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Old 21-09-2015, 13:00   #16
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Re: 0.11 litres per nautical mile

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Originally Posted by Lars_L View Post
During this summer I have at several occasions driven in 4.5 knot and have had a diesel consumption of 0.5 l/h. That is 0.11 l/NM. I don’t know what I shall have, but that don’t feels much. Dashew writes in their book Offshore cursing encyclopaedia that on a circumnavigation you should have fuel for at least 700 NM. That would for me be 80 l. My boot isn’t that large (9.0 m long) but 80 l should I be able to store. The motor is a Yanmar YSB 12 from 1978. For some years ago I hade out the motor and adjust the injector and grind the valves. I do also have an Autoprop propeller.

Now there are many things that have broken on the motor, so its time for me to change it. With a new motor, can I hope to get lower fuel consumption? I plane to have a motor with 18 to 20 hp. I shall also have an Autoprop with the new motor. If I go with the same speed with the new motor, than at least it should not take more. But with a stronger motor will I probably go in a higher speed.
I had a 1982 C&C27 with a yanmar 2gmf20 diesel. It used 1 litre per hour, or less. I think your YSB12 is a one lung (single cylinder) so I think your numbers sound right. Enjoy the fuel economy and range.

I expect a new engine will get about the same. Your boat is light and easy to push. If you get a 2 cylinder engine, it will use a bit more fuel.

It takes a lot more fuel to go just a little faster.

Sailing takes no fuel at all.
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Old 21-09-2015, 14:28   #17
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Re: 0.11 litres per nautical mile

Lars, I have nothing to say about the propeller or diesel fuel thing. But Lars would you be so kind as to state what is the realistic sailing season in the Swedish archipelago?

I lived in Russia on and off for a year so I am familiar with Saint Petersburg weather. I am dying to sail up there. You have a marvelous country and the sailing looks just so interesting with locks, canals, rock inlets, and the water is sweet. Further deepen the spell your sailing area has on me...I beg you
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Old 21-09-2015, 14:41   #18
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Re: 0.11 litres per nautical mile

For those curious about the Autoprop, I have one, it's wonderful. In 6 kts of wind I can motorsail with the engine just ticking over just above idle at 6 to 7 kts. Burn almost no fuel, yet double sailing speed and charge batteries
But I predict I will never save enough Diesel to pay for the thing


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Old 21-09-2015, 14:57   #19
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Re: 0.11 litres per nautical mile

I have a feeling that Dashew give numbers with some margin, but the 700 NM is when you should pass the areas without wind near the equator. During this summer when I hade a diesel consumption of 0.11 l/NM have been when I have hade little or no wind. I have hoped that there should have been some development of motors during this 37 years. Common rail aren’t possible on a one cylinders motor. Today I have a 20 l tank, and I think that when I shall pass areas with risk of no wind, I will have some spare bottles (or maybe something bigger) with diesel and I do not change the tank. With every thing I have put in the boot I think she weights 3500 kg. I do not have any instruments on the motor, there is a control lamp for oil pressure, one for motor temperature and one for charging, that all.

alansmith. During this summer I have been sailing along the Norwegian west cost. May was cold, colder than September, but it has not been any problem. I have hade a lot of time, so when it hade blown too much (or rain too much) I have stayed in the harbour. But even if I have been out in five month I did only come up to Trondheim.
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Old 21-09-2015, 15:20   #20
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Re: 0.11 litres per nautical mile

.11 litre / mile @ 4.5 knots = .495 litre / hour. A Google search suggests the YSB 12 is economical @ 1 to 1,1/2 litre / hour.

My 1998 Yanmar 2GM 20 uses 2 / 2,1/2 litre / hour max. I run it at around 2900 / 3000 rpm with some full throttle bursts every hour for a few minutes at 3,400 rpm, if I'm motoring far. I have an easily driven 30' (9.3m) yacht and normal speed is around 6 / 6.5 knots water speed. Mine is a saildrive which made (replacement of a shaft drive ) installation very easy.


Diesel engines like to be worked reasonably hard. Generally as they are all very efficient the fuel consumption is proportional to the horse power used.
If you are using .5 litre / hour you are only using half the available power.
That seems about correct for motoring at 4.5 knots.


My google search suggests that the YSB 12 is an old design originally by Coventry Climax for WW2 landing craft. It's suggested that these engines shouldn't be run slowly. It appears to be a horizontally opposed twin.
I would be nervous about the risk of water from the exhaust getting in the cylinders of a flat engine. I don't personally know this engine.


If you repower with another Yanmar twin or similar more modern make I think you will be delighted with the improvement, although if your YSB is going well you might have no need. You likely have room for a new motor which might be higher but narrower.
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Old 21-09-2015, 15:31   #21
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Re: 0.11 litres per nautical mile

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.11 litre / mile @ 4.5 knots = .495 litre / hour. A Google search suggests the YSB 12 is economical @ 1 to 1,1/2 litre / hour.

