Maintenance Cost Diesel vs Gas

Cat 3126's can be had in many different hp configurations including 350 / 385 and 420 hp all based on the same engine at the same max rpms.
I do not know which version Dani-Lu has on his boat but I would not apply a straight 60% without knowing his exact version. It is very good that he has worked towards and achieved a max WOT rpm in excess of the rated max rpm's for those engines while the boat is fully loaded on a hot day.
Correctly propping a diesel will go a long way towards a long and less troublesome engine life.
If those 3126 Cats have a total fuel burn around 24 gph at the selected cruise range than I would say that you are in pretty good shape.

Hope this helps
 
quote:
Originally posted by pdecat

quote:
59 Hp per liter of engine displacement.
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cruising!!!

Those rascals are working.

I wonder why Volvo show both 2.5 and 3 exponent when they make the drive?? Are there prop options?

Running at 67% load sounds decent until I realized those are 300 HP WOT engines at 81 hp per liter.!

That is beyond my experience.

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There are diffrerent prop setups. I have the T4 which I recall reading somewhere is the 2.5 number you talk about.

I am gonna do some tests but I still feel best fuel economy for boat seems to be 3100 rpms and mechanic said this was good rpm to cruise at based on the tool he hooked up. He said in 3000 to 3150 is fine. I sometimes back it down if I am in no rush, but I find the fuel burn stays up high so the mpg goes down.
 
Fuel burn high and mpg and speed lower probably means you are coming off best plane angle.

The duty cycle idea introduced above is valuable because it is a way to inform the operator of how to extend engine life. It always yields some startling conclusions about what % of time engines should be run at "cruise".

the CAT idea of total fuel used is very informative because it summarizes all the other ways of looking at the amount of work done by the engine and points out in a clear manner that one particular engine model can be expected to last for 30,000 gallons of fuel. You can run lightly loaded for a very long time or heavily loaded for a shorter time but either way once you use 30,000 gallons an overhaul is suggested. Unfortunately until recently few people had a fuel used counter. It would be super easy to answer the question how hard were these engines run or how much life can I expect when looking at a used boat if fuel burned were available and makers followed CAT with an overhaul number. It also points out that RPM doesn't matter only load which equates to fuel matters.

Without fuel used measurement factors such as RPM, prop curves, EGT, boost, etc are ways we guess what is really going on.
 
Rating 5: LEISURE DUTY
This power rating is intended for leisure (pleasure) craft applications only, which presumes operation by the owner for his/her recreation running less than 300 hours per year.
ƒÞ Full power can be utilized a maximum of one hour per 12-hour operational period. Between full-load operation periods, engine speed should be reduced at least 10% from the obtained full-load engine speed.

Ok, so if I read that right, they say that i can run 1 hour at WOT per 12 hours, which I never do.
And normal load is 10% off WOT? Which is the 3150 RPMS I run at?
 
3100 RPM puts you at about the 200 HP level on the prop demand curve
 
IIRC CAT called duty cycle load factor and it was much lower than 1 hr out of 8 or some such, more like 30%. It was not well known and I don't know if they still use it.
 
quote:
Originally posted by pdecat

3100 RPM puts you at about the 200 HP level on the prop demand curve

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Meaning?
200 out of 300 hp, that's 66%? Seems low.
 
CurrentSea - "Ok, so if I read that right, they say that i can run 1 hour at WOT per 12 hours, which I never do.
And normal load is 10% off WOT? Which is the 3150 RPMS I run at?"

Agreed - the Volvo docs as well as the Volvo tech tell you this is acceptable.

Bruce - "3100 RPM puts you at about the 200 HP level on the prop demand curve"

Agreed - and CurrentSeas monitored and reported fuel burn of 23 gph at the 3150 would confirm a power output of 218 hp at that 3,150 rpm cruise speed. (23/2 X 19 = 218)

I went through this same scenario in the 90's with all the Cummins 6b's in Bayliners as their recommended loading levels in recreational use were quite aggressive - most all of them got passed the manufacturers warrantees without major issues.
There are some great articles on engine loading over at Tony Athens site sbmar(dot)com that may help describe these issues in a bit more detail.

Hope this helps
 
quote:
Originally posted by smitty477

Cat 3126's can be had in many different hp configurations including 350 / 385 and 420 hp all based on the same engine at the same max rpms.
I do not know which version Dani-Lu has on his boat but I would not apply a straight 60% without knowing his exact version. It is very good that he has worked towards and achieved a max WOT rpm in excess of the rated max rpm's for those engines while the boat is fully loaded on a hot day.
Correctly propping a diesel will go a long way towards a long and less troublesome engine life.
If those 3126 Cats have a total fuel burn around 24 gph at the selected cruise range than I would say that you are in pretty good shape.

Hope this helps

Click to expand...


I have the 385hp version. This is a great thread - learning a lot!
 
"I have the 385hp version. This is a great thread - learning a lot!"

Hello Dani-Lu - Do you happen to know your fuel burn at the cruise rpm you use?

Your engines are rated 2,800 at full rpm and are 442 cubic inches (7.2L).
The prop loading curve is likely not tracked exactly by your engines but it shows these numbers as a references only:
2,800/20.1gph, 2,600/15.4 , 2,400/11.8 , 2,200/9.0 , 2,100/7.8 , 2,000/6.8

Hope this helps
 
Smitty I have those motors as well and I think those numbers are real close. I run at just off 2400 with an average of 20 gallons an hour over all. The most I have ever averaged for a 10 hour span was 21 and I was pushing it a little hard with several WOT stents. I have found these motors to be incredibly efficient. The is a big increase between 2400 and 2600 RPMs to me it is not worth the increase in speed hence I stay where I do but open it up from time to time just to exercise the motors.
 
Well Dave that's what the curve says. It also predicts your fuel burn at that rpm. How does that match your results? Fuel burn is a good indicator of power used.
 
"Smitty I have those motors as well and I think those numbers are real close. I run at just off 2400 with an average of 20 gallons an hour over all."

Hello Dave - At 20 gph total you are using about 180 Hp per engine (10 X 18) at 2,400 rpm's.
That would calculate to about 47% load (180/385) at 2,400 rpms which is 86% of your rated rpm's (2400/2800).
Your hp used per liter of engine displacement at crusing speed is 25 (180/7.2).

I can predict that you have lower pitched props than stock and that you can easily exceed the MAX WOT rating of 2,800 as a brief test.
Also you will have lower boost temps and lower EGT's at cruising speeds and that all things being equal you will likely get better MPG at a given cruising speed. You will not be able to extract the maximum power out of each engine even at WOT.

All of these things are very good as opposed to being overpropped and overloaded.

Example only - now if this was a brand new boat with zero extra weight, light tanks, and propped right to the limit on a 60 degree dry day as a baseline things would change fast after it is bought.
60 days later the boat has 5,000 + extra pounds, the bottom and running gear are not perfect , it is 90 degrees out and humid and the boat is run at that same 2,400 rpm's. But now the fuel use is no longer 20 gph it is actually 30 gph but lets say there is no one tracking that. No pyro and boost gages so you cannot tell easily that the boat is overloaded and using more fuel. There is no WOT test done to see that you can no longer reach the WOT rating of 2,800+ and likely cannot even reach 2,600 now.
The boat is now overloaded , getting poor mileage and the symptoms are pretty much masked unless you can track fuel use, boost or EGT in real time - this is how engine life can be greatly reduced.
So if you are close to the edge on these ratios and are not able to monitor the changes in the engines, running gear, and weights it is very hard to know when you are over the limit.

Hope this helps
 
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