Twin outboards.. different fuel consumption

squidslayer

My twin 150hp Hondas on my new Tomcat 255 use different amounts of fuel
ie
160 miles all open ocean
Starboard Honda 150 used 42 gallons
Port " " " used 38 gallons

they only have 100hrs
is it normal for one side to use more fuel than the other all the time??? everyting seems ok....otherwise
is this an excessive difference???

Dick

BrentB · Posted: Wed Jul 04, 2007 12:15 am Post subject:

Just curious
Are you using fuel flow meters?

Sea Wolf · Posted: Wed Jul 04, 2007 12:18 am Post subject:

Dick-

That's a difference of 10%. Pretty significant!

There are differences between the hp output of similar motors. 10% seems a bit large.

Is it possible the difference is in instrument error? Or the way the tanks fill, if using gas imput measurement from the pump?

Joe.

squidslayer · Posted: Wed Jul 04, 2007 1:02 am Post subject: fuel flow

I got the high end Honda digital gauge pkg...with digital computerized fuel management sys

it' really accuate

Dick

jlastofka · Posted: Wed Jul 04, 2007 7:40 am Post subject: Fuel flow matching

Hi,

Don't confuse "precision" with "accuracy". Having a digital readout with decimal places displayed doesn't necessarily mean the system is accurate. A sensor could be in error, for instance. In other words, if you're burning 8 gal per hour, a gage reading "about 8 or so" is more accurate than one reading 7.0032, even though the latter is showing more precision. A good way to check is to see if the total gallons used, added up over a few tank fillups, matches the total fillup volumes added up from the gas pump gages.

Another idea: If you're running the engines at the same RPM, they could still be producing different power and therefore using different fuel flows, due to prop differences or something else. You could swap props between engines and see if the fuel flow difference follows the propeller or stays with the engine. (Unless they're opposite rotation directions, of course)

Jeff Lastofka

Alok · Posted: Wed Jul 04, 2007 9:06 am Post subject:

I have Suzuki 150s on my Tomcat with about 150 hours. The fuel consumption of the starboard motor has always been higher by 5-10%. This has been verified multiple times by the amount of fuel needed to top off the tanks.

I have not been able to pin down the reason. Tomcats list a bit to starboard, and the list is even more pronounced with the helmsman in place. Could it be that the starboard motor is pushing "more" boat through the water, so the rate of fuel flow needed for the starbaord engine to come up to the same rpm as the port engine is more?

Perhaps someone who is more knowledgeable about boat physics will be able to provide a more technical explanation.

drjohn71a · Posted: Wed Jul 04, 2007 9:44 am Post subject:

Hey all, While I have not noticed such extreme variance in the flow guages, I am perturbed that the Honda Speed guage on my boat is just about worthless. I've cleaned the paddlewheel, transducer and yet it is always way off my gps speed (I know "speed over ground" vs. "speed over water") but mine will say I'm going 4 mph when the gps says 37 mph and it will also say I'm goint 14 mph when I'm sitting still. Sometimes it seems to hit the groove and match the gps within reason.

One of my engines has a higher max rev level than the other... so, I guess they are not the "same" totally. If you were just checking the variability on a one way trip, I would think the wind and waves could make one engine work more heavily than the other.

My throttles are not "even", so if you put the handles exactly even, one engine goes faster than the other. Also, I've noticed the revs vary alot depending upon loading, waves and wind.

All in all, I'm very happy with the fuel efficiency so far for such a large, heavily laden boat.

John

Alok · Posted: Wed Jul 04, 2007 10:58 am Post subject:

John:

If I remember correctly, you have your batteries on the port side. If that is so, your boat is better balanced than mine...

Sea Angel · Posted: Wed Jul 04, 2007 4:00 pm Post subject:

Just a silly thought.

How close are the trims and how abt the motor mounting's height on the transoms?

thataway · Posted: Wed Jul 04, 2007 4:30 pm Post subject:

There can be multiple causes--at times counter rotating engines will use more fuel (depending on how the direction is changed --reverse rotation vs difference in lower unit gears).

The turbine sensors can be off by 5% +/- easily. The balance of the boat can be different. Tanks can hold different amounts (positon of the vent, orientation of the tank etc).

Tachometers are often off a few percent.i The throttle handles position will change with time and wear--they can be re-adjusted fairly easily at the engine.

At high speeds it is common for paddle wheels to be off--that is the beauty of GPS...

Lets look at this differently--160 miles and an average of 40 gallons (4 miles a gallon). The difference from the mean is only 2 gallons--5%. This is certainly within reason.

Yes, trim, both of individual motors and the boat (list) will make a difference in the fuel consumption. Although I have not checked the Tom Cats, there are an amazing number of boats which do not have symetrical hulls...

drjohn71a · Posted: Thu Jul 05, 2007 12:04 am Post subject:

Interesting info guys. I wonder if reverse rotation is obtained by adding an extra gear, as in transmissions? Maybe that would throw the numbers off a bit.

Whatever, I do have to say that my experience with the TomCat down here on the Gulf is that it is wonderfully designed for this environment. We are using almost half the fuel of comparable boats running out and back in the same weather.

John

Dora~Jean · Posted: Thu Jul 05, 2007 2:15 am Post subject:

I hate to hijack this thread cause you're talking about the 255, but I have another gas mileage data point on my CD25 with twin Yamaha 80's. Finally filled the tank after the Delta Extravaganza last May, took 60.0 gallons with a GPS total moving mileage of 162.2 on that trip. Varied conditions from rough to smooth throughout the trip, 2 people on board, full gas, water and 5 days of provisions, no raft.

I'll take 2.7 nmpg (3.1 smpg) any day with a boat this size!

squidslayer · Posted: Thu Jul 05, 2007 3:01 am Post subject: MPG

Try a 100 mile to the Farallon Islands or Bodega Bay and back from SF in the open ocean and see what MPG you get.....also being almost 2000 pounds lighter traveling inland waters helps too Neutral

drjohn71a · Posted: Thu Jul 05, 2007 9:08 am Post subject:

I agree, Squidslayer. While I early on touted the benefits of lightweight cruising, reality has sunk in and I have the Tom-a-Hawk loaded to the hilt. Bait freezer, drink cooler, food cooler, bottom fishing gear, trolling gear, light tackle, food, water, grill, air conditioner, generator, extra gas for genset, etc..

I know the 25 is a great boat, but with the winds around here lately, I would not have been able to fish much having to go so slow. It is 28 miles from the dock to the Gulf pass, and then into the really rough stuff out to the oil rigs, etc.. The TomCat has been covering the ground at about 37 statute mph until the waves get 4 foot, then 22 mph until 5-6 foot. I don't think I would have had enough time to slug all those miles out at 16-18 mph.

I do have great respect for the unique performance properties of all the C-Dory line.

John

squidslayer · Posted: Thu Jul 05, 2007 11:12 pm Post subject: Trim

I think I might have solved the reasion Starboard engine gets less MPG that the port. Athough I have not field tested it yet....I found that when I measured trim angle and eyeballed both engines even and level to each other..... my Starboard engine according to the trim indicator bars on the tach gauges are two full clicks lower than the port...when the bar indicators are measuring even..... they are not evenly trimed and are at least 2 inches different than the other ...is there a easy adjustment for this???
Thoughts???

Dick