Center Of Effort

by Dave Acree

One of the first steps in tuning a yacht to sail correctly involves the Center of Effort and the Center of Lateral Resistance. Both principles are important on how a yacht sails. This article will explain the Center of Effort (CE) and the Center of Lateral Resistance (CLR).

The Center of Effort is the geometric center where the wind pressure on the sails exerts its total heeling effect. To determine where the CE is located on any triangular sail plan is relatively easy.

To start, draw lines from any two corners to the midpoint on the opposite. This can be the tack and clew or the head. This is done for the main and the jib. Where the lines cross is the CE of each sail. To find the CE for the sail plan, draw a line connecting the intersection of the two lines of each sail. Some where along that line is the combined CE. Usually around the center. For more information, see Figure 1.

The Center of Lateral Resistance is the geometrical center of the yacht while in the water. To find the CLR, place the un-rigged yacht in a calm pool. Then carefully push the boat sideways with your finger, starting somewhere in the center. If the yacht moves away at an angle, stop, move your finger towards the end which is closes to you and try again.

Be careful and take your time. Once you find the spot on the boat where the CLR is located, mark the spot. See Figure 2. Both the CE and the CLR will vary while the yacht is under way due to heeling and fore and aft trim. These two exercises are used to understand these two important principals.

Once the CE and the CLR have been established now it is time to put the sail rig on the boat. Since this a new sail rig, it is just the matter of setting the mast at or slightly ahead of the forward edge of the keel fin. See Figure 3.

At this point it would be good to explain how the relationship between the CE and the CLR effects how a yacht sails. To start, if both the CE and the CLR are together, the yacht has a neutral helm. A neutral helm is just that, the sailboat sails on a straight course. That is ok but you want the boat to luff up a little.

As stated before, it is desirable when a yacht luffs a little to windward. If the yacht luffs excessively, it has too much windward helm. To correct, move the mast forward 1/2", thus moving the CE forward. Also, if too much weather helm, the rudder is used to such a degree that is becomes a break slowing the boat down.

The opposite is a lee helm, the worst of the three. If the boat falls off wind on a reach, the boat has a lee helm. To correct, move the mask aft 1/2" thus moving the CE back. A lee helm also causes the rudder to be used too much and slowing a boat down.

Well, we have gone over the two principals which makes a yacht sail the way it does. Look at Figure 4 to get a understanding of the three types of helms. Next month we will go over what to do when we put that new sailboat in the water!