A sterndrive combines an engine inside the boat with a steerable external drive mounted through the transom. It is often called an inboard-outboard. The drive contains gears, turns to steer and usually trims up and down.
Sterndrives suit many planing boats because they provide efficient thrust and trim control without an inboard shaft line. They are less common as the primary propulsion system on cruising sailing yachts.
The maintenance burden is concentrated around the transom assembly and submerged drive. Bellows keep water away from universal joints and other internal spaces. Gear oil, seals, shift mechanism, anodes, hydraulic trim, cooling passages and corrosion protection all matter. Fishing line can damage propeller-shaft seals. Water in gear lubricant may appear milky, but a pressure and vacuum test is the proper way to locate a leak.
Never assume all sterndrives use the same oil, anodes, antifouling or winterisation method. Drives differ by model, gear ratio, propeller system and raw-water routing. Follow the paired engine and drive manuals.
For purchasing decisions, compare annual haul-out access, parts support and corrosion history, not just top speed. Also inspect the transom structure around the cut-out for movement, moisture or previous repairs.
Related: Marine propulsion types · Propeller problems