Fuel consumption:Leanburn O/B Vs. Diesel.

rustybarge

rustybarge

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Hi All,

The latest leanburn outboards boast 20% better fuel burn so how do they stack up against equivalent diesel engined boats?

This weeks motorboat owner has the Finnmaster 8 on trail with a Suzuki 300hp leanburn/ 2.5 tons/26' which has a wot speed of 40kts at 22.6gsl/hr and a sweet spot at 24kts/9gals/2.7mpg

Mbm from February 2012 tests the Aquadour 23ht in a tightly controlled test against 3 other diesel engines: 250hp/2.4 tons/24' which has a wot speed of 40 kts exactly at 13gals/hr and a sweet spot at 25kts/5.5gals/4.6mpg

Although the diesel Aquadour is 2' shorter it is a perfect match in 'Wot speed' and 'cruise speed' to the Finnmaster 8 .

wot: 22gals Vs. 13gals at 40kts.
cruise: 9gals Vs. 5.5 gals at 25 kts.

Surprised...?
 
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Not really.

The petrol uses nearly twice as much fuel. And this is the new efficient ones. Imagine how much more a non lean burn motor uses.

So at a cruise the Aquador gets through £20.22 per hour while the lean burn finnmaster gets through £46.98 per hour.

Fuel cost based on MDL berth holder rates last weekend
 
Whitelighter said:
Not really.

The petrol uses nearly twice as much fuel. And this is the new efficient ones. Imagine how much more a non lean burn motor uses.

So at a cruise the Aquador gets through £20.22 per hour while the lean burn finnmaster gets through £46.98 per hour.

Fuel cost based on MDL berth holder rates last weekend
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The Mbm tests where done with specially installed fuel meters which were very accurate, and the Suzuki figures came from the digital instruments which are presumably equally accurate.

sobering!

I was influenced by the advertising hype on the leanburn. In fairness the diesel boat is going to be heavier, and in the above examples the Aquadour is a couple of feet shorter for the same gross weight.

....taking everything into account outboards have a very long way to catch up on consumtion.
 
Just looking at the dipl. Speeds...

suzuki: 6.7 kts/2.3gal/2000revs..........2.9mpg (but at 7.6kts/4gal/2500....,1.9mpg!!!)
Cummins 265hp: 7kts/1.5gal/1500revs.......4.5mpg.


interestingly enough the diesel has a nearly flat fuel consumption graph at 4.5 mpg (except for the hump at 9-15kts)

so which ever way you cut it, it looks like the o/b uses 50% extra at 7kts, 65% more at best cruise of 25kts and 70% more at wot of 40kts.

It starts to get complicated when you take in consideration the purchase costs?
 
rustybarge said:
It starts to get complicated when you take in consideration the purchase costs?
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.. and the longer service intervals, and the lack of a sterndrive to service and repair, and the 5 year warranty you get with most outboards, and the ease of replacement, and the lack of prop fouling because they lift completely out the water. Unless I was doing lots of hours I'd choose the outboard every time.

I have an etec 300 on a 24' sportboat, admittedly a bit lighter at maybe 2,000 kg, and I can get 3.5 mpg at best cruise. That's still not close to the Aquador figures you quote, but a bit better than the Finnmaster/Suzuki.

I also remember seeing a graph that suggested that the Suzuki 300 isn't actually very fuel efficient, the ETEC is better and Yam 4.2L is best by about 15%, so maybe with an optimum set up you could get closer to 4 mpg with an outboard.
 
Nick_H said:
.. and the longer service intervals, and the lack of a sterndrive to service and repair, and the 5 year warranty you get with most outboards, and the ease of replacement, and the lack of prop fouling because they lift completely out the water. Unless I was doing lots of hours I'd choose the outboard every time.

I have an etec 300 on a 24' sportboat, admittedly a bit lighter at maybe 2,000 kg, and I can get 3.5 mpg at best cruise. That's still not close to the Aquador figures you quote, but a bit better than the Finnmaster/Suzuki.

I also remember seeing a graph that suggested that the Suzuki 300 isn't actually very fuel efficient, the ETEC is better and Yam 4.2L is best by about 15%, so maybe with an optimum set up you could get closer to 4 mpg with an outboard.
Click to expand...

I agree, the larger capacity OB's running at less revs are going to be much more efficient than the smaller Suzuki.

3.5 mpg is very respectable in a 2 ton boat, and when you take into account the advantages of an OB as you have outlined above it could tip the balance in favour of the OB's over a longer period of ownership.

