Low fuel economy on my Marlin

[Seacurity]

I have done a search on the subject of fuel economy as well as checked Grady Whites site and noticed that the fuel economy numbers I am seeing are not in line with mine on my boat. Even in the best sea conditions with moderate loads the best fuel economy readings I have ever seen is 1.1 MPG. Grady's website shows the Marlin reaching 29.5MPH at 4000RPMS, in order for me to cruise at that speed my engines would have to run at around 4700RPMs. I have my GPS linked to my gauges so speed is coming from my GPS as opposed to the motors sensors.

I usually run each engine off seperate fuel tanks and noticed that when I top off my tanks after running low my AUX tank which is feeding my port engine will always take 15 to 20 gallons more than my main tank. Anyone know why one engine would burn more then the other?

I have already tried changing out the spark plugs but still have not improved my economy.

My boat is a 2006 and has twin F250's.

 

[ocnslr]

The performace tests done by Grady (as with most/all) other manufacturers are run with completely clean, unpainted bottoms, with light to moderate loads.

You don't indicate if your boat has bottom paint, how clean her bottom may be, and what loading condition you usually operate at.

Your performance numbers of 4700RPM and 29.5mph indicate that you are either running the expected 17" props and experiencing >20% slip, or you might be running 15" pitch props, which I think would be underpropped.

So, how about some more info:
- painted or not?
- how clean?
- which model and size of props?
- WOT RPM and speed? (with engines trimmed up but not ventilating).
- how much load (fuel, water, gear, people)?

Brian

 

[Seacurity]

Brian,

The bottom of my boat is painted and clean. I usually run 1/2 to full on the gas tanks, full on water (15 to 20 gallons) and 3 to 4 people. At the moment I'm not sure about the model and size of my props or my WOT RPMs and speed. I am going down to the boat tomorrow and will get the additional info.

Thanks

Joe

 

[jehines3]

My marlin runs 4200RPM cruise at 24.5knts and burns 12-13gph per motor or 24-26gph. Brand new bottom this season though. I'm right in the 1.0mpg on my old carbed two strokes spinning 17pitch wheels, so yes your mileage sounds way off. jh

 

[Southern Hunter]

MPG

My 305 Express w/F250's gets 1.1-1.2mpg fully loaded.......your boat is 600-800lbs lighter than mine, so something is definitely not right!

 

[Grog]

Something isn't right. Are you trimming the motors up for best results? Do the props look OK or are there dings in them? How is the top end? What props are you running?

Seeing your fuel usage is 20% different between engines, have you checked/cleaned the O2 sensors recently.

 

[ocnslr]

.. have you checked/cleaned the O2 sensors recently.

On F250s ???

 

[Seacurity]

I went down to the boat this afternoon but it was too windy and rough in the bay to run it WOT and feel I could get accurate readings on the WOT and MPH. I did take note that I have 15 1/4 - 19" Stainless Saltwater Series props though. If conditions are good tomorrow I will try again. I also checked the props for dings and to my eyes they look clean.

 

[Tuna Man]

I have the identical boat, 2006 Marlin with 250 four stroke Yamahas. I am also swinging 15.25 x 19" Yamaha Saltwater Series II props. I can spin them up to 6000 lightly loaded, and yes my boat is painted. Fully loaded the engines spin up to about 5800 typically.

Your numbers are off quite a bit from mine. I will post mine later tonight if I get a chance. If memory serves me correctly, in perfect conditions I have seen 1.5mpg in calm water lightly loaded. With full fuel, ice, bait, all the offshore gear I own, six large guys, food, drinks, etc. I still get 1.1mpg or better. The worst I have seen yet was into a decent head sea fully loaded, I intentionally 'buried the nose' and the mpg never dropped below 1.0mpg. This was will full tabs and the motors tucked all the way in at around 30 knots.

As you will see when I post later, my numbers are not quite as good as the factory test (Yamaha or Grady's), but my boat is painted and has more wind resistance - 24' Lee Pro (Senior) outriggers, 3.5' open array radar, etc.

 

[fishie1]

I found that when I pulled off all my gear, took down the outriggers and removed all canvas I came very close to the Grady test boat's performance. That was at the end of last season and with multiple coats of bottom paint on my 02 Marlin with F225s. In season and loaded for bear I average about 1.3 - 1.4 mpg at anywhere from 4200 to 4800 RPMs when sea conditions are good. That is with 15 1/4" x 17 props.

