Where do we get the phrase that will take the starch out of your sails and what is the role of starch in sails?

Starch in sails helps form the heavy canvas into an airfoil shape. A stiff, smooth, consistent sail produces more thrust for the boat than a floppy, unformed sail. Taking the starch out of your sails means to slow you down or discourage you. Besides using starch for this purpose, captains sometimes ordered sailors to pour seawater over the sails. The wet sheets produced a tiny but significant increase in speed.