Van Helsing says that our chance will be to get on the boat between sunrise and sunset.

The Count, even if he takes the form of a bat, cannot cross the running water of his own volition, and so cannot leave the ship.

As he dare not change to man's form without suspicion, which he evidently wishes to avoid, he must remain in the box.

If, then, we can come on board after sunrise, he is at our mercy, for we can open the box and make sure of him, as we did of poor Lucy, before he wakes.

What mercy he shall get from us all will not count for much.