A board forming part of a bucket of a waterwheel, placed at an angle to the float-board so as to prevent water flowing over the float-board into the inside of the wheel.
Origin
Mid 18th century.
Definition of rising board in US English:
rising board
noun
A board forming part of a bucket of a waterwheel, placed at an angle to the float-board so as to prevent water flowing over the float-board into the inside of the wheel.