[Rules] River Battleships rules clarifying

[Juiceman]

Hello, is there anybody familiar with River Battleships additional rules?

I need help with chapter 17.44 Grounding:
"Roll one die, add the depth and subtract the draw."

I guess "the draw" means draught of boats. But I cannot find such characteristic anywhere. Could you explain how to play grounding?

It is in 17.4 Boat Movement, last bullet point.