WOT RPMS for Counter Rotating Twin

For those of you with twin engines with the port engine counter rotating, do you see lower WOT RPMs on the port counter rotating engine?

Anyone have an explanation for why, as a rule, this would happen?

why counter clockwise rotating props are less efficient

I have done some more research and have what I believe is a credible explanation.

1. gearboxes or transmissions are "lossy" elements in a mechanical system. That's why, for example, engine HP measured at the crankshaft will always be higher than HP measured at the prop shaft, or for cars rear wheel HP or RWHP. The differences are the losses incurred by the drive train. Those losses are mainly in the form of friction and heat.

2. Some gearbox designs have more losses than others.

3. since most outboard engines sold are singles, the direction of crankshaft rotation was chosen so that they could use the most efficient gearbox design.

4. when making the counter rotating propeller engine it's too expensive to design, for each model of engine, a power head that rotates in the opposite direction which could then use a gearbox of the same design and efficiency as the clockwise rotating engines. Instead they designed the gearbox to make the direction change. That gearbox is less efficient, by design, than the standard clockwise rotation gearbox and I'll explain why shortly.

5. I have the F225 shop manual and have been studying the differences in the gearbox design between the standard and counter rotating lower units. The differences in design, and specifically where the forward gear and its thrust bearing are located, are significant.

6. First, some basic terminology. The prop shaft is the horizontal shaft the prop is attached to. The drive shaft is the vertical shaft between the crankshaft and the lower unit.

7. Here is why I believe the counter rotating gearbox will always be less efficient than the standard clockwise rotating gearbox. In the counter rotating gearbox, the forward gear is between the prop and the driveshaft. When it comes under load, the distance between the forward gear and the driveshaft gear is decreased ever so slightly, putting a small amount of the thrust load onto the gear to gear interface causing greater losses due to increased friction. This happens because the prop is pushing on the prop shaft, which in turn pushes on the thrust bearing which in turn brings the forward gear ever so slightly closer to the driveshaft gear.

8. Here is why I believe the standard clockwise rotating gearbox will always be more efficient than the counter clockwise rotating gearbox. In the clockwise rotating gearbox, the forward gear is between the driveshaft and the back of the gearbox. When it comes under load, the distance between the forward gear and the driveshaft gear is increased, putting none of the thrust load onto the gear to gear interface. This happens because the prop is pushing on the prop shaft, which in turn pushes on the thrust bearing which in turn pushes the forward gear ever so slightly farther away from the driveshaft gear. The only negative of this is the tooth load is moved towards the weaker outer edge of the gear's tooth but this is obviously something the gear is designed to withstand.

9. By design, as the counter rotating gearbox's thrust bearing wears, frictional losses will increase, so older engines will have greater losses than newer engines.

10. Note that the above analysis is based on the Yamaha F225 and LF225 gearbox design and will likely not apply to inboards or I/Os.

11. Note that there are not more or less gears in one type of gearbox verses the other; each gearbox has 3 gears in virtually identical configurations. The difference is in the location of the forward gear and its thrust bearing.

So bottom line is the counter clockwise rotating engine will always be less efficient than the clockwise rotating engine because the counter clockwise gearbox design is less efficient. That means that for perfectly tuned and matched power heads, the counter clockwise rotating propeller engine will burn more fuel and work harder when running at the same RPMs as the clockwise rotating propeller engine and at WOT the counter clockwise rotating propeller engine will run at lower RPMs. Most people seem to be reporting a 1-2 GPH difference at WOT which for a 20GPH engine equates to a 5-10% loss of efficiency for the counter clockwise rotating gearbox.

I could certainly have made a mistake and I am open to any and all constructive criticism of the above analysis.

Agree 100% that in my analysis, when I say "loss" and "friction" you can infer I mean conversion of mechanical energy into thermal energy. Lower units are well cooled, and after one season my lower unit oil is black. The difference bewteen car transmissions and lower units is the thrust bearing. Tons of force from prop thrust is transmitted through the thrust bearings continuously at WOT. Car transmissions do not provide this, its the rear axle wheel bearings that transmit the force from the wheels to the rest of the car, and unlike a boat, a car on a freeway is using about 10-20 HP so bearing losses are 1-2 HP or less than 1400 watts. And have you ever noticed that automotive differentials often have cooling fins? Lower units do not need the cooling fins since they are submerged in coolant 10% loss of my 225 HP is 22.5 HP or about 17 KiloWatts, a reasonable amount of heat to transfer through the lower unit casing to the sea water.