Fuel consumption

[Grehan]

I've worked out that - on average, over the last few months - Grehan has used about 2 litres of diesel for every 1 hours motoring or motorsailing. Grehan's 11m and 11t and her engine's a 35hp Yanmar.

Is this average/expected?

I don't recall this kind of information published anywhere - ??
Obviously a bit anachronistic ref yachts, but useful nonetheless.
I know consumption's going to vary according to conditions/use, just like cars' do.

 

[andrewhopkins]

You're right that this kind of thing doesnt get looked at but the figures are interesting.

At 25p per litre and assuming you travel around 6nm in an hour, you are probably doing about 30 land miles for a £1.

My car (which is thirsty and runs on petrol) costs about £45 to fill and does 450 miles to a tankful which equates to 10 miles for every £1. so the boat goes 3 x further for the same money.

Bearing in mind that I pay 75p per litre for my petrol which is 3 x more expensive then they are about the same in terms of economy.

Or is my maths flawed ????

 

[kimhollamby]

At absolute wide open throttle, assuming that your yacht has a propeller that allows the Yanmar to develop full power, you'd expect a fuel burn of just over 7lt/hr. At a reasonable cruising speed but not pushing beyond your yacht's waterline speed something under half of that. Given that you motorsail for some of the time (when engine loading/fuel burn less) and perhaps also battery charge and/or potter out from mooring (when again fuel burn minimal that starts to sound about right.

The golden rule for modern four stroke diesels is around 4.5gal/hr/100hp at wide open throttle if developing full power.

kim_hollamby@ipcmedia.com

 

[Guest]

The general (as opposed to 'golden') rule is that a modern four-stroke diesel engine will consume approximately 200 grams of fuel per horse power per hour.

At 2 litres per hour it means you are using about 30% of the Yanmar's available horsepower.

One thing to be aware of is that if you do whack the throttle open in an emergency expect:

o Fuel consumption to increase by 300%.

o Speed to increase by less than 50%.

Trying to get somewhere in a hurry can mean a sudden shortage of fuel well before you get there!!

Best regards )

Ian D

 

[NigeCh]

Yanmar 1GM10 - 0.75 litres/hour at 75% -it doesn't seem to vary whether through 50% to 100% (Prop 2 blade Euro 13 x 12) .... but it would be nice to see how other other engines compare.

 

[Chris_Stannard]

My boat, 8.5 tons plus about a ton for HWMBO's necessary items of stores for the next two world wars, has a 53 hp Yanmar. Overall 37 feet and 32 feet on the waterline. I have kept a check and find that at 2500rpm I am doing about 6.3 knots and my fuel consumption is 2.5 litres per hour, and at about 3200 rpm, boat speed about 7.25 knots, it is about 3.3 litres per hour. Flat out the engine runs at 3800rpm and the boat does about 8.2 knots but as this is limited to 30 minutes max, I have never measured the consumption. Based on the figures given by KH I guess that the flat consumption should be about 10 litres per hour ( approx 20 litres or 4.5 gallons for 100 hp), so I am doing about 30% full power at 3200rpm

Other point of interest is that on 100 litres of fuel I do 260 miles at 2500rpm and 220 miles at 3200 rpm. I guess that if I pushed it up to about 3500 rpm (80%) full power which is the recommended power setting for diesel I would be using 8 litres per hour for about 7.8 knots which would give me about 100 miles for my 100 litres.

My distant Cornish ancestry rushes to the fore and I shall stay at economical speed.

Chris Stannard

 

[Twister_Ken]

Nige, Indigo's 10 hp Bukh seems to do about a half litre per hour at 'cruising' revs. As I've not got a rev counter I've no idea what that means in terms of engine speed, and as I pull in the trailing log when motoring (expensive not to) , I don't really know what the boat speed is either.

 

[Twister_Ken]

Is that 200 gms at full whack?

If so, does it equate to 100 gms at half whack, or aren't these things linear?

 

[Chris_Robb]

I have a 40 ft Ketch 13 tons with a Perkins 4236 75HP. Over the last 2 years the fuels consumtion has averaged out at about 4 ltr per hour.

This is running at an average of 1700 rpm, in all conditions, ging about 7.5knt in flat water and 6.5 into a heavy sea and 30knots over the deck.

I have noticed though that the difference in consumption when heading into a strong wind at the same revs- is quite significant.

Last year we had to motor from Chichester to Brest and motored the whole way into 30 -35 knots over the deck. Each tank holds 60 gallons (273 lt), and I had expected to use about 40 galls (182 Lt) On leaving Brest the engine died, the tank was dry, so I had used nearly 7 ltrs per hour on that journey.

