Answer
Sailing upwind the velocity of the boat relative to the wind is greater than sailing downwind.
Work Step by Step
Let $V_{bw}$ be the velocity of the boat relative to the wind.
Let $V_{bs}$ be the velocity of the boat relative to the sea.
Let $V_{sw}$ be the velocity of the sea relative to the wind.
Let $V_{ws}$ be the velocity of the wind relative to the sea ($V_{ws}=-V_{sw})$.
$V_{bw}=V_{bs}+V_{sw}$.
Now, we put the positive x axis pointing in the same direction of $V_{bs}$.
If I sail upwind, $V_{ws}$ is in the opposite direction of $V_{bs}$. So, $V_{sw}$ and $V_{bs}$ are in the same direction: the positive direction of the x axis.
If I sail downwind, $V_{sw}$ and $V_{bs}$ are in the same direction. So, $V_{sw}$ is in the negative direction of the x axis and, consequently, $V_{bw}$ decreases.
