Engine Rotation
Marine Articles >Boat Engine Parts > Engine Rotation
People are often perplexed when asked by the marine parts man what engine rotation they have.
Parts such as starters, distributors, camshafts and some engine oil seals are specified by engine
rotation.
Engine Rotation
Rotation is always determined by looking at the flywheel end of an inboard engine.
The flywheel of a standard rotation engine turns left, or counter-clockwise, when viewed from
the flywheel. The flywheel of a reverse rotation engine turns right, or clockwise, when viewed
from the flywheel. Most single engine inboard and I/O boats use a standard rotation (L.H.)
counter-clockwise engine. Use the illustration to the right, supplied by API MARINE, to help
understand engine rotation.
If your boat has twin engines, counter rotating engines have long been used to neutralize
propeller torque which improves vessel handling and performance. The port, left side engine, is
the standard rotation engine. The starboard, right side engine, is a reverse or counter rotation
engine. Sometimes it is not possible to view the flywheel so engine rotation will have to be
viewed from the front of the engine. Just remember to reverse what you see at the front of the
engine so it agrees with flywheel rotation. These are the guidelines for engines mounted in the
normal fore and aft position with the flywheel closer to the stern of the boat. Some older boats
used a flywheel forward configuration that can change the rules. Also, some ski boats with single
engines, mostly Ford 302/351 power plants, used reverse rotation engines so it is a good idea to
determine your engine's rotation before visiting the parts department to avoid error.
Due to the higher production costs and the advent of fuel injected gasoline engines bristling with
computer modules and sensors, engine manufacturers discontinued reverse rotation engines in
most cases. Since then, engine outputs have been handled by reduction gears capable of reversing
engine output rotation. Both engines are standard left hand rotation but the reverse reduction gear
changes the starboard engine output to right hand rotation. This is a much simpler, less expensive
way to handle the need for opposite rotating engines.
Starter motors, when mounted forward of the flywheel, will turn clockwise (R.H.) to start a
standard rotation (L.H.) counter-clockwise motor. Starters mounted aft of the flywheel will turn
counter-clockwise (L.H.) to start the same engine. Everything is just the opposite when speaking
of a reverse rotation (R.H.) clockwise engine.
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