In rowing, the primary meaning of the word “scull” is moving the boat with two oars per rower. A rower who uses two oars in this way can be called a “sculler”, and the act of propelling the boat through symmetrical movement of the sculls is called “sculling”. “scull” can also mean a rowing boat in general but in its sense of a symmetrical motion, it is more often used to mean a boat just for sculling.
“Scull” in its rowing sense may have come into English from an old word linked to washing, an activity whose back and forth scrubbing motion is similar to the symmetrical movements of sculling oars through the water. Besides the familiar form of sculling with two oars, “scull” can also mean a single oar mounted at the back of a small boat. Sculling in this way pushed the boat forward through side-to-side movements of the scull.
Scull vs. Skull
New rowers learn quickly that the word “scull” should not be confused with “skull”, another English word that sounds the same but refers to the bones of the head. Ironically, the word “skull” comes from the same root as “shell”, but “scull” seems unrelated.
Whatever the activity, a symmetrical movement seems central to the meaning of “scull”. This is clear in forms of “sculling” in a range of other sports. Examples include swimming (an in and out symmetrical movement of the hands and forearms) and ice skating (an in and out symmetrical movement of the skates).
In rowing, the word “sweep” is used for the method of moving the boat with one oar per rower. Although it is usual to use the term “rowers” for those who sweep, the term “sweeper” is sometimes used. The act of propelling a boat by this means is called “sweeping” or “sweep rowing”.
For those outside of the sport of rowing, the English word “sweep” is most often connected to the long arcing movement of a broom cleaning a floor. Like the arcing sweep of hands on a clock, the movement of an oar through the water is clearly a sweep in this sense. There is also a linked meaning of making something clean (through sweeping).
Other sports that use the term sweep include cricket’s “sweep shot” (swinging the bat in a low horizontal arc) and the similarly executed “sweep pass” in field hocky (low-to-the ground swinging hit of the ball with the stick).
Excerpt from "World Rowing".