Okay. So they need to bind it down.

But...what with?

He doesn't have anything in his pack that could help with that. So instead he looks around, hoping he'll spot something that could be of use...

Aha! There is something - a small box filled with nails. The lid is partly ajar, and many of the nails have likely scattered all over. But there's still a few next. Maybe enough to hold the sail down?

He holds the box out for Donnie.