My 1998 Yanmar 2GM 20 uses 2 / 2,1/2 litre / hour max. I run it at around 2900 / 3000 rpm with some full throttle bursts every hour for a few minutes at 3,400 rpm, if I'm motoring far. I have an easily driven 30' (9.3m) yacht and normal speed is around 6 / 6.5 knots water speed. Mine is a saildrive which made (replacement of a shaft drive ) installation very easy.


Diesel engines like to be worked reasonably hard. Generally as they are all very efficient the fuel consumption is proportional to the horse power used.
If you are using .5 litre / hour you are only using half the available power.
That seems about correct for motoring at 4.5 knots.


My google search suggests that the YSB 12 is an old design originally by Coventry Climax for WW2 landing craft. It's suggested that these engines shouldn't be run slowly. It appears to be a horizontally opposed twin.
I would be nervous about the risk of water from the exhaust getting in the cylinders of a flat engine. I don't personally know this engine.


If you repower with another Yanmar twin or similar more modern make I think you will be delighted with the improvement, although if your YSB is going well you might have no need. You likely have room for a new motor which might be higher but narrower.
I had a 1982 C&C27 with a Yanmar 2gmf20. I ran it at 1800-2000 rpm and it used just less than 1 litre per hour, making 5 knots.

The quote above demonstrates that at 3000rpm it would produce 6.5 knots and use more than double the fuel.

The choice is yours.

IMHO, 2000rpm satisfies the requirement of "running hard" enough to keep the engine in good shape. In the years I owned that yanmar, I never had any issues. I run my current volvo at 1800-2000 rpm, and it is running better now than when I bought it 5 years ago.

There is a line between running hard, and wearing out. Where is that line? That is one argument I would like to steer clear of.
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Old 21-09-2015, 16:32   #22
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Re: 0.11 litres per nautical mile

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....

Now there are many things that have broken on the motor, so its time for me to change it. With a new motor, can I hope to get lower fuel consumption? I plane to have a motor with 18 to 20 hp. I shall also have an Autoprop with the new motor. If I go with the same speed with the new motor, than at least it should not take more. But with a stronger motor will I probably go in a higher speed.
A new 18 to 20 hp motor won't give any better fuel comsumpution as you will still need the same hp as before as the boat is still the same. The efficency of new modern twins is almost the same as earlier motors.

What you will get is a smoother running engine and way less vibration with lower emissions.

Of course you also get the opportunity to use the extra power should you require it but fuel comsumption will rise when using the extra power.

Having owned two single clyinder engines and two twin clinder engines on boats around 30 ft, there is no way I would get another single. The twins were so much smoother.

I concur with the concenus of fuel comsumption in the previous posts for the 2GM20 etc and IMO, I think that 100 litres is ample. I would perhaps keep the tankage smaller say around 30 to 60 litres and carry the rest in containers only when there is some expecation that it will be needed.
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Old 21-09-2015, 16:41   #23
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Re: 0.11 litres per nautical mile

Diesel engines in the sizes we are talking about for leisure boats (i.e. from 10HP to 200HP) will all consume very close to 0.20 litres per HP per hour at their most efficient RPM and load. This is also true of many diesel engines made 80 years ago in the same power class.

Some much bigger slower engines made for constant speed and load operation for use in big ships, power stations and pumps were and are better than that.

What modern diesel engines give compared to those from 80 years ago is a much wider range of RPM and load conditions where you get close to 0.20 litres per HP per hour (as well as being much smaller, lighter and requiring much less maintenance - some old diesels required various bits being manually oiled on an hourly schedule).

Common rail, electronically managed, turbo engines will be within 10% of this maximum efficiency from just above tick over speed to about 95% max RPM and from 20% to 100% load and only a little less than that at even less load. This is the big development. However, maximum efficiency has not changed much at all.
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Old 21-09-2015, 16:50   #24
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Re: 0.11 litres per nautical mile

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I had a 1982 C&C27 with a Yanmar 2gmf20. I ran it at 1800-2000 rpm and it used just less than 1 litre per hour, making 5 knots.

The quote above demonstrates that at 3000rpm it would produce 6.5 knots and use more than double the fuel.

The choice is yours.

IMHO, 2000rpm satisfies the requirement of "running hard" enough to keep the engine in good shape. In the years I owned that yanmar, I never had any issues. I run my current volvo at 1800-2000 rpm, and it is running better now than when I bought it 5 years ago.

There is a line between running hard, and wearing out. Where is that line? That is one argument I would like to steer clear of.
For 3JH5E and same generation of engines, that have max revs 3200, 2900 is highest sustained rev, manual explicitly states:

engines are designed to run LESS than 90% of time below 2800 REV, Meaning at least 10 % of time must run above 2800 REV for optimum life.

My engines run smooth at 2800 so wear hope not too bad.

Presume engine design has changed and your formula may cause issues with new generation of yanmars.
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Old 21-09-2015, 19:24   #25
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Re: 0.11 litres per nautical mile

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I had a 1982 C&C27 with a Yanmar 2gmf20. I ran it at 1800-2000 rpm and it used just less than 1 litre per hour, making 5 knots.

The quote above demonstrates that at 3000rpm it would produce 6.5 knots and use more than double the fuel.

The choice is yours.

