17. As shown in the figure, right $\triangle A B C$ is rotated around the right-angle vertex $C$ to $\triangle A^{\prime} B^{\prime} C$, $P$ is the midpoint of $A B$, and $Q$ is the midpoint of $B^{\prime} C$. If $B C=8, \angle A B C=60^{\circ}$, then the minimum value of $P Q$ is $\qquad$
$\frac{17}{4}$