Gph

This is also correct except when trying to increase range on a given amount of fuel or when you will go several thousand miles a year.

 

Thanks guys and I meant shrimp not crab LOL its been a long week.

 

"This is also correct except when trying to increase range on a given amount of fuel or when you will go several thousand miles a year."

The fuel burn does change , but a DD that creates 16hp from a gallon VS an electric nightmare that creates 20 will still take a long time to justify.

Needing 60 hp would be perhaps 4 gph from the DD , 3 from the new engine.

ONLY that 1 gph difference will be paying off the new motor.

10,000 hours at 1 gph will be the $30,000 difference .

But then there is the maint cost of $9000 computer boxes to keep operating.

FF

 

In this case we're dealing with a fixed market....the unlicensed ex-fishing vessels available in BC. Jimmy's are probably the most common power in the available (older) fleet, there are a scattering of Volvo's and Gardner's, a very few Cummins and Cat's, and the odd newer Lugger or John Deere or Isuzu.

Asking price is based on vessel size, age, and overall condition...with specific equipment making a small (if any) difference. The desperation of the owner is a big factor as well......

By temperament the Gardner powered boat was owner operated by a fussy and meticulous caretaker.....While the Jimmy powered boat was more likely owned by a company who hired punks to run her into the ground.....which would you rather trust your life to?

I totally agree that the Cummins 855 and the J and H series before them are beautiful engines...but for some reason they are rare out here, fewer around every day.....

Looking at the data that I have from Detroit Diesel and Gardner, the LXB 150 turning at 1350 RPM is producing about 65HP at the prop, they claim consumption of 2.5 imp gallons (3 usg) per hour. The fuel map I have for the 6-71 states 65 HP (prop) at 1400 RPM and consumption of 4 usg/per hr.. A reduction of 25% seems pretty significant if the coast of admittance is minimal.....

 

TAD,
To qualify those burn numbers one would need to know the load. "Specific consumption" numbers are good for efficiency comparison but manufacturer HP/consumption curves are generated by WOT operation at various or all engine speeds to 100rpm. When a boat engine is operated at less than max rpm the fuel consumption drops off sharply. I once had a 120 Sabre and the "book" on that engine presented a graph of fuel burn that was presented as typical burn rates with the engine operating at typical loads in a boat. They pointed out that all the numbers except WOT were not exact but average for a typical installation. I think this kind of presentation is not common. On the hp curves is the hp listed different or the same as the "data you have"? If the numbers you presented were not from WOT operation but from typical use and loads the numbers could be misleading. Hull resistance is of course quite different from boat to boat and over propping was very common when these engines were popular. If one example was over propped and the other was not and one switched the over propping from one example to the other fairly large numerical differences in burn rate would occur ..no?

Easy Rider.

 

Easy...No....

No boat, gearbox, shaft, bearing, resistance, or speed information is involved because there is none. I am only comparing the fuel required to produce 65HP from either engine according to the manufacturers published data.

Both figures are from the manufacturers "propeller power curve", which I believe is what you are referring to as "typical loads". This information should be presented on any data sheet for a marine engine. In my experience it almost always is.

You can only get a fixed pitch and diameter propeller to match the engine output at one point, usually this is at max output (HP and RPM). Note in the curves attached they both reach an apex at max power. The prop power curve is created using a fixed exponent, often 2.7 for average loads, or 3 for high-thrust tug type loads. Over-propping means too much pitch or diameter, and the power curve crosses the engine output curve below the peak. This is generally fine if you never push the RPM beyond that crossing point. On the other hand with too little pitch or diameter the prop power curve never crosses the engine output curve. So it just spins uselessly and may overrev and tear itself to bits.

 

Thanks TAD,
OK good. What I think I see is at 1400rpm (6-71) wer'e burning 4gph w a typical prop load making about 60shp. With the 20hp per gal rule we'd see 3gph but since it's a Jimmy we burn 4. Whereas at 1400rpm propped for WOT (at that engine speed) the 6-71 should burn 11gph and make 165hp. It looks like the engine becomes more efficient at or near WOT probably due to prop slippage. If I were to look at the hull power requirements (curves) as seen in "Voyaging under Power" I could predict how various boats would perform as to fuel consumption at any speed covered w both sets of curves. Yes?

