Twin outboards unequal fuel consumption

S

squidslayer

New member
I think I posted this once before but....am still having the same problem and am taking to the dealer soon but would like anyones take on this issue.

I have twin 2007 Honda 150 Four Strokes with 300hrs on my C-Dory 255 Tomcat.
I get 10% to 15% more fuel consumption on the starboard engine than on the port engine.
I’ve tried trim adjustments….both are at the same rpm…
.For example on my Honda fuel management gauges…both engines.@ 3300rpm…the port engine reads 4GPH and the starboard engine reads 4.5 GPH. The starboard consumes more than the port at any speed.
Refueling I have confirmed this by measuring how much actual fuel the starboard tank takes as compared to the port tank. On full tanks if I need to fill 50 gallons to the port tank..I need 55gallons to fill the starboard. Both engines run fine but….. on a cold start the starboard engine is harder to start.
I know that I cannot expect EXACTLY the same fuel consumption in both due to factors like boat design ect …but 10% more fuel consumption in one engine over the other is excessive in my book..
I know someone who has the SAME boat as mine….. Tomcat 255… with the SAME 2007 150 Honda engines…with the same Honda Fuel Management Guages..he says he has double checked and that both his engines use the same amount of fuel equally.

Any help or advice or knowlege on this subject would be deeply appreciated.

Dick Enderle
Orinda, Ca
 
Wild guesses on my part, but any chance your friend has counter-rotating props and you don't? I'll leave the real analysis to the engineers in the group, but here's a thought.

With the sideways torque induced by prop rotation, I'm thinking one engine will always have a slightly higher power requirement to keep the boat tracking straight. Counter-rotating props might negate this minor difference.

Another thought - any chance there is a slight mis-alignment in your engines? I'm assuming you have both set at the same RPM when cruising, but if one engine is straight and the other slightly offset, the offset engine will require more fuel for the same RPM's, no?
 
I have a Tomcat 255 with twin 150 Suzukis and counter-rotating props. I have noticed the same thing- the starboard engine uses up to 10% more fuel than the port engine.

This has to be a characteristic of the Tomcat. Does it have something to do with the list to starboard that all Tomcats seem to have? This might affect the angle of the motor while running and increase the load on the starboard prop...

What is even more surprising, just as Dick states, the starboard engine on my boat also gave me trouble in cold weather last winter. A couple of times, it stalled within minutes of starting. I remember calling the dealer concerning the problem. This has to be a coincidence! Interestingly, this winter it has been OK.
 
Is the boat running completely level? As others have suggested/hinted at, it may be that one engine is doing more work even though it is at the same RPM's. Someone recommended a dash mounted level a few months ago, it would be interesting to see if simply trimming the boat to level would even out the consumption.
 
squidslayer":1iv4dq47 said:
I have counter rotating engines...plus permatrims..hmmmm I wonder? could the permatrims be the issue??

Dick
Click to expand...

I added Permatrims last year. This issue was present before the Permatrims were put on.
 
rogerbum":1p0l5x98 said:
Is the boat running completely level? As others have suggested/hinted at, it may be that one engine is doing more work even though it is at the same RPM's. Someone recommended a dash mounted level a few months ago, it would be interesting to see if simply trimming the boat to level would even out the consumption.
Click to expand...

It will be interesting to figure this out. I'm thinking out loud, but if one engine is doing more work, you would need to add more throttle to maintain the same rpm as the other engine....

Unless the throttles are not adjusted properly, and unless you are positioning one throttle farther ahead of the other, it does not make sense to me that it would be a work load issue.....

What would affect fuel burn efficiency that would not be reflected in the throttle position/adjustment?
 
Maybe the port engine is running 10% leaner than the starboard, slowly committing suicide :wink: Maybe a prop is dinged, more weight on the starboard side, maybe your friend with two engines that use exactly the same amount of fuel hates you?
Have a good one!! :mrgreen: :beer
 
I don't have a specific answer for this either but my "gut" reaction is that since the counter-rotating engine is typically installed on the starboard side on the Tom Cat that it's something to do with that. The only reason the counter-rotating engine rotates the other way is there is a small intermediate gear in the lower unit of the counter-rotating model that makes it shift the opposite direction. So it seems it would either be that the engine for whatever reason is a bit less efficient in that mode, that the LH prop doesn't work quite as well (I've noticed I can almost never get exactly the same WOT rpm from the engines), or that the list on the boat is creating more drag on the starboard hull. Of course, there are lot more possibilities...this is just a start.

You could, really without much trouble, swap the two lower units and see if that changes anything. There isn't anything to connect or disconnect, when the bolts are removed the lower unit slides off.

You might also make sure the two props are really pitched the same. In having several new props checked it's amazing sometimes how not alike they really are (or aren't).

Ah...and the toe-in and/or toe-out of the engines could cetainly change the load on the props as well.
 
Maybe if you cruise in reverse half the time, it will even things out.

Thank you, thank you...I'm here all week.
 
Les Lampman":2uuh9583 said:
... The only reason the counter-rotating engine rotates the other way is there is a small intermediate gear in the lower unit of the counter-rotating model that makes it shift the opposite direction. So it seems it would either be that the engine for whatever reason is a bit less efficient in that mode....
Click to expand...

I was thinking about the same...do you think this lack of efficiency would cause greater fuel consumption without having to compensate in the throttle position or adjustment?
 
Dick,

If you trust that the combustion on both engines are equal, the following may contribute to the extra fuel consumption:

1. Props from the factory are probably not pitched and dynamically balanced as a pair. Gary at Wilmington Props (Southern California) use to tell me that two new props, out of the box, will be different enough if you need to use them as a pair. He recommends any set of props purchased to be run as a pair should be sent in to identically set the pitch and cup size. The props should also be dynamically balanced (balanced under running speed).

2. C-Dory puts the left-hand rotation gearbox on the starboard side. Depending on the make of your engine, the left-hand gearbox will generally use more fuel because of extra gears. In addition, the counter-rotating gearbox of some engine manufacturers require more oil, which relates to more friction.

3. Using the Permatrims to adjust for level running attitude will cause uneven fuel consumption. If you have a starboard list, the starboard engine will have to be trimmed down compared to the port side. This requires more torque on your starboard engine and you will also have to increase throttle to match rpms. Thus, starboard engine uses more fuel.
 
Here's another view of the information. I looked at the probability of an engine-related cause vs. hull-related:

3 TC-255's have reported the same problem so far on this thread (Squidslayer, Alok and Discovery) -- all that the STARBOARD engine is using more fuel than the port, 4th (Matt_unique) didn't report.

Assuming there's a 50-50 chance that one engine over the other will have a performance, lower end or prop-specific problem (not CW vs CCW, but rather a poor cast), we have a probability of 1 in 8 so far that it is engine-specific related (.5 x .5 x .5).

The sample size is small though to be a reliable indicator, but if true, it leans more towards something consistent among TC-255's (or at least those that reported), like hull or engine configuration related. Just my 2c (about what it's worth!).

(Note: I had to edit this slightly -- realizing that this really doesn't prove or disprove the efficiency of a CW vs CCW prop or lower end, oh well, it was a thought...)
 
Dora~Jean":2ywjfx25 said:
....
3 TC-255's have reported the same problem so far on this thread (Squidslayer, Alok and Discovery) -- all that the STARBOARD engine is using more fuel than the port, 4th (Matt_unique) didn't report.
...
Click to expand...

I take delivery in March, will let you guys know when I'm splashed.
 
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