A suggestion. Unsupported, but it makes sense to me. Ships often had to beach themselves in order to take on cargo (Wuz ye ever in Miramashee, where ye tie up to a tree?) The tide would go out, cargo was loaded and the tide came back in but the ship had settled, aided by the extra weight of cargo, into the mud and to free it they had to roll the vessel in order to break the seal. A rope is passed around an 'immovable object' on shore and the capstan on board, and the vessel is rolled. They are free to go. Roll and go.

Ian