On boats and ships around the world, shanties were work songs whose rhythms helped synchronise tasks. So for example a haul shantie was sung when hauling in a line. If the job was expected to be quick, a short-haul shantie would be sung - one long stress per chorus; if long, with less force perhaps two pulls each chorus, a long-haul shantie was called for such as the ubiquitous Blow the man down (beloved of music teachers).
Other shanties would be developed for other jobs and might involve stamping as a kind of group percussion. And it is the driving rhythm that may encourage us to recognise shantie or shantie beats in poetry. Or simply hear the sound of a work task hidden in its structure.
There is more information here http://www.jsward.com/shanty/index.shtml and here http://en.wikipedia.org/wiki/Sea_shanty