Center of Effort vs. Center of Lateral Plane

For the bow to turn to weather, then there must be a force to lee, aft of CLR. This force is CoE.

At least we can agree on that?

Sure there is problem going from 2D stationary to 3D under-way, especially with (on paper) CoE being ahead apparently .. But I'm no expert, I'm asking too, so please correct if I leading everyone astray?

Let's list variables and their possible effects?



Most mono weekend-racing captains are really poor sailors, and so boats are set up to accommodate them.

The current practice is to increase the weather helm on a J24, by faring the keel: forwards. Most sailors seem to need more help in light airs, when the CoE is small.

Weather helm is reactionary, observing the wind and moving the helm BEFORE in anticipation is faster, and needs retraining.

No lapses allowed, "keep your eyes in your boat" and let the tactician tell of the positions of the competition.

Practice on a neutral-helm performance tri, and they'll soon figure it out. And then in light airs they'd be taken school for a second time; without any ballast to keep one moving.

But that's for racers, cruisers want to relax.

 

I understand the safety aspect of a small degree of weather helm so that the boat rounds up if the helm lets go.

Is 2 to 4 degrees of weather helm when close hauled not beneficial in that the rudder is actually providing more lift to weather than detrimental drag? That's what I've always been told, and it makes sense on a full keel boat where the rudder can act like a trim tab. Is this not valid on a fin keel/ spade rudder hull as well?

Chris

 

Unfortunately the terminology gets confusing in a hurry. There is what is commonly called the "CLR" which is a geometric construct, and defined as the center of some or all of the underwater side profile. Useful from a purely emprical standpoint but would be better if called something other than CLR.

Then there is the CLR which MikeJohns is refering to and which has fundamental physical significance, which is the point at which the side forces on the boat can be resolved into a simple side force without a moment; or to put it another way, the point where a side force applied would result in the boat moving sideways (as well as continuing to move forward) without yawing. This is the point which matters in terms of the balance the aerodynamic and hydrodynamic forces and moments so that the boat will go forward without yawing. It will be dependent on heel angle and rudder angle, and also on speed. Unfortunately it's not simple to determine, and requires either careful tow tank testing or complex CFD analysis.

Likewise for the CoE of the sails.

 

There is a lot about this on other forum threads. There are two notable systems here. Firstly stability. By this I mean yaw-stability. We first define the safe state, which for a yacht is head-to-wind. We now ensure that the boat tries to attain this heading with no helm. This results in weather helm, which is reasonable. A neutral helm, or lee-helm is actually potentially dangerous, as skipper failure will not cause a return to a "safe state". At best it will be undefined, at worst downwind and uncontrolled. The latter will likely result in a large roll angle (ie. capsize).

The second system at work is what ACTUALLY determines the required rudder angle. This is the relation between the true yawing moment of the rig, and the true yawing moment of the hull, keel and rudder, usually taken about the CG. Unfortunately, this is a very difficult relation to calculate with any accuracy, which is why we have rules based on centres of area, which are, quite obviously on inspection, total rubbish.

The amount of rudder angle which constitutes "reasonable" weather helm depends more on how much "reserve" rudder authority is required, than the drag produced. You must also remember, that the rudder angle relative to the centreline is of little relevance unless the leeway angle is taken into account.

Ultimately, the hull,keel and rudder will provide a sideways force, equal to that of the sails. The rudder angle will make a small difference to the total drag (for small angles), but the rudder drag (trim drag) is a result of balancing the yawing moment of the keel, hull and rig.

So, quite simply, you must balance the stability against the trim drag. I haven't yet heard of anyone doing the full solution properly.

Cheers,

Tim B.

 

The actual balance of forces and moments is very difficult to understand if considered only from a 2-D, side view perspective. Trying to do so is usually mis-leading at best.

Consider a boat with a cat/uni rig running downwind with the boom at almost 90 degrees. The actual "CoE" of the sail will be to leeward and ahead of the actual CLR, and the boat will have a strong weather helm.

