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Outboard Engines
  • An outboard is a portable, self-contained package of an engine, gear case, and propeller that is attached to the transom of a boat.
  • A growing number of outboard engines are of four-stroke design, but many are still conventional two-stroke engines that burn oil as a lubricant along with the fuel. New-technology two-stroke outboards are direct-injection engines and burn over 75% cleaner than conventional two-stroke outboards.
  • Steering of outboard boats is controlled by a tiller or steering wheel that swivels the entire engine to direct propeller thrust.
Outboard engine
Outboards have more power per pound of weight than do inboard engines.
Inboard Engines
  • An inboard is a four-stroke automotive engine adapted for marine use. Inboard engines are mounted inside the hull's midsection or in front of the transom.
  • The engine turns a drive shaft that runs through the bottom of the hull and is attached to a propeller at the other end.
  • Many personal watercraft (PWCs) have two-stroke inboard engines that burn oil as a lubricant along with the fuel. New-technology two-stroke PWC engines are direct-injection engines and burn cleaner than conventional PWC engines.
  • Steering of most inboard vessels, except PWCs and jet-drive boats, is controlled by a rudder behind the propeller.
Inboard engine
Inboards have automotive engines adapted to operate in marine environments.
Stern Drives
  • Stern drives are known also as inboard/outboards (I/Os) because they combine features found on both inboard and outboard engines. Stern-drive engines are four-stroke automotive engines adapted for marine use and are mounted inside the boat.
  • A stern-drive engine is attached through the transom to a drive unit (also called an "outdrive") that is essentially the lower unit of an outboard. The engine turns a drive shaft that is attached to a propeller at the other end.
  • Steering of stern-drive boats is controlled by the outdrive, which swivels like an outboard engine to direct propeller thrust.
Stern drive engine
Stern drives have quieter and more fuel-efficient engines.
Jet Drives
  • Jet drives propel a vessel by forcing a jet of water out the back of the vessel. Directing this jet of water steers the vessel.
  • Personal watercraft are the most common type of vessels that use a jet drive.
  • Jet drives also may power larger vessels (jet boats) and are used commonly for vessels designed for shallow water conditions. Jet boats can have inboard or outboard jet drives.
Outboard, inboard, and PWC jet drives
Jet drives use an engine to power a strong water pump, which sucks up water and then forces the water out the back to thrust the vessel forward.