A person of mass \(m\) stands at the left end of a uniform flatboat of mass \(M_1 = 7m\) and length \(L\) that is initially at rest on a frictionless lake. The person walks to the right end of the boat, causing the boat to shift to the left by a distance \(d_1\) relative to the water. In a second scenario, the same person stands at the left end of a flatboat of mass \(M_2 = 3m\) and length \(L\), also initially at rest on the water, and walks to the right end, causing the boat to shift to the left by a distance \(d_2\) relative to the water. What is the ratio \(\dfrac{d_2}{d_1}\)?