In order to protect the joint between two bamboos, one bamboo should be cut longer than the other so as to shift the point where the largest force is applied from the center. For this reason, a balance should be used because the entire balance would be disrupted when the sail is spread unless the center of the yard matches the center of gravity. The length of thin and thick bamboos should be made three to seven, and the center of the yard should be on the thick one.