The boat engine’s lower unit is responsible for actually moving the boat through the water. While the combustion engine creates the power, the lower unit houses the driveshaft that transfers that power to—and turns—the propeller, as well as the water pump and gearbox, plus more. These units, which are engine specific, are usually sold as one piece and rarely require maintenance though may occasionally need new o-rings or seals, or perhaps additional lube to keep the parts in smooth motion. Replacing the parts of a lower unit requires disassembly . In addition to the driveshaft, the boat motor lower unit also contains other parts that help the boat run more smoothly and efficiently. The skeg, for example, juts out from beneath the lower unit and helps protect the propeller by deflecting anything that may approach it. It also acts as a rudder, allowing the boat to turn more precisely and accurately in the water. A hydrofoil is also commonly attached above the propeller. The size and shape of the hydrofoil allows the boat to lift higher in the water as it gathers speed, which ultimately reduces drag and makes the outboard motor more efficient. A boat’s lower unit is vitally important in making sure the engine moves efficiently through the water. Use our Marine Engine Parts Selector Tool to find the right engine parts for your boat.