Inboard

Inboard refers to something situated toward the center or interior of an object, in contrast to outboard, which denotes a position toward the exterior or edge. In nautical and marine contexts, inboard most commonly describes propulsion equipment that is mounted inside the hull of a vessel.

Inboard propulsion consists of an engine, transmission, and drive shaft located within the boat’s hull. The drive shaft runs to a propeller, typically beneath the vessel, providing power through a shaft-driven connection. Common in powerboats and yachts, inboard propulsion can use direct-drive arrangements, where the engine crankshaft drives the propeller directly, or geared systems that match engine speed to the propeller through a gearbox. Engines are typically diesel or gasoline and may feature integrated cooling and exhaust systems designed for installation within the hull.

Advantages of inboard propulsion include a cleaner and more navigable deck area, improved balance and interior layout, and often quieter operation due to isolation from the propeller and wake. Disadvantages include higher initial cost, more complex installation and maintenance, and potential risks associated with fuel, coolant, or exhaust within the hull. Inboard setups can also require deeper hull drafts compared with some outboard configurations, potentially limiting where the vessel can operate.

Beyond nautical use, inboard can generalize to internal components within machinery or devices, as opposed to external or removable parts (the outboard). The term thus describes location relative to the exterior or edge of a system.