Fuel Consumption Estimate

[alandav123]

Hi all, I am looking at moving my 31 foot by 11 foot beam 4 foot draft MFV Dignity weighing about 12 tonnes from Arbroath to the Thames estuary. It has a Merc OM352 6 cylinder marine diesel engine fitted and runs along at about 6 or 7 knots maybe a tad more. I have never really got to know fuel consumption on her but am now seriously comparing likely fuel costs versus getting her taken down on a low loader then lifted into Thames. Would anyone hassard a likely fuel costs for a trip such as this assuming the engine runs OK and there are no major fuel issues? I have been quoted about 2K to move her plus lifts out and in therefore it would be nice to get some rough idea of likely fuel costs making it easier to make a concrete decision as to best way of getting her down there. Regards Alan

 

[Tranona]

Fuel consumption is a direct function of the amount of power demanded from the engine. You can work it out more accurately by finding how much power your engine produces at cruising revs and then looking up the fuel usage graph. However, this information may not be readily available for your engine, but a good rule of thumb is 1l per hour per 10hp maximum rating. So, if your engine is rated at a maximum of 80hp and good estimate would be 8l per hour (probably very generous!). So averaging 7 knots, 100 miles would use about 115 litres @£1.2 a litre is £140 worth of fuel. Guess passage is about 400 miles so £550. Much cheaper than road, but of course you might not do it in one go so there may be other costs, and take a longer time. Have to assume the boat and engine are reliable and up to the passage .

 

[alandav123]

Thank you so much for your helpful reply.....I should have said that the om352 IS A 120 bhp engine but its apparantly very good on fuel as it does not have to rev that hard....but even as you say with your generous amounts it way cheaper that a lift out , lift in etc etc.....if you had any other thoughs please reply as I have just had a berth in the thames confirmed so Dignity is going down there in July , one way or another.....Regards and thanks..BTW I will need to start sea trialing her very soon indeed....but mechanically she is fine, mainly cosmetic needs doing.....

 

[Tranona]

"good on fuel" is a bit misleading, as the efficiency (amount of power produced for each gram of fuel) varies little from engine to engine. It is the power you are using that determines fuel consumption, and the amount of power needed is in turn determined by what the propeller demands to move the boat.

 

[jerrytug]

Hi, if it's an old MFV, the fuel tanks will contain muck which will definitely block your fuel filters when the swell starts rolling her, so take off the inspection hatches and get the insides of the tanks clean, blow the lines through with compressed air, renew all filters and take plenty of spares.

Got any photos, lots of MFV fans on here..

 

[burgundyben]

It depends how the prop loads the engine.

I would think a boat like that at 6 or 7 knots would do 4mpg.

I reckon you get about 22hp per hour per gallon.

I should think at 6 or 7 knots you'd be pulling about 1/3 of the 120hp, say 40hp, so maybe 2 gallons per hour. Which at 7 knots is 3.5mpg. Oh look, nearly back where I started.

 

[westernman]

At 5 knots (i.e. speed to length ratio of < 1.0) you should need no more than about 12HP. This works out to about 12/20 UK gallons per hour. i.e. 0.12 gallons per mile. i.e. 8 miles per gallon.

At 7 knots you will consume about 3 times the fuel per hour for going just 2 knots faster. I.e. a little more than double the fuel consumption per mile. I.e. a little less than 4 miles per gallon.

 

[Bitterend]

I'd disregard all the conflicting gubbins above and do a test run outside Arbroath But the advice about ensuring clean fuel tanks is paramount. And you should test the engine, fuel and other systems close to home before embarking on a long journey.
There are 2 easy ways to do this:
You could get 3 or 4 20 litre plastic fuel cans mark them off in 2 litre gradations , fill with diesel and stick the fuel line and return in one of them then ensure a gravity feed to the main filter before running the engine over a measured distance on gps and note time taken. You may be surprised at the amount of fuel returned to the container.

My rationale for the above is that its an easy way to check accurately the amount of fuel being used per hour in known conditions without any likelihood of **** blocking the system. I did it on a 60 ft yacht with a 120hp engine because I didn't believe the fuel calculations I was achieving - they were correct and frugal!!!!

Or you could top up the fuel tank, run for about 3-4 hours outside Arbroath, there must be many other things you can check while doing this, fill the tank again and do your own time / distance / consumption calcs.

Better you know what is happening practically than trying to get a theoretical result which isn't so simple as you can see from the above.

 

[Davegriff]

Can't think of anything much more misleading about boating than refering to miles per gallon. Gallons (or litres) per hour is the only reliable consumption figure to be considered and is a direct computation between boat shape, weight, LWL, engine hp and prop efficiency.

Any 'miles per gallon' figure would need to include the effects of tide, wind and sea conditions - virtually impossible to compute in reality.

 

[westernman]

Can't think of anything much more misleading about boating than refering to miles per gallon. Gallons (or litres) per hour is the only reliable consumption figure to be considered and is a direct computation between boat shape, weight, LWL, engine hp and prop efficiency.

Any 'miles per gallon' figure would need to include the effects of tide, wind and sea conditions - virtually impossible to compute in reality.

Gallons per hour depends on speed (an awful lot) !!!!!!

 

[Davegriff]

Gallons per hour depends on speed (an awful lot) !!!!!!

True, and if the OP had a planing boat I'd wholeheartedly agree. But he doesn't, hence the 'shape, weight, LWL, engine hp and prop efficiency' formula. Sort of limits speed somewhat, eh?

And 5 knots against a 2 knot tide is way different to 5 knots against a 4 knot tide, mpg-wise....

 

[Spi D]

FWIW we used 0.98 liters per nm in a 10,5 metre 10.5 t steel motorboat, powered by a Yanmar 4-cyl 75 hp, over 95 nm cruise at some 6-7 knots. And that includes running the Eber heater 7 nights and a number of manouvres approching berth/lock/bridge. All done in the Netherlands (Friesland - HIGHLY recommended, btw.)

 

[westernman]

FWIW we used 0.98 liters per nm in a 10,5 metre 10.5 t steel motorboat, powered by a Yanmar 4-cyl 75 hp, over 95 nm cruise at some 6-7 knots. And that includes running the Eber heater 7 nights and a number of manouvres approching berth/lock/bridge. All done in the Netherlands (Friesland - HIGHLY recommended, btw.)

At 5 knots you would have used half that.

 

[Spi D]

Not so sure. Judged by the shape of the boat, lwl and the little power needed to move it through calm waters.

Did a lot of <5 kn some days on narrow canals and 'faster' cruise over the lakes withut noting much difference.