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Old 20-09-2015, 13:42   #1
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0.11 litres per nautical mile

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 donít know what I shall have, but that donít 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 isnít 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.
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Old 20-09-2015, 13:51   #2
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Re: 0.11 litres per nautical mile

That sees low:
On my CSY 33 with a Perkins 4-108 I averaged 0.6 Gallons per hour at 5.3 knots. 2,2 liters per hour.
(1 U.S. Gallon is 3,785 liters.)
A modern Diesel engine may be more efficient, but the numbers you have now seems way low and impossible to beat...
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Old 20-09-2015, 14:05   #3
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Re: 0.11 litres per nautical mile

Claims seems well beyond the BTU/KWh of diesel fuel. But I'm not going to try and prove it.
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Old 20-09-2015, 14:46   #4
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Re: 0.11 litres per nautical mile

it is 1.3L for 3JH5E 40 HP a 1600 RPM that gives you around 4.5 kn on L 4000. So 0.5L is not out of question with even lower RPM and smaller engine.

Yanmar curve for consumption starts at 1600 RPM for 3JH5E so I guess lower than that is not recommended. However, at least 10% of time one should run at 2800 RPM where consumption is 7L to keep motor in good shape, according to manual.

Assuming above recommended practice consumption will still average around 2L ph on long motoring trip.
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Old 20-09-2015, 16:44   #5
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Re: 0.11 litres per nautical mile

Hej HŚkan,

You have a H 30 then your displacement is (?) 1,5t empty and you will want approx .2 of a liter per each hp you use.

To drive your boat (empty!) in flat water and no wind, at a good clip but below her hull speed you will need approx 2 hp per each 1000 kg. This implies about 3 hp which in turn implies 0.6 of a liter per hour. I can imagine you are motoring at about 4 knots then this translates to about .15 liter per mile. Going slower hugely lowers this figure. (look into HR website where they quote ltrs/hour/mile for various revs - it is an eye opening read!)

So your calculations seems very close to what the books say.

Now whether you can further improve on that or not will depend on how efficient and how well matched with your engine the new prop is. You may gain some or you may lose some. Try to make sure your prop is matched to you boat/engine and planned cruising speeds as best you can. Also make sure the prop is clean and the hull is clean too. If you can have a feathering or a folding prop this will greatly affect your sailing speeds! Now think about how much you sail and how much you motor. The implication is clear.

Do NOT get too hung up on the Dashews. Are you going to ever motor over 700 miles?

We are sailing a boat that displaces empty 3t and our motor is 13 hp. Our tank is about 40 liters. We never ever needed extra fuel. Your boat is much more like ours than like the Dashews'. I think 80 liters will be an overkill BUT if you are planning on a VERY long voyage then it sure is nice to be able to stock 80 liters, if only to save on fuel or to avoid buying diesel where it is known to be very dirty. Still I would somewhat prioritise water tanks volume as a sailing boat can go without diesel but her crew can't go without water.

Take care, good luck in your undertaking,
b.
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Old 20-09-2015, 17:01   #6
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Re: 0.11 litres per nautical mile

Diesel efficiency is almost flat as to what it was years ago, or said another way, new non common rail Diesels aren't very much if any more efficient, plus if you go to a bigger motor but use the same HP due to friction loses and BSFC, you'll burn more fuel.
The other thing is the Autoprop is very good at pitching itself to get most out of a drop of fuel, it's exceptional at Motorsailing for instance, it has a unique ability to add or subtract pitch as loads vary, where a fixed prop has a sweet spot and is less efficient on either side of that curve.
I think to get as good as your getting now your getting a slight boost from something, current of maybe just sailing a tiny bit from your windage or something.


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Old 20-09-2015, 17:35   #7
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Re: 0.11 litres per nautical mile

I get a burn rate of 1/3 a US gallon/hour on my 15 HP Yanmar. Consistently, to the point I can closely approximate the amount of fuel I need to take on by looking at the hour meter. I have a 30 foot boat, which is close to 9 metres.

Miles/gallon tell you nothing, as one needs to consider the effect of current/wind.
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Old 21-09-2015, 10:17   #8
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Re: 0.11 litres per nautical mile

All depend on load. On flat water with no wind at 4.5kn I get around .5l/hr. (7.5 ton boat)for ever knot of extra speed or 10kns wind I reckon to double the consumption. A bigger engine if matched to a larger prop and set up for low revs tends to be more efficient than a small high revving one but that more to do with prop efficiency than the engine
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Old 21-09-2015, 10:21   #9
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Re: 0.11 litres per nautical mile

As a PS. About 10% below your design speed will be your max efficiency so for me that's 4.75kn
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Old 21-09-2015, 10:31   #10
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Re: 0.11 litres per nautical mile

Quote:
Originally Posted by avb3 View Post
. . .
Miles/gallon tell you nothing, as one needs to consider the effect of current/wind.
Exactly. The efficiency of the engine or the prop is a very minor consideration compared to how fast you are trying to go and into what wind and sea conditions. Miles per gallon or liter will vary tremendously depending on this.

