Balancing the sails may be viewed by some as sailing Nirvana, a perfect state which you know exists but can never quite reach. The truth is, balancing your sails is very simple.
When a sail fills with air, it begins to power up as its shape takes on the form of an airfoil. When the sail is properly trimmed and shaped like an airfoil, it will generate lift which will be transferred to the sailboat via the rigging and in turn move the sailboat through the water.
Perfect trim will have the air flowing evenly and smoothly over the both sides of the sail. This state will produce the maximum amount of power out of the sail. This is why when sailing, you want to trim the sail until the tell tales are streaming evenly on both sides of the sail.
There are various sails on the boat, located in front and behind of the mast. If the sails are all powered up to their maximum, balance will probably not be attained. The combined force from the sails will be localized into the "Center of Effort" or CE. This force is going to be counteracted by the "Center of Lateral Resistance" or CLR which is produced by the underwater profile. If the CE is ahead of the CLR, the boat will have lee helm and veer to leeward. If the CE is behind the CLR, the boat will have weather helm and veer to windward. If the CE is directly in line with the CLR, the boat will be balanced!
For purposes of balancing the boat, the rudder will be locked midship and not altered. This will keep the rudder in line with the keel and thus keep drag to a minimum. With the rudder center lined and locked, the CLR will not change. The relationship of CE to CLR can be altered by trimming the sails.
If the boat wants to turn to windward, this means that the CE is behind the CLR.
If the boat wants to turn to leeward, this means that the CE is ahead of the CLR.
These are the only two options present when the helm is locked, and it no longer seems that complicated when you break it down to just two choices with no gray area.
Your sails will generally develop more power if you sheet them in, and less power if you ease them out. This means that you can generally alter your CE by either sheeting your sails or easing your sails.
If the yacht wants to turn to windward, you need to move the CE forward to meet the CLR and balance the boat. To do this, simply ease the main. Easing the main will reduce its efficiency and cause the CE to move forward as the headsails have not been altered.
If your yacht wants to turn to leeward, you need to move the CE aft to meet the CLR and balance the boat. To do this, simply ease the headsails. Easing the headsails will reduce their efficiency and cause the CE to move aft as the main has not been altered.
That's it! Doesn't seem so complicated now does it?
If you ease a sail to the point that it starts to luff or develop leach flutter, you should sheet the sail in a bit until it calms down and then sheet in the opposite sail.
For example, if you have lee helm and you ease the headsail until it flutters, sheet it in a bit and then sheet in the main. Easing the headsail will move the CE aft, but flutter will destroy your sail. Sheeting in the main will also move your CE aft, and not cause premature destruction of your headsail. In the end, you can move the CE aft until it meets the CLR and the boat becomes balanced.
After you achieve perfect balance and are sailing along with your rudder center line and causing minimal drag, you may feel inclined to look at your sails once more. You will probably see that your sails are trimmed terribly. Tell tales will probably be twirling on one side of the sail, indicating that the sail is not trimmed to utmost performance. You may feel the urge to go trim the sail until all the tell tales are flying perfectly, but that would upset the balance of the boat by moving the CE either forward or aft.
You need to remember that sails are an individual component that is tied together into the rest of the boat via the rigging to form a sailboat. The goal is not to have a perfect sail, but instead a perfect boat. If an over trimmed sail is needed to balance the sailboat, then that is what is needed to sail along properly.
Being able to zoom out and see the big picture is an important ability. This will show you that the tell tales don't really matter, and what is important is that the boat sails along through the seas.