Twin engine, Match fuel consumption or RPM

J

Johnc_3

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C Dory Year
1998
C Dory Model
24 Tomcat
Vessel Name
White Knight
So I have a 24' TomCat with twin Mercury Command Thrust 115hp Out board engines, starboard engine is counter rotating. I use Mercury VesselView mobile for electronic engine feed back. When on step I usually match RPM but noticed the counter rotating (starboard) engine uses a lot more gas than the standard rotation (port). My son says just match the fuel consumption when trimming the engines. That works and fuel consumption matches but... That doesn't seem the proper way to trim the boat. Anyone have any reasons why the counter rotating engine uses more fuel? And what is the recommendation for trimming out the engines? fuel consumption or RPM?
 
Johnc_3":1c0lz5aj said:
So I have a 24' TomCat with twin Mercury Command Thrust 115hp Out board engines, starboard engine is counter rotating. I use Mercury VesselView mobile for electronic engine feed back. When on step I usually match RPM but noticed the counter rotating (starboard) engine uses a lot more gas than the standard rotation (port). My son says just match the fuel consumption when trimming the engines. That works and fuel consumption matches but... That doesn't seem the proper way to trim the boat. Anyone have any reasons why the counter rotating engine uses more fuel? And what is the recommendation for trimming out the engines? fuel consumption or RPM?
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I would start with "Are the fuel flow counters calibrated correctly?" You might run the engines off separate tanks and see if the fuel used matches the fuel consumed numbers.
 
we do have separate fuel tanks and indeed when matching rpm the counter rotating outboard uses more fuel.

The Mercury electronic Vessel View uses the on board computer in each engine for RPM and fuel consumption feed back.
 
Is the gear ratio different in the counter rotation? This is sometimes the case. I don't know about Mercs.
 
Gearcase ratio 2.38:1 is what is listed for both lower units. So should be the same. I wouldn't think there would be much loss in gearing to reverse rotation.
 
I run Yamaha 150’s but the hull is the same. Engine info is on the N2k backbone (I’m not aware of any other TC255 owner with this setup). MPG and GPH display as analog gauges, so it’s not easy to see trivial differences under 15%.

Official YamaWorld pronouncement is that if 2 consecutive engines built by the same girl on the same day on the same assembly line differ in fuel use by 15% that is NORMAL. (We don’t find this on the all-robot Porsche Panera assembly line for a reason).

Likewise, They pronounce that any counterrotater will consume MORE fuel, but not how much more or why. If 15% is NOTHING, maybe 30% more could be expected.

My understanding (perhaps wrong) is that automated engine ‘synchronizers’ on large boats synchronize RPM’s, not fuel consumption. I tend to synch manually by RPM, but my fuel consumption per engine is within 15-20% (14 GPH is within 15% of 16.1 GPH, which I now regard as trivial. 14 GPH for #1 vs 16.8 GPH for the counter rotator would only be a 20% difference).

Is your variation around this same 15%?

I see no harm in following your son’s rec’d to synch by fuel consumption, as long as you chill out for differences in the 15-20% range and chalk it up to another of the Mysteries this Universe is entitled to have, with or without us.
It all seems more art than science.

Enjoy your boat! My VesselView experience was that if the engine computer didn’t shut ‘em off, it’s all Good and nothing to worry about.

Cheers!
John
 
Johnc_3":9qv57gnt said:
Gearcase ratio 2.38:1 is what is listed for both lower units. So should be the same. I wouldn't think there would be much loss in gearing to reverse rotation.
Click to expand...

Some engines have an extra gear to reverse.--and this maybe true in engines with same gear ratio-- which will add more friction. Long time ago some outboards were run "backward" (counter clockwise) for the counter rotating engine.

Incidentally your engines are set up properly for a cat, with the counter rotating on the Starboard side--the reverse of what is recommended for monohulls.

The engines should be synced for RPM--and prop thrust. For example in some boats with different ratios, different pitch props will be used to give same RPM.

I agree with John, it would be interesting to run curves to see exactly how much extra fuel the engine is burning at different RPM. Info from the engine computer should be accurate. I have computers on my trucks which give "current" fuel contusion in gallons per hour. At the same RPM, it can vary depending on load.

Would there be more load on the counter rotating engine? I don't think so--but for example if there was more fouling on that hull it might give that effect (and I doubt that there is any fouling...Just an example.
 
Sounds clear as mud to me :shock:

On my Tomcat 255 the starboard Honda 150 consistently uses more fuel than the port counter-rotating motor. Best guess is an average of 10% fuel usage.

As a long time pilot I prefer to match engine RPM's over fuel flow. Do you not sometimes hear slight propeller cavitation when matching fuel flow versus prop rpm?
 
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