Whereas an automobile has a drive shaft that runs from the transmission to the rear end, an inboard/outboard boat--sometimes called a "stern drive" boat--has a short "drive shaft" that may not even look like a drive shaft. It's called a coupler, and it carries the full load of the motor across the gap from the back of the engine to the plate at the front of the transmission to drive your vessel efficiently.
The engine coupler links the engine to the stern drive on inboard/outboard engines. In form, it's a steel plate or even a short shaft, with holes drilled though one end so that it can be bolted to the flywheel of the boat's engine. It has six additional holes on the opposite end, or side, that allow you to bolt it to the stern drive. Even though it may resemble a round, square or even triangular plate, it still amounts to a drive shaft that connects the boat's power source to the propulsion system in the same way that a drive shaft connects a car's engine to the drive wheels.
Most marine engine couplers look like a short, stocky shaft with a plate on each end, reinforcing the image of the drive shaft. The shape of the coupler is dependent on what type of motor. General Motors-based engines, such as Mercury Marine's Mercruiser, shares its engine block with the Chevrolet Corvette. This is a Mercruiser coupler. The second type is called a "flexplate" type of coupler, identified by a shaft that's bolted to the engine and a triangular plate called a yoke that may be removed if the stern drive uses a shaft-type attachment, rather than a a shaft-type connection between the engine and the drive system. The "flatplate" style of coupler resembles the flexplate coupler, except that its yoke cannot be removed. The shaft-type connection mounts to the engine's drive shaft as the shaft penetrates the flywheel. The flatplate--for which it's named--is then bolted to the drive system using the bolts provided with it.
The coupler is found behind the flywheel--that is, away from the front of the engine--and ahead of the stern drive on your boat.
While some marine engines, such as the Mercruiser, are similar to their automotive counterparts, they are not the same: the marine engine requires a special flywheel designed to allow the coupler to connect directly to the crankshaft. Another misconception relates to heavy vibration in the drive system: many boaters blame this vibration on a bent propeller. In fact, the vibration may result from a poorly aligned coupler, causing the propeller shaft to vibrate.
Because a misaligned coupler can cause a vibration that's both bothersome and damaging, particular attention, such as that given by a boat motor shop, is required when reinstalling the engine of an inboard/outboard boat. The couplers are designed to operate with 4/1000-inch clearance on all sides.