WOT RPMS for Counter Rotating Twin

[choogenboom]

Yamaha tech support told me it is normal for the counter rotating engine (ie the port engine) in twin installations to run 200-300 RPM lower at WOT than the normal clockwise rotating starboard engine. I have not found any other sources that confirm this, and have found no explanation for what the science is for this. Does anyone have either science or experience that confirms or refutes this? Seems to me that if they have the same pitch props they have to run at the same RPM otherwise the higher RPM engine will be loaded by the slower prop of the lower RPM engine. Comments??

[jb123]

i would seriously find a new tech

[choogenboom]

i would seriously find a new tech

I am guessing that you are saying you disagree with what Yamaha Tech support is saying. If so, can you explain why you disagree?

Thanks,
Chris

[choogenboom]

The answer to this question can be found here:

Grady White Boat Owners :: View topic - WOT RPMS for Counter Rotating Twin

Bottom line is that counter rotating propeller engines are less efficient, by design than the same clockwise rotating propeller engines.

[jb123]

"here is why i believe" are the keywords there

ive owned 4 pairs in the last 15 years and all, unless running improperly would run the same exact rpm, but then again maybe its just me

[choogenboom]

"here is why i believe" are the keywords there

ive owned 4 pairs in the last 15 years and all, unless running improperly would run the same exact rpm, but then again maybe its just me

I am always careful to preface my opionions with "I think" or I believe" or "I am guessing" ... I learned many years ago that I am often wrong

If you look at how the gearcase is built all of the clearances are con*****ed by shims. On the counter rotating gearbox, the clearance between the forward gear and the driveshaft gear is set by a shim. I am guessing that there is a large variance in losses in the counter rotating gearbox, from no difference to 10% difference, which is directly related to the gear to gear clearance and the shim that was chosen when the gearbox was assembled. Shim it too big and you'll get premature gear failure, shim it too small and you get larger friction losses in the gear to gear interface. I am sure that Yamaha prefers the err on the side of the latter. The other variable is how the gearbox's shims, thurst bearings and gears have worn which is dependent on how frequently the lower unit oil was replaced. My theory is that as the shims and thrust bearing wear and get seated that the gear to gear clearance for the forward gear is reduced leading to higher frictional losses.