Helmsmanship can be described as the art of sailing a yacht to the best advantage, and of knowing instinctively whether the sails are correctly balanced and trimmed for the desired course or whether too much or too little sail is being carried for the current weather conditions. To become a good helmsman takes practice and experience, but it is made easier if the principles of sail balance are understood.
The underwater profile of a yacht is designed to create a resistance to sideways movement caused by air pressure on the sails. This is known as Lateral Resistance, and the point through which the resultant of all the lateral forces is designed to act is the Centre of Lateral Resistance (CLR). Similarly, the forces of wind acting on the sails can be reduced to a single resultant acting through one point being called the Centre of Effort (CE). If the CE is aligned exactly above the CLR, the yacht will be perfectly balanced, but if the two points are not aligned a turning moment is created and the yacht will react accordingly. If the CE is aft of the CLR the yacht will tend to turn towards the wind, this is called Weather Helm. If the CE is forward of the CLR the yacht will tend to turn away from the wind, Called Lee Helm.
Normally a yacht is designed to carry slight weather helm, which gives a positive feel to the tiller and makes it easier to sail close to the wind. A yacht with lee helm is very uncomfortable to sail, and under certain conditions can be positively dangerous.
The position of the CLR in a fixed keel yacht cannot be changed except by structural alteration, but the position of the CE can be varied by changes in the sail plan, and to some extent the rake of the mast. Excessive weather helm can be corrected by reducing the size of the mainsail or increasing the size of the headsail, thus moving the CE forward and closer to the CLR. Lee helm can be corrected by reducing the size of the headsail and increasing the size of the mainsail, or, if the condition is inherent, by raking the mast further aft. Correct sail balance must be maintained when sail is reduced in heavy weather, thus it may be necessary to set a smaller headsail when the mainsail is reefed