Could I just point out that the ballad 'Captain Ward And The Rainbow' is in Child (no. 287)? It is, apparently, a black-letter ballad from the end of the 17th century.
I've recently learned a version of this ballad, called 'Ward the Pirate', from Roy Palmer's excellent book 'Folk Songs collected by Ralph Vaughan Williams' (Dent, 1983 - and more recently re-issued as 'Bushes and Briars', I think). RVW collected this song from a Mr James Carter, a fisherman of King's Lynn, Norfolk, in 1905.
I recommend that anyone who is interested should read Mr Palmer's note to this song. Briefly, he tells us that Ward was a fisherman, from Kent, who became a pirate in the Mediterranean, eventually dying of the plague, in Tunis, in 1622. Some years after Ward's death a captain in the British Navy named 'Rainsborrow' or 'Rainsborough', commander of the royal ship 'Samson' had some success against Barbary Corsairs in the Med. Palmer hypothesise that the later ballad writers may have got the two stories confused - hence brave Captain Rainsborrow was transformed into a ship called the Rainbow!!
Ewan MacColl did record a Scottish version of the ballad on a Critics Group LP ('Ye Mariners All' Argo ZDA 138). This version always makes me smile for the following reason: The old black letter version starts with, 'Strike up, you lusty gallants, with musick and sound of drum, For we have descryed a rover, upon the sea has come:...'. Mr Carter's version opens with: 'Come all you gallant seamen bold, all you that march to drum, Let's go and look for Captain Ward, for on the sea he roams..'. Ewan's Scottish version starts: 'Cam all ye jolly mariners, that love te tak a dram...'! Obviously Scottish mariners have different priorities!