From Mechanics

From part b) we have

\displaystyle \mathbf{v}=\left( 2-\frac{t}{5} \right)\mathbf{i}+2\mathbf{j}

If the boat is travelling north, the \displaystyle \mathbf{i} component of the velocity must be zero.

\displaystyle \begin{align} & 2-\frac{t}{5}=0 \\ & t=10\ \text{s} \\ \end{align}