It's weird that Volvo et al only offer a 12 months guarantee as standard, and yet Honda offer a 6 year g'tee on their bigger outboards.
 
Diesel has a higher calorific value, so should in theory always be better mpg. The health and environmental impact of diesel is currently very understated however and that may change - if it does then higher tax differential will follow. I'm asthmatic and drove diesel trucks/vans for years, but since dispensing with van+car in favour of a petrol/lpg 4x4 condition of lungs has improved significantly. Anyone who drives a diesel vehicle will be aware of the black film everywhere and your lungs will be similarly coated - this is carcinogenic! Petrol is much cleaner than diesel and LPG much cleaner than petrol. Not sure I'd want LPG powered boat for other reasons, but on the emissions alone I can see a higher % of duty on diesel in the not-so-distant future...
 
rbcoomer said:
Diesel has a higher calorific value, so should in theory always be better mpg. The health and environmental impact of diesel is currently very understated however and that may change - if it does then higher tax differential will follow. I'm asthmatic and drove diesel trucks/vans for years, but since dispensing with van+car in favour of a petrol/lpg 4x4 condition of lungs has improved significantly. Anyone who drives a diesel vehicle will be aware of the black film everywhere and your lungs will be similarly coated - this is carcinogenic! Petrol is much cleaner than diesel and LPG much cleaner than petrol. Not sure I'd want LPG powered boat for other reasons, but on the emissions alone I can see a higher % of duty on diesel in the not-so-distant future...
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There is a company in Holland that Convert Honda 250hp outboards to LPG.

http://www.cheetahmarine.co.uk/en/deliveries/lowland_success_as_cheetahs_charge_the_continent
 
rbcoomer said:
Diesel has a higher calorific value, so should in theory always be better mpg. The health and environmental impact of diesel is currently very understated however and that may change - if it does then higher tax differential will follow. I'm asthmatic and drove diesel trucks/vans for years, but since dispensing with van+car in favour of a petrol/lpg 4x4 condition of lungs has improved significantly. Anyone who drives a diesel vehicle will be aware of the black film everywhere and your lungs will be similarly coated - this is carcinogenic! Petrol is much cleaner than diesel and LPG much cleaner than petrol. Not sure I'd want LPG powered boat for other reasons, but on the emissions alone I can see a higher % of duty on diesel in the not-so-distant future...
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Robin, I've driven diesel cars for about 20 years, have never seen the black film you describe. Is it possible your Diesel engines were not tuned correctly?
 
..... to add to Nick's list.

The subsidised price of UK marine diesel seems unlikely to remain as low for too much longer. Other EU members seem intent on disrupting our arrangement.

In an outboard powered boat, there is no need for an engine room. More space left inside the boat for accom.

If the outboards are more reliable and have longer service intervals, I can spend less time trying to get work done by service/fixing people.

I think that outboards are cheaper to purchase than marine diesel engines with a stern drive.

Outboards boats are easy to repower after a few years.

Running a petrol outboard at tick over, say to power the inverter to use the toaster whilst at anchor, is a bit less invasive than running a diesel engine.

The costs are certainly in favour of the diesel inboard if one only considers fuel consumption and the current price of fuel.

I think I started to believe that the considerations may be more numerous when I noticed some commercial fishermen using petrol outboards every day for their fishing on the south coast of the UK and in Northern France.

Garold
 
Not sure I would be keen on lugging a couple of hundred litres of petrol about in 25l containers, I think there are rules about buying and carrying large quantities of petrol. As you say though, commercial guys do it. They get the tax back on petrol which makes it much cheaper though.
 
longjohnsilver said:
Robin, I've driven diesel cars for about 20 years, have never seen the black film you describe. Is it possible your Diesel engines were not tuned correctly?
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Hi John, cars tend to have better seals on doors etc, but the particles are still produced. Modern diesels are better and many cars have particulate filters, but diesel is still a dirty fuel when looked at alongside others. A quick search of the web for 'diesel particulates' or 'diesel particulate matter' (DPM) will reveal all sorts of research and evidence. It can surely only be a matter of time before we (the consumer) end up paying for this via taxes.