In very calm conditions or with a tailwind and motoring very slowly -- say 6 knots -- my fuel consumption goes down to a liter or two an hour giving me 3 to 6 miles per liter or 0.3 or 0.166 liters per mile.

But motoring hard into short head seas and a 35 knot wind -- I might need 3200 RPM to go 2 knots. That would be maybe 15 liters per hour so what -- 7.5 liters per mile?

And everything in between. A very rough average on my boat for economical motoring in reasonable conditions might be one liter per mile, so 700 miles out of my tank, but that is really theoretical. It all depends on the speed and conditions and will vary tremendously from that in real life.
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Old 21-09-2015, 10:39   #11
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Re: 0.11 litres per nautical mile

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Originally Posted by roland stockham View Post
As a PS. About 10% below your design speed will be your max efficiency so for me that's 4.75kn
I think efficiency continues to go up sharply at less than 90% of hull speed.

Hull speed on my boat is 9.3 knots.

To motor at that speed in calm conditions takes about 2800 RPM and burns 12 liters an hour or so.

The consumption goes down by half or more at 8 knots.

My usual economical motoring speed is more like 7 knots, which is about 3 or 4 liters an hour.

At 6 knots, consumption goes down even much more.

The slightest bit of sea state or headwind pushes those numbers way up.
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Old 21-09-2015, 10:40   #12
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Re: 0.11 litres per nautical mile

Quote:
. The other thing is the Autoprop is very good at pitching itself to get most out of a drop of fuel, it's exceptional at Motorsailing for instance, it has a unique ability to add or subtract pitch as loads vary,
Hmm, I thought a fixed pitch prop was the most efficient for motor sailing if the prop is tweaked just right for the boat and the engine.
An auto prop or folding prop would shine when the blades fold or feather for sailing. I was not aware that these props were highly efficient for motoring.?
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Old 21-09-2015, 11:22   #13
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Re: 0.11 litres per nautical mile

Quote:
Originally Posted by CSY Man View Post
Hmm, I thought a fixed pitch prop was the most efficient for motor sailing if the prop is tweaked just right for the boat and the engine.
An auto prop or folding prop would shine when the blades fold or feather for sailing. I was not aware that these props were highly efficient for motoring.?
I think you're confusing different kinds of props.

There are three:

1. Folding. Autostream, etc. The pitch is fixed, but the blades fold back to reduce drag when sailing.

2. Feathering. Maxprop etc. The pitch is fixed, but the blades feather to reduce drag when sailing.

2. Feathering/Variable Pitch. That's the Autoprop. It feathers to reduce drag when sailing, but it also changes the pitch of the blades depending on the load -- a completely different function.

It is like having different gears versus a one-speed transmission. The benefit when motor-sailing is that when you are moving at a given speed and the sails are providing part of the drive needed, the blades will pitch up and give you the equivalent of a higher gear. And they will pitch up and down as needed as the wind blows harder or less.

It is not only more efficient in terms of fuel consumption, but you can also slow the engine down while still keeping a reasonable load on it, which makes it much quieter.

It is absolutely brilliant for motor sailing, but for just motoring in different conditions it is also valuable because you need a different amount of power in different condition.
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Old 21-09-2015, 11:39   #14
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Re: 0.11 litres per nautical mile

To the original question, whatever your consumption was a new 20hp Yanmar will make almost exactly the same consumption. To an earlier point, nothing significant has changed about these engines, and the prop differences are negligible. My 5 knot consumption is about the same as yours in calm water with my Yanmar 30.

I personally think that 700nm is a good minimum motoring distance capability for a circumnavigation. If you lose your mast you will want to be able to motor to landfall, and that distance is based on the half-way point between the longest legs you would take. U.S. West Coast to Hawaii have a 2500NM leg, the halfway point of which is 1450NM. However, there's a lot of traffic on that route so it's very unlikely that you wouldn't find help. Tankage will also help you bypass some of the more expensive fuel stops in the world (Like Hawaii and the rest of the Pacific Islands).

Just my 2 cents.
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Old 21-09-2015, 11:43   #15
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Re: 0.11 litres per nautical mile

Quote:
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 donít know what I shall have, but that donít 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 isnít 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.
Lars,

I envy you your Autoprop and have been exploring putting one on my boat.

In preparation for that I have been doing similar research.

In addition to this forum, another source I find very valuable is Attainable Adventure Cruising; the Offshore Voyaging Reference Site. [No affiliation other than being a satisfied subscriber...] It does have a modest subscription fee [less than $2US/month] but I find it well worth the cost of entry.

Regarding your topic there was a recent article entitled Understanding An Engine Fuel Map which I found highly enlightening.

In case you find this helpful.

Cheers!

Bill
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