A sterndrive, or stern drive, is a form of boat propulsion also known as inboard/outboard or I/O. The upper unit engine is located inboard, just forward of the boat stern, producing power by way of a shaft that goes through the stern to the lower drive unit (also known as the outdrive) located outside the hull, which resembles the bottom half of an outboard. The lower drive unit carries the boat propeller and contains gearing for the engine system. This unit controls the steering of the boat, as with an outboard motor. No rudder is necessary. The sterndrive engine itself is usually the same as those used in true inboard systems. The most popular sterndrive engines have historically been Chevrolet and Ford V-8 car engines that have been adapted for marine use. The most popular brand of sterndrive is Mercruiser by Mercury Marine, who also manufactures outboard motors and Mercury oil. Advantages of the sterndrive versus the outboard include higher available engine horsepower, a clean stern with no cutouts for outboard motor installation and no protruding powerhead, making for easier egress and ingress for pleasure boat passengers and more convenient fishing. Advantages of the sterndrive system versus outboards include simpler engineering for boat builders and space savings with engines mounted all the way aft, allowing more occupancy space for the boat’s passengers. Performance with sterndrives is better than inboards, due to the inherent loss of force with inboard propellers due to the angle of the prop shaft.
The main disadvantages of sterndrives versus inboards is that their components–namely oil lines, hoses, and rubber bellows, some of which are in the water–are more vulnerable to corrosion and damage from being exposed to the elements. Maintenance on sterndrives is more complex than outboard maintenance in several ways. Sterndrive oil changes may necessitate complicated pumping mechanisms, and engine repairs are often difficult to perform due to tight engine compartment spaces. In some sterndrive boats, the entire engine must be removed to perform even simple repairs, whereas with an outboard motor, only removal of the cover is necessary. With both inboards and sterndrives there are fire and explosion hazards from fuel vapors within the engine compartment. It is recommended that these boats run a blower for several minutes prior to starting the engine, while idling, or moving below cruising speed. The blower circulates the fresh outside air inside the engine compartment and vice versa. Sterndrives have become sought after for use in pleasure boating due to their practical advantages and attractive costs, and there are many current boat product lines that include models for which sterndrive power is the only option for available propulsion.