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Generally, it isn’t as complicated as you might think. The center of effort (CE) is a point within the sail plan through which the resultant of all wind forces is assumed to act. In a sail plan of a vessel each sail is assumed to have its center of effort at its geometric center, as Jack described, and on a drawing of a vessel's sail plan the resultant of these forces is assumed to be the center of effort of the whole sail plan. In practice, the sails are never completely flat as shown on the plan, and the actual center of effort moves with the re-trimming of the sails.
The center of lateral resistance (CLR) is a point assumed to lie at the geometric center of a sailing vessel's underwater profile. On a vessel's design plans this is indicated with the hull floating upright on its designed waterline. In practice, however, with sailing vessels heeled under a press of sail and lifting and pitching over seas, the actual center of lateral resistance is constantly shifting.
Thus, because the metaphysically exact CE and CLR change with the rolling of the boat and trimming of the sails, those points should be considered approximations. When weather helm increases as the wind strength increases, it is primarily because the relationship between the CE and CLR is changing.