It could refer to a piece of tackle, such as a large block used in heaving up an anchor, but I am stretching here, don't know for sure. The King spoke on a schooner helm was the one marked at the top when the rudder was centered, so it might be connected to this usage. The Buddhists have a legend about a King called the Wheel-Rolling King. Not related to the sea though. A song called "Rolling King" is listed as coming from the British Isles, no further data given, at Mappamundi. But a fragment of South Australia was published in Songs of American Sailormen by Joanna Colcord, under the title "Rolling King". She relates the tune to the well-known shanty that begins "Gloucester girls have got no combs; they comb their hair with jellyfish bones".

But still no explanation for what it is!! The version sung as "traditional" by Captain Jesse Schaeffer says, "Heave away, you rolling Kings", which sounds like a slang term for the deckapes at work rather than some part of their equipment -- so to speak....

A