Engine rotation

Does anyone know whether my starboard engine is counter rotating? I doesn’t run and I can’t find a number that confirms it.

1990 Cooper Prowler 10 meter
7.4L twin Mercruisers.

I would think that if your engines are counter rotating your props would be pitched opposite. Can you check them?

Look at your transmission. Push both helm levers to the forward position. Now go look at the gear selector on each transmission. If they are both in the same position you have a counter rotating engine. I believe counter rotating engine is always on port side.

Bump the starter over while a friend watches the rotation on each engine.

Maybe crack open the raw water pump and take a peek at the direction of the rubber vanes?

CooperBrewer said:

Does anyone know whether my starboard engine is counter rotating? I doesn’t run and I can’t find a number that confirms it.

1990 Cooper Prowler 10 meter
7.4L twin Mercruisers.

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Interesting question .... I would think that all engines rotate only in one direction and the rotation is changed by the gearbox ... no ?

My twin Cummins rotate the same way ............. fb

Most diesels rotate only one way and counter rotation is done through the gearbox. With gas engines you could actually make the engine run counter rotation by changing the distributor, cams and starter motor.

Several years ago a friend with twin gas engines needed a new starter motor. He was a cheapskate and bought a rebuilt unit from the local auto parts store. When he tried to start the engine it sucked water into the cylinders. Guess what? It was a reverse rotation engine.

All modern engines (gas or diesel) turn counter clockwise (viewed from the flywheel) with just a few exceptions.
A few older engines such as the early Volvo MD series diesels turned CW. Some early Detroit's could be set up with the output on either end.

Detroit 2 strokes can run in either direction as well a chose the side for servicing.

If you have the Mercruisers that are in your signature, you could tell if the engine is CR by the firing order. Look for it stamped on the intake manifold or just trace the spark plug wires to see if they are in different positions on the distributor.

They could be either rotation. Usually marked with an arrow at the bell housing or crank pulley.
Did it used to run in its current configuration? If yes then crank it a second and you will know.

18436572 is normal rotation 12756348 is counter rotation

The coming of hydraulic transmissions ended the need for counter rotating engines. The prop rotation is controlled in the transmission by diverting the oil flow.
Because it simplifies manufacturing and cuts the number of engine parts, there is no incentive to produce counter rotating engines. For operators that carry major spares, it cuts the inventory in half.
My Detroit Diesels turn in opposite directions. They use mechanical transmissions. One engine requires a different crank and cam. In other parts, different starter and water pumps on some models. But they use the same head, pistons, sleeves, etc.

One easy way to tell if your engines were set up properly to counter rotate is to look at the alternator fins on the belt pulley. If they are opposite, the engines are set up to counter rotate.

You don’t have to open anything up or turn anything on.

My Perkins 6.354 twins are counter rotating and the alternator cooling fins are facing in opposite directions.

Ex Sailor said:

Interesting question .... I would think that all engines rotate only in one direction and the rotation is changed by the gearbox ... no ?

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That is not the case. Many diesels were counter rotating. Many of those engines, the counter rotating ones, are still around, in twin engine boats.

Although I have a single diesel, older engine, my engine model has standard and counter rotating models depending upon whether the boat was a single or a twin.

That has changed in most cases so now the newer engines all turn the same way with the shaft rotation change done by the gearbox.

The same goes for many gasoline engines. My previous boat had both types, standard and counter rotation.


The suggestion to try to crank the engines will tell you if you have a counter rotating engine. Just a bump will be all it takes. If one turns one way and the other turn the other way you have a counter rotating engine.


While looking at the FRONT of the engine where all the pulley/belts are:
-- if the engine rotates clockwise the engine is standard or L.H. rotation.

--if the engine rotates counter clockwise then it is counter or R.H. rotation.

THe determination is actually done at the flywheel but since most of us cannot see the flywheel this is acceptable just keep in mind which way those pulleys turn on which engine.

I am surprised by the question. What it tells me is the obvious that different boats are different or at least their operators are. When I do my engine room checks underway I can see the shaft rotating so I was surprised that this would not be the case with others.

Shafts may turn in different directions than engines. They always turn in different directions from each other. That is achieved by changing engine rotation or in the transmission. So what you see on the shafts may not help determine engine rotation.

Some clever methods I had not thought of here!

ghost said:

Some clever methods I had not thought of here!

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Both of my big block V8 Mercruisers , fire in the same direction. They're virtually the same engines. The counter rotation of one prop, is handled by the hydraulic transmission.

This post was from long ago. Since this time, and with some help from this forum... my learning curve is almost a straight vertical line now. I have learned quite a bit, about my boat, and boats in general.

Good to hear. Does that engine run now?

High Wire said:

Good to hear. Does that engine run now?

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Yes, my port side engine is running good. Working on the starboard now.

tiltrider1 said:

Look at your transmission. Push both helm levers to the forward position. Now go look at the gear selector on each transmission. If they are both in the same position you have a counter rotating engine. I believe counter rotating engine is always on port side.

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This is definitely the easiest way, and I have determined that the gear selectors are opposite, and that both of my engines are standard rotation. Thank you.