Twin outboards unequal fuel consumption

Bad Boy · Posted: Fri Jan 25, 2008 7:17 pm Post subject: Unequal Fuel Consumnption

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.

dogon dory · Posted: Fri Jan 25, 2008 8:28 pm Post subject: Re: Unequal Fuel Consumnption

Sorry, Folks - Post Deleted By Author

Dora~Jean · Posted: Fri Jan 25, 2008 10:13 pm Post subject:

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...)

Doryman · Posted: Sat Jan 26, 2008 1:23 am Post subject:

The Yamaha Command Link gauges only report total gallonage used so I have no way of comparing the two engines' consumption.

matt_unique · Posted: Sat Jan 26, 2008 7:04 pm Post subject: TC

matt_unique · Posted: Sun Aug 03, 2008 10:44 am Post subject: Starboard engine

I know this thread is old but I too concur my starboard engine uses more fuel. I also need to push my starboard throttle farther ahead of the port to get the same rpm. Also, the starboard engine has a top rpm of about 5800 when I'm trimmed for top speed, my port engine will hit 6100.

snal · Posted: Sun Aug 03, 2008 3:56 pm Post subject: Re: Starboard engine

drjohn71a · Posted: Sun Aug 03, 2008 7:05 pm Post subject:

I have the twin Honda 150's, but only notice a very small difference in fuel consumption at slow cruising speeds, i.e., hull speeds of 4-9 mph. My last fill up showed about a 12% greater fuel usage by the starboard motor. However, there could easily be a difference in how rapidly the fuel nozzle is shut off.

I start off and end each cruise with 30 min. or so of high speeds, so not always going slow, though. I use the autopilot extensively and it groans back and forth at times.

I do have the same difference in top rpms reported above. I keep the Permatrims pretty well flat most of the time. The steering for the engines looks to me as though they are often "toed in" or not exactly parallel. This seems to be worse at slow rpms - hull speeds.

Does anyone know if the engines not looking parallel is a sign of SeaStar steering problems? ... installation? ... or normal operation? ...or low fluid? ...or air in lines?

Alok · Posted: Sun Aug 03, 2008 7:09 pm Post subject:

Just another piece of information to add to the above:

The Suzuki 150 motors now have excess of 250 hours. Last week we took the boat down from Clear Lake to Corpus Christi. Full gas, full water, full provisions, two adults, bimini, permatrims, Texas flag on top of the cabin, ensign on the stern, bottom paint.

After a non-stop 107 mile first leg at 28-31 mph, fuel economy (judged by refilling the tanks) was exactly 2 mpg.

The starboard tank took only 6% more fuel than the port tank. In fact, the difference between the two motors, while still significant, seems to be gradually declining as the motor hours are increasing. Either I am getting better at trimming the boat, or the counter-rotating gear is running more freely...

Bad Boy · Posted: Mon Aug 04, 2008 1:49 pm Post subject: Uneven fuel consumption

Dick,

Are you able to get the same RPMs on both engines at WOT? If not, I would check out the pitch on both your props. When I ordered a spare set of props from the Prop Gods, I paid extra to have the props checked out and balanced right out of the box. One prop was 3/4" different in pitch.