Let $L$ denote the required length of the ship. Assume Emily's speed is $1$ and let $v < 1$ denote the speed of the ship relative to the riverbank. When Emily walks from the back to the front, it takes $210$ steps, so $210(1 - v) = L$. Similarly, when Emily walks from the front to the back of the ship, it takes $42$ steps, so $42(1 + v) = L$. Solving this system yields $v = \frac{2}{3}$ and $L = 70$.