Sterndrivejärjestelmä

Sterndrivejärjestelmä, often simply called sterndrive, is a type of marine propulsion system that combines features of both inboard engines and outboard motors into a single unit. The system consists of an internal combustion engine or electric motor housed within the hull, a gearbox, and a propeller shaft that ends in a drive unit resembling an outboard. The drive unit is mounted at the stern of the vessel and can be angled by a hydro- or hydraulic thrust lever, allowing the boat to be steered without a separate rudder.

The main component of a sterndrive is the drive unit, which contains the gearbox, transfer case, and the propeller. When the drive unit is raised, the propeller lifts out of the water, reducing drag for high-speed cruising or for passing clearances. During backward motion or low-speed maneuvering, the unit remains in a lower position so the propeller can be used for thrust and steering.

Sterndrives were first introduced in the 1950s as a solution for medium‑size pleasure craft that required the power and efficiency of an inboard engine with the ease of maintenance and maneuverability of an outboard. Today, sterndrives are common on boats ranging from 12‑ to 24‑metre sailboats, as well as many motor cruisers. Modern models include hydro‑drives and power‑thrusters, which eliminate the mechanical link between the engine and drive unit and allow for vertical still‑on and still‑off operation without the need for a separate prop‑lift mechanism.

Advantages of the sterndrive system include reduced weight, lower fuel consumption compared to traditional displacement engines, improved handling at high speeds, and quieter operation due to the location of the gearbox. Disadvantages can include more complex maintenance, higher initial cost, and susceptibility to damage from underwater obstructions because of the protruding drive unit.

Typical maintenance tasks involve regular inspection of the drive shaft, bearing clearance, protector plates, and hydraulic fluid levels (for hydro‑drives). Marine certification bodies require sterndrive vessels to meet specific safety standards, which cover spill containment, fire suppression, and emergency shut‑off systems.