Inboardoutboard

Inboard-outboard, often abbreviated as I/O, refers to a boat propulsion system that combines an inboard engine with a stern drive (outdrive) mounted on the transom. The main engine is located inside the hull, while propulsion is provided through a drive unit that extends through the transom into the water. This arrangement aims to blend the benefits of an inboard engine with the versatility and space efficiency of an outboard drive.

The stern drive contains the gearcase, propeller shaft, and propulsion gears, and it is steered and trimmed to optimize handling and planing. Power is transmitted from the inboard engine to the stern drive via a driveshaft and a marine transmission. Steering is typically hydraulic or mechanical, and many systems offer power trim to adjust drive angle for performance and efficiency.

I/O propulsion became widely popular in the mid-20th century as a compromise between traditional inboard setups and full outboard motors. It is commonly found on sport boats, runabouts, and family cruisers in the small to mid-size range, with manufacturers such as MerCruiser (Mercury Marine) and Volvo Penta playing major roles in its development and support.

Advantages of I/O systems include preserved cabin and deck space since the engine remains under cover inside the hull, relatively straightforward access for certain maintenance tasks at the transom, and good shallow-water performance in typical recreational boats. Disadvantages include more complex maintenance for the stern drive, potential for water intrusion and corrosion at the drive unit, and generally higher initial cost compared to some outboard configurations.

Typical applications are boats under about 30 to 40 feet, though some larger vessels use stern-drive arrangements. Regular maintenance, including gear oil changes, anode replacement, and winterization, is essential to prolong drive life.