IMHO, 2000rpm satisfies the requirement of "running hard" enough to keep the engine in good shape. In the years I owned that yanmar, I never had any issues. I run my current volvo at 1800-2000 rpm, and it is running better now than when I bought it 5 years ago.

There is a line between running hard, and wearing out. Where is that line? That is one argument I would like to steer clear of.
Engines are a bit like our bodies. Never use them and they rust away. Over rev them and they wear out. I've been advised by a Yanmar mechanic that 1500 rpm is a bad speed for my motor. There can be harmonic oscillations in any motor especially 1 or 2 cylinders. More or less than 1500 is OK. And that it can run all day at 3000 rpm and mine seems happy at 2900. I have a Kiwi Feathering prop with adjustable pre set pitch.

Working out consumption figures accurately is a bit complicated. I just look at my hours meter and how much diesel I've bought over a period and then how much I might need for a long motoring voyage with a safety factor.

Working in litres per mile is very very approximate as if you are motoring at 3 knots into a 3 knot tide you go no miles and consume maybe a litre of fuel. Likewise if you go with the tide you travel 6 miles appearing very economical in miles / litre.

Without doubt fuel consumption increases non linearly with the increase in speed. I would suggest if you double speed you consume 4 X the fuel. That can be even more if you try to exceed hull speed and open the throttle with no speed increase. It also depends on the pitch of the prop. If you have a large prop on a small motor it could be overloaded and never reach maximum power even if you run at full throttle, consuming a lot of fuel for not much speed.

Fuel consumption is usually scientifically measured in pounds weight of fuel per brake horsepower per hour. (or metric equilivant) Skeene's Elements of yacht design suggests .4 lbs diesel / HP / hour. A gallon of diesel ( US is 3.8 litre) weighs 7.13 lb.

You only need to consider this to get an idea of where it's heading. Your displacement is of course a factor as the propellor has to move that weight of water for every boat length travelled. That's not allowing for wind resistance with or without a head wind. Double the wind speed = 4 X the wind resistance. Generally I can motor at 4.5 knots directly into a 30 gusting 40 knot headwind in a choppy harbour until a wave might almost stop me.

Also horsepower figures are a little vague. My "20Hp" from memory develops around 13 HP. Your 12 might be only 6 or 8 HP. And then what is the HP at the propellor?

Then there's "indicated horsepower" calculated by. PLAN over 33000. That is pressure length and area of a cylinder / piston X the number of cylinders. That's theoretical but helps in design.

If you really want to get into fuel consumption calculations they are all in Skeene's Elements of Yacht Design page 303. It is possibly on Kindle.
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Old 21-09-2015, 19:36   #26
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Re: 0.11 litres per nautical mile

We are 36 tons, 115 HP Westerbeke 6-cyl. We cruise at about 7 knots & 1100 rpm. Performance is about 4 miles/gallon. We carry 380 gallons.
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Old 21-09-2015, 19:45   #27
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Re: 0.11 litres per nautical mile

Thanks Lars...you live in an exquisite cruising grounds. I pray to God to be allowed to sail a season or two there before the arterial corrosion gets me...
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Old 23-09-2015, 13:27   #28
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Re: 0.11 litres per nautical mile

One of the problems with my motor is to keep up the oil pressure. I let the motor go at a speed where the oil pressure lamp stops flickering. When the motor gets warm, it is quite high speed. At the moment I have solved that problem with a thicker oil. I know that the problem is the bearing of the oil pump. And the bearing is direct in the motor block. But I think I have found an area in the Sognefjord in Norway where I can dump the motor. It is 1200 m deep.

Yanmar YSB 12 is a four-stroke one-cylinder motor on 510 cc. I can both hear and feel every explosion. With a two cylinders motor I hope that it should go smoother. There are even three cylinders motors with 20 hp. Do they go much smoother? Do they take more diesel than a two cylinders motor with 20 hp?
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Old 23-09-2015, 14:23   #29
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0.11 litres per nautical mile

Smoothest engine design is an inline six cyl, but an inline three can be surprisingly smooth.
In line fours although very prolific in number is not a smooth running design.
Often packaging drives engine design, a V6 is a terrible design in my opinion, but there is a huge number of them in automobiles.
I'm not including boxers and radials etc as I haven't seen them in boat motors, so not much point in considering them.
But if I had the option, I'd definitely pick a twin over a single, and inline three over a parallel twin, based on smooth running

Now if your using say 6 hp to cruise, using 6 hp from a 20 hp motor will burn slightly more fuel than 6 hp out of a 12 hp motor, but probably not that much.
More cylinders due to frictional losses will consume more fuel, maybe so little more that you would be hard pressed to measure it, but a three cylinder turning slower and making less noise and vibration would probably be a lot more pleasant.
If you can afford an inline three, I think you would be way more happy, even if you had to carry slightly more fuel.


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Old 23-09-2015, 15:12   #30
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Re: 0.11 litres per nautical mile

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Thanks Lars...you live in an exquisite cruising grounds. I pray to God to be allowed to sail a season or two there before the arterial corrosion gets me...
can you enlighten me.. why is place so desirable as sailing ground ?

Presume there is less pollution... anything else ?
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