As most model boat sailors know from experience, the forces acting upon the boat are dynamic - they are moving around pretty much all the time. Accordingly a boat may be balanced (by which I mean a state in which the boat can sail forward, with the rudder centered, and without futher input from the helm) for a few seconds or so, and then something changes which causes the boat to go out of balance. Perhaps the most obvious example is when a gust of wind causes the boat to heel further, and there is a sudden onset of weather helm, which leads to the boat rounding up towards the wind.
Some boats seem to have great difficulty staying in balance, whereas others seem to have a wide “zone” within which balance changes are minor and tolerable. On a full sized boat we can alter the balance on any point of sail and wind strength buy trimming the mainsail and headsail separately. On a dinghy, we can also change the balance by raising/lowering the centerboard. These trim adjustments move one or both of the CoE/CLR, with the objective of bringing them back into the same vertical line. But on most of our RC sailboats (at least those limited to two channels) we are stuck with the settings made before the boat leaves the dock, and from that point on limited to sheeting both sails via a single winch.
Thus in designing an RC sailboat, I believe considerable emphasis must be put on the effects of hull shape on balance. Lester has a good article on his website about this. What we are talking about is not the position of the CoE cf the CLR - but instead the effect on the balance of difference angles of heel - through changes in the underwater shape of the hull.
Imagine your boat, upright, on a windless pond, with the helm centered. If you could give it a nudge from behind, then hopefully it will move acros the water in a straight line. Now imagine the same windless environment, but through the magic of imagination, you have the power to set the heel of your boat to, say, 30 degrees. If you could now give your boat a nudge from behind, would it still move in a straight line, or would it describe an arc across the water? Pretty much every boat will move off in a curve. Even if the underwater sections remained exactly symetrical at heel, you still have the influence of a keel fin and bulb now canted to one side.
The tighter the arc, the bigger the impact on balance through the hull shape of the boat - even in the absence of the major factors of CoE and CLR.
The hull shape for which balance is least affected by heel would be a symmetrical bulb shape, round at every vertical cross section, and also symmetrical laterally. For such a hull, the only poisoner of balance (in our windless scenario) would be the keel and bulb, and rudder. Unfortunately, such a hull shape is probably not the fastest design we could come up with - hence that wonderful neccesity in sailboat design - compromise.
I have a little 24 inch sailboat, which I built off a plan many years ago. When first launched it showed wild swings in balance between lee helm and weather helm, with every gust of wind. Over time, I modified rig design, mast position, mast rake and rudder size. I got rid of some of the problems but the boat was always “unbalanced”. This is because the underwater hull shape changes a lot as the boat heels. The Center of Boyancy moves a long way, and the underwater hullshape becomes very asymmetric.
So, in addition to talking about CoE and CLR, any discussion on balance must include hullshape - especially in models.
There’s another influence too - but I’ve already written too long a post. Think about the turning effect created when the lift generated by the rig, is moved away to leeward as a boat heels. Even if the CLR and CoE are in the same vertical plane when viewed from the side of the boat - they are not when viewed from above. Consider this against the advantages (aerodynamically) of a high-aspect ratio rig v a low-aspect ratio rig. But that’s another story.