Easy Rider

 

As Tad pointed out


"Both figures are from the manufacturers "propeller power curve", which I believe is what you are referring to as "typical loads". This information should be presented on any data sheet for a marine engine. In my experience it almost always is."

The "prop curve" is all you usually can get from the engine folks.

A FUEL MAP is what is required to actually set up a boat for best performance at your chosen speed range.

Its far easier to get the plans for a T-88 small nuclear weapon than a fuel map from most engine marinizers.

FF

 

Yes Fred,
I recognize the shape of those curves from engine shopping days. Since engine marinizers all have a keen interest in staying in business I'm going to assume the fuel map is not often required. Do you have any idea how the engine marinizers create the "propeller power curve"?

Easy

 

Do you have any idea how the engine marinizers create the "propeller power curve"?


They fit the engine with a prop it will pull at full bore , then use math to generate the entire curve.

Its a power of something , onlt the peak output might be measured , the rest is pure speculation.

BUT if you look at the graph it is very self serving for the mfg , because loading it with their curves will assure the engine NEVER (except full throttle) works very hard.

There is ZERO concern for Your fuel consumption , engine wear , cylinder glazing or slobbering due to under loading.

It will die off warentee , not their concern.

Sometimes if you lie to the mfg and tell them you build gen sets , you might get a realistic fuel map.

As genset mfg do worry about fuel use and loading at modest 1200 ,1500, 1800 rpm .

FF

 

FF,

The engine manufactures do not fit a prop and do any test of any sort to arrive at the "prop load curve" (Or hull resistance curve). As you point out it is a mathematical excersise. Correct, scientific and a well established formula - Yes, but still theoretical and mathematic.

What one has to be observant of is the "exponent" that different manufacturer uses (They typically note this exponent in their data sheet).

For example: CAT uses 3.0, Cummins and Yanmar 2.7, Volvo appears to be using 2.5 for their pleasure rated engines and 2.7 or 3.0 for heavier duty, commercial, engines.

3.0 is a displacement hull.
A modern planing hull would have prop load curve exponent in the 2.0 - 2.3 range. 2.7 for a planing hull is not realistic.

One could wish for an industry standard: "For a planing hull we will all publish data using a xx exponent, for a displacement hull we will all use yy exponent".

I don't see that they can do this any other way but what the buying public needs to understand is that there is no way for you to know what exponent (Hull resistance) your particular vessel has. These curves can provide an indication of engine load and fuel burn swinging a prop but chances are that it will not match your boat exactly.

Also, you can not compare fuel burn of two (say, 300 hp engines) at propload using these charts if Engine A is using an exponent of 3.0 and engine B is using 2.5.

Karl

 

I have, for some time been against over propping. My criticism of improper propeller loading has always been on the over propping side but yesterday I met a marine mechanic that says one should be able to (while underway) attain the same rpm that one can against the governor in neutral gear. Never heard of such a thing. Any comments???

Easy Rider

 

That would be way, WAY, under propped.
Neutral and wide open (governed rpm) supplies the engine with just enough fuel to spin it at that rpm. The actual hp it can produce at this rpm may be 4-6% of rated hp. Again, under propped.

Suggest you take marine diesel advise from a different mechanic.

Karl

 

Yeah, one...

..... send him back to shool!

regards
Richard

 

http://www.frontierpower.com/library/national_fisherman_reprint_Deere.pdf

Here is another opinion, not one I subscribe to , as it is highly inaccurate , but a different point of view.

In researching any engine remember few small (under 1000hp) are actually built for a marine use , most are industrial , pump ,generator , and vehicle (big truck) units.

Go to the marine web site and there will usually be 4 loadings , power boat lifting it self on to a plane full bore ,1- 2 min ,,to 24/7/360 for a work boat.

HUGE difference in HP!

Happily many will give the fuel burn at the different loadings , so you will have some information , other than full bore output and a guess.

The same engines will usually have a truck or coach (bus) site and also a series of ratings , HP and fuel flow.
Some will have lower gov settings for Skool buses , which will help figure the fuel burn at lower throttle settings.

And then there may be a pump or gen set site , excellent low RPM info, if you can find it.


ALL the HP curves will have a bottom, some (usually turboed) at 1500rpm others at 1200rpm. You must observe these limits when working out the boats normal LRC , long range cruise 9 usually .9 SL) .



FF