I found this on the web.
"Boat builders manfacture their crafts' in three basic transom heights. The transom height is measured from the top of the transom, (where the engine bolts on), straight down to the keel, (bottom middle), of the boat. A 15'' transom would require a shortshaft engine. A 20" transom would need a longshaft outboard, and a boat with a 25" transom, would require a extra longshaft engine." From WikiAnswers.