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Old 09-23-2018, 10:27 PM   #1
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Fuel usage last trip

Our last trip we burned 46 gallons of fuel over three hours using both engines. Cummins 6BTA 300 Kept rpm between 18 and 2000. By my old math that works out to about 15 gallons an hour or 7.5 per engine.

What I don't know is if that is normal, above average, not so good....whatever. Of course I would assume that if I speed up my fuel usage would also climb.
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Old 09-23-2018, 10:55 PM   #2
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What was your speed?
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Old 09-23-2018, 11:09 PM   #3
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Try 1500 RPM for the next 3 hour trip. Compare speed and NMPG. Bet you'll be a lot happier at 1500 RPM.
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Old 09-24-2018, 05:24 AM   #4
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Seems you are about where you should

https://www.sbmar.com/docs/performan...M-90208%5D.pdf

SBMAR is a great source for Cummins knowledge, parts, accessories. Not affiliated but impressed with his dedication to marine diesels.
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Old 09-24-2018, 09:19 AM   #5
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We were running between 8 and 9 knots depending on current.
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Old 09-24-2018, 09:41 AM   #6
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Is speed more important or fuel consumption?

At 7 knots I would bet you're under 5 GPH for both engines.
At 8 knots I would bet you're under 10 GPH for both engines.

Those numbers are based on no current. Remember, it takes a lot of HP / fuel to make water stand up (big wake). Is taking another hour on that trip worth saving 25+ gallons of fuel?

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Old 09-24-2018, 10:21 AM   #7
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Quote:
Originally Posted by wwestman View Post
Our last trip we burned 46 gallons of fuel over three hours using both engines. Cummins 6BTA 300 Kept rpm between 18 and 2000. By my old math that works out to about 15 gallons an hour or 7.5 per engine.

What I don't know is if that is normal, above average, not so good....whatever. Of course I would assume that if I speed up my fuel usage would also climb.
In your avatar, your boat is a 45 Hershine Jefferson, so a 45000 lb boat?
Your stated speed of 8 to 9, depending on current, near Bellingham Wa, suggests that you are trying for 8.5 through the water.

In nearby waters, my own 44000 lb 44 footer cruises happily at just over 8 knots, on 4 gph, so 2 mpg. Your figures of 0.56 mpg, over 3 x the fuel burn, suggests that you should slow just a little, for a very significant improvement in fuel economy.

If you do speed up, keep an eye on the size of your wash. every extra inch = higher cost in fuel. Alternatively, keep an eye on the depth of the hole your stern is digging. Same rules of physics apply.
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Old 09-24-2018, 10:32 AM   #8
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Good points Keith and Ted. Then the issue of "is the vessel correctly or over propped" raises its head.
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Old 09-24-2018, 10:54 AM   #9
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Slow down one knot and see.

Trim tabs fully retracted?
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Old 09-24-2018, 12:44 PM   #10
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I go slower

Quote:
Originally Posted by wwestman View Post
Our last trip we burned 46 gallons of fuel over three hours using both engines. Cummins 6BTA 300 Kept rpm between 18 and 2000. By my old math that works out to about 15 gallons an hour or 7.5 per engine.

What I don't know is if that is normal, above average, not so good....whatever. Of course I would assume that if I speed up my fuel usage would also climb.
My OA 456 is fully loaded with about 50k pounds. She is powered by twin 6bta 330s. I run about 1500 travel 7.5 to 8.5 and burn 5 gal/ hour. If i ran 1800 to 2000 i would make a lot more noise, leave a deeper wake, and go a knot faster. Knot worth it to me.
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Old 09-24-2018, 01:06 PM   #11
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Quote:
Originally Posted by Gordon J View Post
My OA 456 is fully loaded with about 50k pounds. She is powered by twin 6bta 330s. I run about 1500 travel 7.5 to 8.5 and burn 5 gal/ hour. If i ran 1800 to 2000 i would make a lot more noise, leave a deeper wake, and go a knot faster. Knot worth it to me.
Agreed, our GB 42 is around 44,000 loaded. With 250 hp twin Cummins 6BTA's it took a while for us to find that 'sweet spot.' Minimum RPM to reach hull speed is around 1,300-1,500. This gives us 3.5 gph burn total. Adding 300 rpm and barely a knot in speed burns 50% more fuel.
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Old 09-24-2018, 01:43 PM   #12
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Whoops, I went back and checked and the 3 hour figure was just the return. Total time was actually 5.75 hrs for the same 46 gallons. So my hourly burn was 8 gallons with both engines or 4 gallons per side.

This is the first time we have done a trip where we did not use the genset or the diesel heater, so I have never been able to do the calculations before (and obviously I suck at it).
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