Don't be fooled by assuming the same revs will give a constant fuel consumtion - the fule pump will supply the fuel to maintain the revs with the increased work load on the prop

 

[Guest]

Sorry for the delay.

It's virtually linear at 200 grams per horse power per hour.

You need to check the power curve for the engine.

An engine that delivers 20HP at 100% revs will be eating 20 x 200grms per hour (i.e. just over 4 litres per hour) at full whack.

When running at whatever revs equal 50% power (i.e. 10HP) the consumption will drop to just over 2 litres per hour.

The reason it is "just over" is that the specific gravity of diesel is slightly less than water.

i.e.
1,000 grams (weight) of water equals one litre (volume).
1,000 grams (weight) of diesel equals about 1.15 litres (volume).

It has to be remembered that the 200 grams is only "rule of thumb" and as a general rule consumption will be higher as a result of poor tuning, worn piston rings, carbon deposits, injector wear etc. etc etc.

On the subject of diesels - especially for displacement boats - it is generally considered to be much better to have a smaller HP engine running at 60 to 70% of full throttle than a large HP engine ticking over for long periods at 20% or so.

Diesel engines don't like running at "tickover" for any length of time as it tends to breed carbon deposits in the cylinders and exhaust!!

If you have a big engine and only need to run at low revs then you should get into he habit of opening her up every so often to blow the soot out.

Hope this helps.

Best regards )

Ian D

 

[Guest]

Chris

You are correct - and many people mistakenly relate "rpm" to "horsepower" and forget the "torque" requirement.

However, one thing that was in error was the statement "the fuel pump will supply the fuel to maintain the revs with the increased work load on the prop".

This is not strictly true - unless you have a speed controller fitted to the engine!!

What happens when my diesel van comes to climb a hill is that I "put my foot down". The engine uses more fuel to deliver more horsepower to allow the van to climb the hill at the same speed. The revs stay the same - but I am using more fuel - as I need more horsepower.

On your trip to Brest (assuming that you don't have a speed controller fitted) what happened was that YOU adjusted the throttle to maintain the 1700 rpm - against an increased workload.

All the fuel pump did was to respond to your instructions and deliver the higher horsepower required to maintain the requested 1700 rpm.

However if you read my original reply it says that fuel consumption is more or less linear for HORSEPOWER - not revs per minute.

Best regards )

Ian D

 

[davidhand]

For what it's worth motor sailing my Beneteau First 375 from San Diego to San Francisco I burned almost exactly 1/2 USgal. per hr. (abt 2 litres/hr I think). I was at about 2200 rpm and have a Volvo 2003 28HP, boat is 17 tons gr.

 

[Chris_Robb]

Ian - I'm not sure that I agree with you on this. The Perkins fuel injector pump is governed by revs, therefore I can start off in flat calm at 1700 rmp and still be running at 1700 with 30 knots accross the deck without touching the contrals

 

[Chris_Robb]

US Galls - the colonies

Please don't confuse me even more! Imperial or Litres will do please. When will the colonies come into line!

 

[dickh]

Perkins Perama M20(same as VP 2020) - ¾litre / hour at 2000 RPM, 5 knots in calm water, Jaguar 27, prop 15x8; at max 3200 RPM = 6½knots - but don't know consumption as I rarely run it continueously above 2700

dickh
I'd rather be sailing...

 

[davidhand]

Re: US Galls - the colonies

I've been trying to tell them this for years, but fact is there are more people speaking American English than Brit. English,
and are imperial gallons still used officially? The Brits. now use metric, was the battle of Waterloo in vain? But I agree back to pounds shillings and pence, twenty ounce pints of beer, and buy your petrol in big gallons, and f*** the bureaucrats in Brussels.
I remember a quote from Margaret Thatcher, "in my lifetime most of our problems have come from continental Europe, and most of our solutions have come from the English speaking world.

 

[Twister_Ken]

Heavy man!

!7 tons for a Benny 375?

They left the builders weighing about 7 tons

 

[charles_reed]

Yanmar2GM20, mean about 1l/hr, with lots of motorsailing with an Autoprop.

Mastless (whereby hangs a tale) at 75% revs 6.8knots and 2.2l/hr.

 

[Guest]

Interesting thread this. Yanmar publish "specific fuel consumption" curves which for the 4JH3-HTE engine (rated 100hp) at 2400rpm (about 33% power) a figure of 154 g/hp.h Is anyone out there able to tell me what that equates to in gallons per hour?