If you want to understand the actual physics there are a number of good books available, including:
"Principles of Yacht Design", Larsson & Eliasson
"Aero-Hydrodynamics and the Performance of Sailing Yachts", Fossati
"Sailing Theory and Practice", Marchaj
"Sail Performance", Marchaj (cursory about balance, lots on aerodynamic forces including their effective location and direction)

 

Thanks for all the great comments. I hope that others have learned form this discussion and I have had conformation of my basic understanding.

 

I think the issue is that in the geometric design the lead should be positive, but that this is not representative of the location of the actual forces on the boat. Given the rules of thumb when designing a boat on paper should yield a boat with a practical negative lead.

 

The location of the CE along the centerline of the boat is irrelevant in this case. It is because of the lateral position of the CE, either to port or starboard of the CLP that the boat wants to head to wind. You need to consider a point of sail where the sails are actually pulling, ie the apparent wind is forward of the beam. If the actual CE were ahead of the actual CLP the boat would bear away. Only if the actual CE is aft of the actual CLP will a boat have weather helm, although on paper this may be represented by the geometric CE being forward of the geometric CLP.

 

The aerodynamic forces (and hydrodynamic forces) can be resolved into a vector which can be thought of as located along a line in space. Any point along the line/vector can be regarded as a "CE". What is your preference?

 

As someone who has raced multis and monos, it's hard to find any evidence for the idea that weather helm is slow on all boats, or that those who sail on "neutral helm performance tris" are better sailors.

On some boats, weather helm creates superior windward performance by loading the rudder - therefore it is NOT a matter of average sailors needing weather helm, it's a case of weather helm being faster - quite a different thing.

About the tri point -please provide us with a list of world-class sailors who have come out of racing "neutral helm" tris. Compare that to a list of sailors who have come out of classes like J/24s (in a breeze) or Lasers that feel as if they have significant weight on the tiller. Using the classes I sail as a sample, there is a huge variety of weight on the helm, and the best sailors come out of the class with the heaviest helm - hardly an indication that weather helm is "reactionary".

I'll bet the J/24 and Laser sailors have done a lot more winning than those from "neutral helm tris".

BTW most of the classes I sail have little or no weather helm, but those that DO have weather helm certainly do NOt produce inferior sailors.

 

I'm not a NA, but it always seems weird that the rudder isn't counted, when we know that many (all?? most??) separate rudders carry a very large amount of the lift.

Something as simple as a Laser can't be sailed effectively unless the rudder is providing sideforce, and a windsurfer depends enormously on the "rudder" blade for CLR. It seems odd to ignore that.

 

Chris
As you say a sailboat can get a lot of lift from the rudder, most if not all racing boats would look for weather helm within the drag bucket of their rudder profile for this reason to help counter leeway.

The extreme version of this are the small Spanish and Portuguese fishing sailboats that are all rudder and no keel, the boats are designed to be beached and sit flat. All the lift is from a large deep rudder.

The whole CLR COE area guide is really just a rule of thumb for designers who need to start somewhere.
Cheers

 

Here are some high performance boats where the rudder is more involved in developing lateral resistance than on most traditional designs....

Bill Roberts "Shared Lift" concept: http://www.aquarius-sail.com/images/arc21/index.htm

pictures L to R: 1) Mirabaud a 26'+ monofoiler-note position of daggerboard forward of the mast, 2) CBTF(Canting Ballast Twin Foil) where the two foils
together simultaneously act as rudder and lateral resistance, 3) the Arc 21 with its daggerboard forward of the front beam-see "shared lift" link above:

Click on image-

 

Remove ALL foils, weight the hull so that it is heeled and push.. she will turn to weather, so some force (rudder or CE lead) is needed to keep her tracking straight. Hence some weather-helm has nothing to do with CE versus CLP, but the asymmetry of the heeled hull (including the shape presented by the bow). Thus wider hulls and the more they heel increases weather-helm.

So thin hulls, like multi-hulls and those that don't heel much, like tris, don't (may not) have as much weather-helm.

Increasing the light-air weather-helm of a J24, just is not fast as someone who can sail her without it. Like someone who sails tris?

Increasing weather-helm is adding drag. Part of the reason why "sail flat" is faster.

 

Several years ago I read Martin Simmons book "Model Aircraft Aerodynamics".The section on pitching moment seemed to describe a very similar situation if you can visualise the foils rotated ninety degrees and apply the idea to keels and rigs.