Engines are the primary propulsion power source in cruising boats, either in single or twin inboard applications. Most cruising boat engines are inboards, but some use stern drives or outboard engines, too. Mounted beneath the cabin sole, and like a car or truck, marine engines have transmissions that help shift the engines in and out of gear, as well as reducing the revolutions to a speed usable by the propeller, which they are linked to with a stainless steel shaft. Engines in cruising boats are typically diesel engines, which are safer than gasoline engines because the fuel used is not readily combustible.