With F250s on your 06 I would have expected better performance than what I get so something is not right with your setup.

 

[Tuna Man]

Here are my results:
75 degree air with 70% humidity. Water temp. 60 degrees. Seas less than one foot in Barnegat Bay. Bottom painted with Petit Ultima ACP. All curtains in place, 24' outriggers, 3.5' open array radar, antennas, etc. Normal gear on board including safety stuff, light tackle, tools, water tank half full, two adults, sixty gallons of fuel.

2006 Marlin with 2006 Yamaha 250F motors with approximately sixty hours swinging 15-1/4" x 19" Yamaha SWII stainless props.
RPM SPEEDO-MPH GPS-KNOTS MPG TRIM GPH TABS
600 3 3.2 2.2 2 1.3 0
1000 5 4.9 2.3 2 2.4 0
1500 8 6.7 2.1 2 3.8 0
2000 10 7.7 1.8 3 5.7 0
2500 11 8.6 1.4 2 8.2 100
3000 15 13.7 1.4 3 10.6 100
3500 21 18 1.5 4 13.9 70
4000 30 24.6 1.6 6 18.8 20
4500 34 29.2 1.4 7 24 0
5000 38 32.8 1.3 7 28.3 0
5500 44 36.8 1.2 7 35.5 0
6000 46 38.8 1 8 42.3 0

*3700 28 22.8 1.7 5 16.4 0
*4200 31 26.1 1.6 5 18.7 20 stbd
*6000 47 39.2 1.1 8 42.2 0

Numbers with asterisks were the best I could get when averaged in both directions. Please note that the speedo readings are NOT gps.

 

[Seacurity]

Thanks for the very detailed post Tuna Man, I hope that this weekend I could run the boat at WOT and see what my numbers are. I'll post the details when I do and hopefully I could nail down what my problem is.

 

[seasick]

It seems that the issue here is not just fuel performance but rather why there appears to be a big difference between the two engines. Assuming the method to estimate fuel usage is correct, there appears to be a large difference bewteen engine economy. What I need to know is what is the difference. 15 or 20 gallons difference consumption over what total consumption per tank> If you are burning 40 gal on one and 60 on the other that's a big difference. If it;s 20 gals over 150 gallons, there could be simple explanations, as simple as synchron1sation issues or general motor tune. perhaps even prop differences.
So tell us the total numbers for a trip by tank. When you state how much fuel is used, do you use the gauges to estimate a full tank or do you fill up until the flow stops?

 

[Seacurity]

Seasick, I see the 15 to 20 gallon difference in fuel consumption after burning most of the fuel in my tanks (100 - 120 gallons per tank) at the fuel pump when I refill until it stops.. The last 3 times I topped off my tanks the Aux tank which feeds the port engine always takes 15 to 20 gallons more than the main tank which I have set to feed the starboard engine.

I should have WOT speed and RPMs this weekend if weather permits.

 

[ocnslr]

With regard to the fuel burn difference between port and starboard.

Don't you have Yamaha fuel management installed? If so, you should be able to look at the GPH of each engine separately, as well as the total.

Just get on a steady run, then alternate looking at the port and stbd GPH and see if you can tell a difference.

You may or may not be able to see the difference.

 

[seasick]

Seasick, I see the 15 to 20 gallon difference in fuel consumption after burning most of the fuel in my tanks (100 - 120 gallons per tank) at the fuel pump when I refill until it stops.. The last 3 times I topped off my tanks the Aux tank which feeds the port engine always takes 15 to 20 gallons more than the main tank which I have set to feed the starboard engine.

I should have WOT speed and RPMs this weekend if weather permits.

Based on the consumption, something is definitel amiss. You should not have a 15% to 20% difference in fuel burn rate. Have diagnostics been run on the suspect engine? Something is causing that motor to run a bit rich.

 

[BobP]

Unless I missed it, did you say the boat is burning that much more fuel than it had in the past under near identical conditions for sea, wind, and load?

 

[Grog]

.. have you checked/cleaned the O2 sensors recently.

On F250s ???

They're somewhat new but worth a check.

 

[Gary M]

2001 Marlin with EFI 250s

1) 1.3 Nmpg at 4,000 and 24 knots VERY light load, timmed up.....
2) 1.0-1.1 Nmpg at 4,200 and 24 knots at a heavy/full load, trimmed up..