Why some whalers carried a boom below a square sail

Why do some nineteenth-century whalers show a boom beneath a square course when older square sails did not need one?

The boom let the crew control the sail's lower height and move it clear of work taking place on the whaler's deck.

The fitting is not simply a reversal of older square-sail practice. It gives the crew more control over where the foot of the sail sits while handling the equipment and heavy work associated with whaling. The ship's commercial function therefore reshapes its rigging details.