What a great thread Jerry, very, very interesting.

"What about sea chanties? If two guys sang the same note when they were hoisting the lanyard (or whatever they do) would they be keel-hauled."

Actually there are those that feel strongly that harmony singing is not correct to the chanty singing tradition, and that it was introduced by "Black crews" on American vessels during the latter part of the 19th century. Frederick Pease Harlow in his book Chanteying Aboard American Ships makes the comment, "The southern negroes are not gifted to sing a chorus in unison and consequently they employed their harmonious faculties on the chantey, with the result that the whites soon began to imitate them." Today it is quite common to hear harmony in a chanty chorus, but there are folks that feel strongly that it's not "traditional".

As for singing the a "unison during the coarse on a harmony line', that appears to me to depend on where your from. It is far more common in the United States. The American Chanty groups that I'm thinking of right now that are known for their harmony arrangements, (Forbitter, Nextradition, Windlasses, The Johnson Girls) tend to lean toward what you'v described, (no two singers ever on the same note, parallel motion being quite common). However British, and European Chantey singers tend to lean more toward (for lack of a better comparison) the style of singing that reminds me of the Copper family. In this European version of harmony singing, the parts go in and out of unison going to the 3rd, 4th or 5th then back to the unison and very rarely moving in parallel motion.

As for me, I like the sound of male voices singing in unison, but most of the people I sing chanties with would rather not sing then sing the same note as someone else. I always thought this was an ego thing.