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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Keep your hooks wet & your Butts dry also make sure your whistle stays wet

Suggestion: Be careful assuming that your engine rotation matches that specified in the article. In my 1984 39' FB Sedan (with 350HP, 454cid, Crusaders), the port engine is the counter rotating (RH) one - when it was disabled, I determined the direction of rotation with the help of a directional arrow on the engine near the flywheel then verified by bumping the starter. I actually had my engine rebuilt but if I'd taken the "standard" specified in the article as gospel and bought a standard rotating replacement I would have been in lots of trouble. From some other posts I've seen on this site (and the one before), I think maybe my boat is somewhat of an oddball but .... and I'm sure everyone knows what they say about ASS-U-MEing

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Richard

Richard:

You wouldn't have a photo or two of your boat and it's props as it sits out of the water? I have a theory that I'd like to test.

Jeff

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Only 40 days 'til splash time!

It appears that the port prop on Miss Cleo rotates CCW so the tranny must be reversing - seems to be a funny way to do it - to me, it seems that would be extra drag on the engine. So I think that Doug & Peter were right - maybe Jeff too. Anyway ... my point in the "suggestion" was that we all need to check before just buying an engine according to the "standard"

Jeff: I sent you a pic to an email address I had for you - if you don't get it, PM me with your address.

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Richard