Sterndrive boat engines are known by several different names. They are inboard engines placed at the stern of a craft, connected to an outboard drive system. They are also called inboard/outboard motors or simply "I/O." Outboard boat engines are a single unit of both motor and drive system mounted on the stern or transom of a vessel. Outboard motors steer by pivoting the entire engine/drive unit. They are manufactured as separate units from their craft whereas sterndrive engines are generally manufactured as a part of the craft.
There are two design elements that favor sterndrive boats: aesthetic and weight distribution. Aesthetics are subjective, but there's nothing subjective about the visibility of an outboard motor. Outboard motors can be seen; sterndrives are hidden. So, outboards restrict the freedom of the boat's lines; they must be included in the profile. Another "must" is the weight distribution. An outboard will place the weight over the transom. Sterndrive designs give designers some freedom of where to place the weight, and the forward weight is generally preferable: It is more balanced and closer to the center of the boat.
Beyond the favorable aesthetic and weight distribution, praise for sterndrives dwindles, primarily because they are more expensive in nearly every measure. One example is maintenance. Accessibility of sterndrive engines is usually more difficult. Rather than removing an engine cover to expose the engine, it is usually recessed in an engine bay below the aft deck. This makes the majority of normal maintenance such as oil changes more difficult on sterndrives.
Durability of the engine favors neither design. Each can be excellent or poor. Durability of the drive, however, favors the outboard for one simple reason. An outboard can be tilted forward, lifting the drive and propeller out of the water when it's not in use -- even if the boat remains in the water. With a sterndrive, the drive is submerged the entire time the boat is in the water. This can increase the frequency of major drive overhauls by a factor of three or more for sterndrives.
Sterndrives and outboards can each be removed from one boat and installed on another. But outboards are designed to be removable. A sterndrive's engine and drive is not more interchangeable than a car's engine and driveline. For used boats, in particular, this means it's easier to buy and sell independent outboard motors than sterndrives. Sterndrives are generally sold with their boat because installing one is no small task.
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