Rowlock

Rowlocks, also known as oarlocks, are pivoting crutches that support and guide the oars of a rowboat, acting to transfer some of the thrust to the boat. They are normally "U" shaped with a pin protruding below. Some sporting ones have a retaining bolt across the open top.

Originally rowlocks were two wooden posts, that the pole of the oar nestled between. They act as a fulcrum and also as a thrust block. Rowing would be nearly impossible without them.