I agreee with Amos's final comment. The chanteyman seems to be calling the sailors rolling kings rather that refering to any piece of equipment. And given that "rolling" seems to be a common metaphor for "sailing" (cf. Rolling down to old Maui, Roll the woodpile down, Roll the old chariot along, etc.) I would guess that he is calling them "sailing kings" i.e. great sailors. There are a number of chanteys which have lines expressing the idea of "What a great crew we are." and I think this falls into that category.

-Bill