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Diesel consumption when in neutral

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I worked on a 109' sports fisher uite a few years back.  Went from Auckland to Opua at about 25kts.  Used about 5 tonnes of fuel - it took longer to fill up than it did to get there....

I would use about 60l for the same trip.  Apples and oranges.

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Nephew brought a 28ft ply launch,150hp detroit. 12knots full noise 15lt ph   8 knot cruise 10lt,only had it 2 weeks and sold it. now he has a 1850 stabicraft 115hp etch outboard thing,cruise at 30knots 10lts ph or 45knots 12lts ph.

cover big distance in a short time but hes in to gamefishing . Not for me thump thump thump all day and dragging lures around 6hrs must be the most boring thing I have ever done on a boat.

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Your diesel engine is governed - you set the RPM and the governor controls the fuel to match.

Your fuel use at idle ("no load") without the alternator charging, represents some sort of base friction in the system - the amount of energy needed to keep the engine turning over. This "wasted" energy is probably proportional to engine speed. Double the engine speed in neutral, you could expect your fuel consumption to double.

As you add load, the revs start to fall, the governor responds with more fuel, the revs rise back to the set level.

As you remove load, the revs start to rise, the governor responds with less fuel, the revs fall back to the set level.

This happens really fast - you may not even notice it apart from a change in the noise.

So when you turn on your alternator, this adds load and the engine will use more fuel. The efficiency of this extra conversion (thermal to mechanical to electrical) has a theoretical maximum around 70%, but is probably far less. The actual load  will depend on the engine speed, your alternator, battery and state of charge. And the efficiency will depend on the engine speed and the alternator.

An engine output, fuel consumption graph like this one: https://betamarine.co.uk/wp-content/uploads/2020/07/B20-HE-Power-Graph.pdf assumes that you are actually driving a propellor and boat. If you run your engine at max revs in neutral you will not use 5L of fuel per hour. If you run your engine at low revs under a large load, you will not use 0.5L of fuel an hour.


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On 26/02/2021 at 11:13 AM, wheels said:

It was a long time back and the exact figures would be a bit fuzzy now. I may have the 27kts part wrong.
His was a Bayliner 3388.
Mine has the Perkins 6.354 engine. Usual consumption was around 10lt/hr if I remained conservative with speed. But that figure would climb fast if I pushed it.

The Bayliner is 33 foot and displaces 7 tons, versus your girl at ????

This for sale listing has a Bayline with twin cummins 155 hp diesels using 28L of diesel at cruising speed of 10kt and gives a max speed of 15kt.


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2 hours ago, Tamure said:

When you start talking fuel burn in weight, you have now entered the big leagues! Was that the sport fisher that YDL had for a while?

No idea, the details have been rinsed from my brain....  Possibly there was more than one though.

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I used to sail on a big Salthouse Motorsailor 74ft - big Mann engine (I think 250hp ish plus 2x Northern Lights gensets etc). They had filled up Europe before leaving and only needed to refuel in NZ after 18 months so we filled up in Opua. We took on 5 tons too. Beforehand I thought I should sign up for the loyalty scheme for whichever brand it was (Shell at the time I think). But I calculated even 5 tons of diesel would only entitle me to get a toaster or a set of 6 wine glasses. I didn’t bother.

Another tune I was on a Palmer Johnson in Sydney Hbr when we had to refuel after 12 months. Cummins 350hp. Was about 7 tons this time. They bought the fuel barge out to us at anchor in Rushcutters. I didn’t even bother to look up the loyalty program.

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