A couple of questions for diesel consumption

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cmedsailor

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Theory says that a 10hp inboard diesel engine will consume around 1lt/hr, 20hp 2lt/hr. 30hp 3lt/hr and so on.
1. At what "speed" this is achieved? Cruising speed which means around 80% of maximum or less?
2. As long as you keep the same rpm (cruising) will consumption be affected by waves, tide or whatever changes your SOG?
Thanks
 
Figures are very dependent on propellor type.

autoprop will significantly change fuel consumption depending on forces acting on the vessel.

large fixed three blader will have least difference, but wind and wave resistance will make quite a difference, again depending on hull shape and streamlining.
 
I have found that my 28 footer with a 16hp engine consumes 1 gallon every 3 hours at 5 knots (through the water). In heavy seas this does not increase noticeably but you cover less distance per hour.
 
I'm afraid I can't answer that as I do not have a rev counter. I usually just open the throttle till the engine sounds comfortable probably about 75% max unless I'm in a hurry.
 
The problem with your question is that there is no such "theory". In practice the consumption will vary with many factors. What you state is only a very rough approximation at best and has no direct calculated theoretical basis.
 
Not "theory" just a rule of thumb.

Look in the workshop manual or specification sheet for your engine and you will find the performance curves that include a specific fuel consumption curve which shows amount of fuel consumed per KW/HP plotted against RPM.

For example a Yanmar 2GM produces approx 9KW (just over 12 HP) at 2600 rpm, 70% maximum (typical cruising speed in flat water). At this speed it consumes approx 280grammes (.28 litres) per KW in an hour. Actual fuel consumed will be therefore .28*9 = 2.52 litres per hour.

This is very different from your "rule of thumb", although one can see how it comes about. The Yanmar is described as a 20 HP engine, but never of course produces that! The relationship is, however, consistent in that a 1GM uses half as much fuel at the same RPM, but only produces half the power. Specific fuel consumption does not vary much within families of similar engines such as the GM series, although generally improves slightly the larger the engine.

Consumption at any given revs is unaffected by sea conditions, although, of course speed is, and consumption in relation to SOG will vary. If you compensate in heavy weather by running at higher revs then consumption will rise, probably at a greater rate than the increase in speed.

Hope this helps
 
I had the same setup on my previous boat - Westerly Fulmar, but dumped the original 2 blade as ineffective when push came to shove. Replaced it with a 3 blade fixed and ran it at 2k revs with consumption about 10% less than your figure, and no loss in hull speed. A 3 blader converts revs more efficiently.

PWG
 
On a recent trip (Falmouth to Almerimar) I found that diesel consumption only varied a small amount - between approximately 1.8l/h and 2.1l/h.

I have a Nauticat 35 with a 43HP Volvo Penta 2003T engine, and a 14" three bladed fixed propellor.

Consumption seemed to vary less with the conditions - although we were happy to keep RPM fairly constant, rather than alter speed to push against wind and tide - but more with whether we were motor sailing or not.

The engine has a full speed range of 2800 to 3200 RPM, and we generally run it at 2200 to 2500 RPM
 
Very interesting figures. Are you sure that you are measuring in litres and not gallons? At cruising revs your engine is producing between 30 and 35hp, and yet from what you say it is using less fuel than a 2GM producing 12 HP? and the latter is one of the most fuel efficient engines on the market.

If your figures are correct it gives a specific fuel consumption of around 75gr per HP -h compared with the most efficient small Yanmar 3HM35 which is just under 200!
 
Depends on the engine and the speed the designer has in mind when designing it from a clean sheet.
There is a point in the rev range where the engine is designed for deriving the maximum energy from each pound / kilogramme/ gallon of fuel.
This is usually about 15 - 20% below the point where max BHP is found.

The further from this sweet spot, the less efficient the engine is at converting the fuel into useful work.

Higher revs equal higher friction losses, although a turbo engine may recoup some of this by having a positive pressure in the cylinder on the intake stroke.
Lower revs equal less friction loss, but pumping losses increase, although a mechanical supercharger may help reduce this.

Not even getting into form drag/ parasitic drag and lift drag on the prop & hull.
 
Not necessarily. The Yanmar specific fuel consumption curve is almost flat from 2200 to 3400 and only rises slightly for the last 200 revs after that. Its most inefficient speed is around 1200 where it is using about 20% more fuel per HP/KW.

As you rightly point out the inefficiencies in a boat are to do with the "boat" bits not the engine, starting with the inefficiencies of the propeller and ending with the physics of max hull speed, with weight, drag, dirty bottom etc in between!
 
Much of this is academic as there are so many variable in practice. The engine maker may publish a brake specific fuel consumption curve. Units are generally g/kW-hr. This is noramlly taken from the full load power curve and will vary considerably at part loads. There's a curve commonly refered to as an "onion plot" which shows part load bsfc's superimposed within the torque curve. Great fun for engine designers but pretty meaningless once on the water.

The best results tend to come from long term monitoring of fuel consumed. Even that's not simple given the limited amount of time we spend under power.

The engine is only half the story. The load is the other half. A prop loads an engine in a very different way compared to, for example, the way road and rolling resitance loads a car engine. Then there's the effect of the hull and so on. Sea state also makes a huge difference to the load applied to the engine.

Yes, at constant engine speed fuel consumption will be significantly worse as the conditions deteriorate and the engine is required to deliver more power, hence work harder.

David
 
Have been unable to find any fuel flow figures on mike's link so cannot calculate the SBFC.
However according to my pitiful arithmatic, purely as a performance comparison I get the following BMEPs

2000 rpm - 115.32 psi.
3000 rpm - 106.45 psi.

We lost almost 10 psi in 1000rpm, yet I presume we are injecting more fuel to overcome the losses and increase the revs.

Yanmar engineers are positive magicians to keep the BSFC completely flat in this rev range.

I wish I had listened more at college, my head hurts.
 
Theory is exactly that - educated calculated guesswork.
Practice is the only answer, and it will vary (on the same boat) with seastate, headwind, fouling etc. Even if you always run at "n" revs, the load sensed by the governor will vary and rack more or less fuel as it feels proper.
FWIW, I run a 4108 (40-odd HP?) at about 1600 revs giving on average, 6kn for 3Lt/hr.
 
Also depends on how clean the bottom is .. We moored at Titchmarsh last weekend and it was interesting to see the amount of weed on some of the boats. Had ours on the hard at Ramsholt and was surprised how much muck was stuck on .. Mind you the CopperCoat has kept the barnacles off .. Just left more room for the weed .. /forums/images/graemlins/cool.gif
 
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