As usual, governments tax us to make us change our ways, then realise they got it wrong and tax us more to revert back to where we started... Cynical perhaps, but true! :rolleyes: With the EU in control and against the duty relief we have in the UK, unless we get out then I suspect there's a price adjustment coming. Looking at how the 'general public' look through rose tinted specs at things like Studland/SHT/MCZs, I can't imagine much public outcry at duty rises for 'rich yachties' either?
 
rbcoomer said:
Interesting link that - thanks :) I run the car on LPG and Torquay is one of the few places that sell LPG quayside, so I have thought about it for the Arrowbolt rebuild (5L V8) - just not quite sure I'd be comfortable with LPG in a boat!
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They say a half empty petrol tank is a bomb waiting to go off; so maybe a tank of LPG would be no worse.
you don't often hear of cars exploding like they do on the movies.
 
rustybarge said:
They say a half empty petrol tank is a bomb waiting to go off; so maybe a tank of LPG would be no worse.
you don't often hear of cars exploding like they do on the movies.
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True, but the difference is the the vapour in the petrol tank isn't under 4.5 bar of pressure - big difference to the rate of escape if you get a leak... :ambivalence:
 
I had it, and apart from some issues with the quality of the workmanship, never worried about safety.

As long as there is a blower running in the LPG storage area, and you run the usual bilge blower for a while before starting you'll be fine.

Of more concern is that salt-water running through a vapouriser will wreck it - so you are better with a freshwater cooled engine if poss
 
ontheplane said:
I had it, and apart from some issues with the quality of the workmanship, never worried about safety.

As long as there is a blower running in the LPG storage area, and you run the usual bilge blower for a while before starting you'll be fine.

Of more concern is that salt-water running through a vapouriser will wreck it - so you are better with a freshwater cooled engine if poss
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How did it save in fuel costs, and was it worth the installation cost?
 
Brilliant and rubbish in that order.....

Actually truth be told it wasn't as simple as that....

I had a Mercruiser 5.7V8

It burned around 18gal / hr at WOT

When the LPG was first installed, it was buring 25 gal / hr at WOT so would never have saved much money - however I refused to let that one go, and demanded tests and so forth or I wanted my money back due to false advertising.

After much research, we got the gap down to about 18 gal / hr on petrol and 22 gal / hr LPG which would have been better.

I also got a big refund which helped!!

But then I found out that the data for the timing on the Mercrusier was American and based on using 92 octane petrol - which meant that the ignition settings were for that.

LPG has an octane of over 100 and so it wasn't running anywhere near as well as it could have.

By advancing the timing well over the max in the manual we got the LPG engine to run at about 20GPH at WOT which compared well to the 18 of the petrol.

Back then, Petrol was 75p per litre on the Marina, and LPG was 38p - so the boat went from costing £62 per hr to around £34 per hr at WOT - and a fair bit less at cruise - so at the original conversion cost of £3500 it wouldn't have been worth it - but after I got £1500 back - yes it was worth it.

I note that several years later the same boat came up for sale on eBay and the owners had removed the LPG purely as they didn't have it locally to them so wanted the space back in the mid-cabin.

If you can get a car system fitted and can actively expel the vapours in the bilge, then I think you'd do fine - I would suspect a simple vapour LPG system to cost around £300 + fitting and you could fill up at any Autogas forecourt which would save a lot of money.

You will need to be trailing the boat though - anything marina based wouldn't work
 
ontheplane said:
Brilliant and rubbish in that order.....

Actually truth be told it wasn't as simple as that....

I had a Mercruiser 5.7V8

It burned around 18gal / hr at WOT

When the LPG was first installed, it was buring 25 gal / hr at WOT so would never have saved much money - however I refused to let that one go, and demanded tests and so forth or I wanted my money back due to false advertising.

After much research, we got the gap down to about 18 gal / hr on petrol and 22 gal / hr LPG which would have been better.

I also got a big refund which helped!!

But then I found out that the data for the timing on the Mercrusier was American and based on using 92 octane petrol - which meant that the ignition settings were for that.

LPG has an octane of over 100 and so it wasn't running anywhere near as well as it could have.

By advancing the timing well over the max in the manual we got the LPG engine to run at about 20GPH at WOT which compared well to the 18 of the petrol.

Back then, Petrol was 75p per litre on the Marina, and LPG was 38p - so the boat went from costing £62 per hr to around £34 per hr at WOT - and a fair bit less at cruise - so at the original conversion cost of £3500 it wouldn't have been worth it - but after I got £1500 back - yes it was worth it.

I note that several years later the same boat came up for sale on eBay and the owners had removed the LPG purely as they didn't have it locally to them so wanted the space back in the mid-cabin.

If you can get a car system fitted and can actively expel the vapours in the bilge, then I think you'd do fine - I would suspect a simple vapour LPG system to cost around £300 + fitting and you could fill up at any Autogas forecourt which would save a lot of money.

You will need to be trailing the boat though - anything marina based wouldn't work
Click to expand...

like everything to do with boats there's never a clear solution; just a bunch of compromises! ;)
 
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