Small diesel working hard or Large diesel working easy?

Easy peasy, - - Horsepower necessary to achieve efficient cruise at just below maximum torque for the engine which seems fairly closely aligned with best fuel consumption expressed in volume of fuel, (grams or 'mils') per-hp-per-hour at the rpm so just do your number crunching from there...

I achieve 7 knots from a pair of nanni-sail-drives, (3 cylinder 20hp rated at 3600rpm of about 722cc displacement each engine), at 2500rpm and the propellers are spinning a lazy 1000rpm and burn about 1.75 liters/hour each engine for a total fuel economy of around 2 miles per litre...

 

If there was low load damage it is in the upper cylinder.

The link is bull crap about saving fuel and co2 Maersk say they were in the forefront of slow steaming technology but says nothing else about how.

Maersk and I do not use the same engines.

 

I looked at the charts for the stry motors that micheal referred to and they show the torque curve and hp curve. The torque curve peaks way before the hp curve. So is the optimum rpms for a boat based on the torque curve or the hp curve.

It would seem that running the engine in the torque sweet spot would be the most efficient and thus the least stress ful on the engine. Does that make sense?

 

Yes and no ---its not always possible to keep an engine on its sweet spot. Between Hp and torque peek is fine.

Infact im not a glaze believer --not to the extent some people take it.

I,---like Van Han have run largeish engines at low RPM with no strange stories to tell.

Im more prepared to go off my own experience than a book

 

We run lots of stationary diesels at low load, even no load, for long times.
The rig I'm on right now, has 44,000 hours on it. It's not new either, it was a rebuilt shipped out to us. Its a detroit diesel ( international / navistar).
That is untouched except for 2 air starters and injectors.
Both could have been avoided without retards looking after it.

I think it's terrible when guys in the marine community talk about 1000-2000 hours on engines getting a rebuild.

I think maybe a lot of the engines are set up to run high RPM's, but theres no reason you couldn't gear them a little more to low RPM use.
Retard the timing a little, use a smaller turbocharger, ?


Wether low RPM use is more fuel economical or not, how much do diesel engines cost? I don't see why so many repairs need to be done with so few hours.....
The point of installing a diesel is to save money, how do you save money by rebuilding an engine within a few thousand hours?

 

Its the high output diesels and poor maintenance that cause ....connecting rod thru the block.. issues.

Engine rated continuous duty, correctly maintained, are incredibly rugged.

Again,,, no need second guessing engine operation..CONSULT THE TECHNICAL DOCUMENTS FOR YOUR ENGINE.

 

Although Im not a glaze believer I know that ships and rigs keep lights on during the day to keep load on the gens. I suppose if its running might as well put something on it.

 

Not necessarily,

Our rig has a genset that provides power to 2x 30 hp electric mix pumps, probably 50 flourecent 2x75 watt fixtures, and a 12 hp electric hydraulic pump, and 4x 4500 watt heaters, and a 6 hp electric flyght pump.
Also, power for 3 office trailers for computers, water pumps, lights etc. Furnace motors, but are propane.

At times, it is overloaded, and definitely losses a few hundred RPM while the turbocharger catches up.

In the summer, while power consumption is minimal, and times when everything just happens to be off, i've seen it under 30 amps lots.

It might run like that all day, or even days.

 

A good source of practical info on diesel engine life cycle wear are the engineers who service the engines for companies who rent diesel gen sets to the construction industry.

Gensets on a construction site never operate to the correct load.

 

Folks down in the tiny HP area (15-30) would doo well to talk to the reefer truck folks.

They regularly service hundreds of truck fridge setups that basically operate coast to coast with Zero maint.

As a source of good used small engines , these cant be beat!

Over sized oil pans and larger filters than the similar Kubota or Yanmar boat engine is the norrn.

 

There is a valid electrical reason to do that.
Without any load the output voltage reaches undesirable levels that can damage the 1th piece of equipment when switched on.
Load regulation on most gens is quite primitive and requires 10% minimum load to stay within specs.

 

Also what about the 3/4 ton and 1 ton diesel pickups?

My Dodge 300-350 hp Cummins would run for hundreds of thousands of kms getting 20+ mpg.
Which is 3 gallon an hour or maybe 50 hp...with only occasional turbo boost loads to pass or start out.

 

ok so it looks like the general observation from experience is that a larger engine doesnt neccesarily have a shorter life if run on a lower load. This brings me to 2 more questions.
Frosty i have seen pics of your boat and i did read your thread where you wanted foils for some economy at speed but how do you find your fuel consumption when running your larger engines at a low speed? Would you swap your big engines for smaller ones to get better economy?

Masalai, i have seen your boat when it was parked at Spinnaker Sound some time ago and although you have a different type of cat to Frosty do you find that your engines although being quite ecconomical lack power in some conditions? Would you go for bigger engines running on less load to have reserve grunt? Or do you find that for the majority of time your engines are big enough?
I am quite happy to putt along at a slower speed but as Michael mentioned earlier on the extra HP can be very usefull when conditions are harsh.


Thanks Don

 

QUOTE=Don H;
Frosty i have seen pics of your boat and i did read your thread where you wanted foils for some economy at speed but how do you find your fuel consumption when running your larger engines at a low speed? Would you swap your big engines for smaller ones to get better economy?

Thanks Don/QUOTE



It is an economical boat in the first place a semidisplacemet catamaran with surface drives. It wont go slower than 16 knots. If it does RPM is still very high and water is just thrashing.

It has one speed and anything off that is wasting fuel.

Foils were to get it faster than what it does, Im not sure if it would be more economical,--I guess it might be.

With both engines at 3800 RPM 40 liters per hour a 14 ton boat doing 16-20kts is fine by me.

Turbo boost guages hardly ever come off Zero--I saw them lift of zero once.

If I had a propellor factory I would try some 28pitch and load it some more.

 

Don:

I use bursts of power to run a 22 ton boat at 30 knots to minimize exposure to harsh ocean conditions,outrun storms,and to deal with 7-14 knot running tides and currents-yet need to be very wary of whales,logs,dolphins,orca, etc. The rest of the time I'm at 8-10 knots.

If I had an 8 knot "trawler" I'd waste more fuel trying to fight the tides/currents and move over the ground burning gallons an hour while making no or 1 mph headway.
Or spend half my time waiting for tides to change,or getting up at 3 am to avoid them.

At 14 or 30 knots it's the same mpg.