An inboard motor is a type of marine propulsion system wherein the engine is permanently mounted within the hull of the vessel, typically located towards the center of the boat for better balance. The engine usually drives a fixed propeller that is connected via a shaft extending out to the stern. The transmission is attached to the engine and it controls the rotation direction of the shaft and hence the propeller. Unlike the outboard motor where the engine and propeller system are located outside the boat, in an inboard motor setup, only the propeller is immersed in water while the engine is housed inside the boat. The shaft propeller setup is usually under a protective cover to prevent any water leakage into the vessel. Due to the larger size, complexity, and cost, inboard motors are generally used on larger vessels. They also require a rudder separate from the propeller for steering the vessel. These motors can offer greater speed and are more efficient fuel users compared to outboard engines, making them suitable for vessels designed for extended travel. The inboard motor differs from the sterndrive or inboard/outboard (I/O) motor in that in an I/O motor, the engine is also housed inside the hull but the drive unit is outside, making it adjustable according to the boat's requirements or conditions of the waters. See also sterndrive and outboard motor for comparison.

1. A large fishing trawler uses an inboard motor, housed securely within the hull of the ship. Despite the harsh conditions encountered at sea, this internal placement helps protect the engine and ensures smooth operation to drive the fixed propeller. 2. A luxury yacht is another example of a vessel that uses an inboard motor. Along with its size, the vessel demands a powerful engine that can be best served by the sturdy construction of a well-protected inboard motor. 3. Large houseboats rely on inboard motors for navigation. As they are designed for leisure activities and not for speed, such boats need a motor with steady performance, such as that provided by an inboard motor. 4. Most commercial freighters utilize inboard motors as their main propulsion system. These engines are powerful enough to propel extremely large and loaded ships across oceans, and their inside location in the hull ensures their longevity and safe operation. 5. Navy destroyers, which are large military vessels, typically rely on inboard motors. These motors are designed to withstand the rigors of naval operations, efficiently driving the shaft and propeller of these large vessels.

The inboard motor is incredibly efficient and offers significant advantages in certain types of vessels. One interesting fact is that while this type of engine is typically found in larger, commercial vessels, it's also popularly used in competitive water skiing and wakeboarding boats. This is due to the fact that inboard engines, with their weight centralized in the boat, can create larger and more controllable wakes. So, when an athlete is water skiing or wakeboarding, the wake generated by a boat with an inboard motor offers a much larger "ramp" than an outboard motor would, allowing them to perform more impressive jumps and tricks.