Yawl usually refers to a sailboat with a particular mast configuration:
  1. A mainmast somewhat forward of amidships, and
  2. A smaller mizzenmast placed at the stern, aft of the helm.
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The most famous yawls are the fabled 103 made by the Concordia Company of Padanaram, Massachusetts, between 1938 and 1966.  Designed by Waldo Howland, they were built entirely from wood and came in 39 and 41 foot lengths. My little diagram can hardly do these beautiful vessels justice.

Yawls are frequently confused with ketches; however, a ketch is a slightly larger boat with its mizzen forward of the helm.