A perfectly balanced yacht is a dream that most long for. The thought of setting the sails with the rudder straight and locking the helm as the yacht sails herself through the seas towards their intended destination.
Then reality comes when you set the sails and find that it keeps turning to windward and you end up dragging the rudder to leeward as you fight the forces and try to keep on course. If the winds build up too much, you will soon feel overpowered as your arms turn into spaghetti noodles with exhaustion and your yacht rounds up into the wind.
The fight is over, and the forces of wind and balance have won! But why does this happen?
Weather helm is caused by two forces: wind force and hydrodynamic force. The wind forces on the sails add up into what is referred to as the "Center of Effort" which is none other than the geometric center of each sail, and then the combination of each sails Center of Effort (also referred to as CE) to give you the yachts CE.
The hydrodynamic forces are the same concept, being the geometric center of the underwater profile when viewed as a silhouette. This is called the "Center of Lateral Resistance" and is also referred to as the CLR.
CE can be moved around by changing the sails that are flying and adjusting the trim of the sails, but CLR does not change.
Basically, you have two variables that control balance, one is fixed, the other is in your hands to control!
If the CE is aft of the CLR, the yacht will want to turn to windward. This is the case if too much sail is being flown aft and not enough sail is being flown forward.
If the CE is forward of the CLR, the opposite will occur and the yacht will want to turn downwind. This is the case when flying a giant spinnaker, as the large sail area forward will pull the yacht downwind and will not allow the yacht to travel upwind.
Balance occurs when the CE and CLR are in the same position fore-aft. When this occurs, the yacht will sail straight with no desire to turn, requiring no input from the rudder to keep it on track.