Boat balance refers to the way the boat tracks through the water. If the boat tends to go in a straight line without any pressure on the tiller it is said to have a balanced helm. If the boat tends to turn upwind when no one is holding the tiller, then it is said to have weather helm. If it tends to turn the other way it has lee helm. When steering straight, the tiller has to be held to the weather side of the boat when there is weather helm. When there is lee helm, the tiller has to be held to the lee side. Sailboats are usually designed to have a slight amount of weather helm under normal conditions. Ths is primarily a safety feature.
CENTER OF LATERAL RESISTANCE
Each Sail has its own center of effort. (C. E.) The two centers of effort combine to make an overall center of effort along a line extending between each C. E. The exact point where the overall center of effort falls depends on the relative trim between the two sails. When the sails are trimmed so that the overall center of effort lines up on the center of lateral resistance, the boat is in balance.
In the above illustration the leftmost boat has its main trimmed very tight compared to the jib. As a result the main is pulling harder and pulls the combined center or effort aft of the center of lateral resistance. This causes weather helm.
The center boat has sails pulling evenly resulting in a balanced helm. (Most real boats will have a slight weather helm when sails pull evenly.)
The rightmost boat has a very loose main. The jib pulls the combined center of effort forward ahead of the center of lateral resistance. This causes lee helm.
Controlling Balance - The skipper and crew have two ways of controlling balance. One way is by controlling the relative trim between the jib and mainsail. The other is by controlling the heeling of the boat either with the Sails or by moving crew weight from one side of the boat to the other. By controlling balance the crew can actually steer the boat without the use of the tiller.
Relative Trim - Controlling the relative trim involves moving the center of effort forward or aft. Each sail has its own center of effort located at the geometric center of the sail. The overall center of effort travels along a line running between the centers of effort of the two sails. If the main is eased relative to the jib, the overall center of effort will move forward. If the jib is eased, the center of effort will move back.
The boat also has a center of lateral resistance. The center of lateral resis- tance is a line running vertically through the boat and represents the area where, if you were to push sideways on the boat, it would not have a tendency to turn. For instance, if a boat was being pushed sideways by the current against a piling, it could rest on one point where the boat would not be turned by the sideways pressure of the water. On a sloop this point will be near the area of the mast.
If the combined center of effort lines up over the center of lateral resistance, the boat will have a balanced helm and have a tendency to keep going in a straight direction. If you move the overall center of effort aft behind the center of lateral resistance by trimming the main, the boat will develop weather helm and tend to turn into the wind. Moving the center of effort forward by easing the main and trimming the jib will cause lee helm making the boat tend to turn away from the wind.
Heeling - Heeling affects the balance of a boat by creating a tendency to turn in the opposite direction from the way it is heeled. As a result, the more a boat heels away from the wind, the more it tries to turn up into the wind. As the wind builds while sailing, the weather helm has a tendency to increase. Reducing weather helm by moving the crew weight to the windward side of the boat will make steering easier and the boat go faster. It will also reduce leeway.
When the wind is strong enough that you can't reduce heeling enough by moving crew weight, you must reduce power in the sails. Flattening the sails by increasing luff and foot tension and easing the traveler will reduce power without cutting down on speed. If flattening doesn't reduce heeling enough, you must reduce sail area. You can also reduce power in the sails by easing them. But easing should not be used if it results in the sails luffing badly.