for it to turn the bow into the wind, ie pivot around CLR. Dosen't moving the mast aft increase weather helm?

Multihulls- hulls that heel little, or are narrow (quite symmetrical when heeled) will have less lead or maybe none?

 

Moving the mast aft increases weather helm. Moving the keel/centerboard forward increases weather helm. The question is where to locate the them relative to each other.

What is commonly refered to as "CoE" is NOT where the aerodynamic forces act while sailing. Likewise for the "CLR". So trying to figure where CoE should be relative to CLR on a first principles physics basis doesn't work. Lots of boats have the CoE ahead of the CLR, and have a weather helm. However putting the CoE in a similar relation to the CLR as other similar boats known to be successful is a good pragmatic, emperical way to locate the sails. Fabio Fossati in "Aero-Hydrodynamics and the Performance of Sailing Yachts" in discussing the use of such methods has said "they [lead method] are only valid for producing a boat design that we can consider balanced with respect to a series of previously analysed projects; otherwise they are excessively disconnected from the fluid dynamics governing the problem, and from this point of view there is room for in-depth research, ..."

 

DCockey has posted what we've forgotten, that is the CE and CLP locations commonly noted are really for comparative purposes, when looking at different design specifics. In this regard the actual lead may be quite different then the assumed lead. In the end, rig type, hull form and appendage "qualifiers" will push the lead percentage around. It should be noted that I've never seen a modern yacht, that didn't require a healthy amount of weather helm to preform well upwind, which suggests the black art of this relationship still exists.

 

The figures I gave before are for the real hydrodynamic CLR from physical testing.

The other issue is that you want lift and that requires an angle of a keel to the flow. A well separated spade rudder won't provide any more drag if its countering weather helm or lee helm to provide that angle it's more a matter of safety. A boat should round up and stop in irons ideally if the helm is released.

A full keel with attached rudder has to have a lee rudder angle to get decent flow lines around the lower body.