Sterndrives, I/O, inboard outboards are all the same item.  They are the brainchild of Charles Strang, who in 1948, was working toward his master's degree at M.I.T.  He was an avid outboard racer enthusiast and the rest is history.  They were designed to be the best of both types of engines and relatively inexpensive to purchase.  They are a wonderful creation as long as you stay up on the scheduled maintenance.  Stay away from the rocks
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