Electronic Diesels & Fuel Consumption vs. Load

Boatingfool

Boatingfool

New Member
Feb 13, 2008
389
Annapolis, MD
Boat Info
560 Sedan Bridge
Engines
V10 1050hp Mans
This is something I've been debating for a while...I am better trying to understand the "best" crusing speed for my boat and hopefully this will be fuel for thought for anyone else who has larger engines and is wondering how to maximize their fuel consumption.

The manufacturers typically (especailly MAN) publish the engine output (horsepower & torque) at "Full Load".

The Fuel Consumption is published at "Propeller Demand".

So my hypothesis is this... theoretically you may use 100% Load to get up out of the water, but as the boat planes the water resistance of the boat should decrease slightly. Becasue this load decreases, the load on the engine should decrease. This decrease in engnie load should causes the engine to burn less fuel at a given RPM.

I unfortunatley do not have a load meter on my electronic diesels.

So, the question for those of you who do have fuel flow & load meters, have you compared what your Fuel Consumption the gauge tells you vs. the Fuel Consumption listed on the diesel manufacturers data?

And, once you are up on plane does the boat run at 100% load or does it back down to say 70-80% load?

****************************************

What am I trying to establish? How much load decrease, if any, you see to help me better understand the correlation between the published fuel consumption data.

The only reference point I currently have is that the MAN literature says the engines consume 84gal hr. at Propeller Demand @ 2100rpm (which gives me 30-31knots). Sea Ray says their recommended crusing speed is 2100rpm (they say 35mph) and at 2100rpm they say the engines are burning 68gal hr.
 
I have to admit that I have never watched the load % while accelerating, but the only way I can get my engines to run at 100% load is to run at 100% throttle. I cruise in the 70-85% load range.

at 2000 RPM MAN says I should be burning 64 GPH - my numbers show 66

At 2300 RPM MAN says I should be burning 96 GPH - my numbers show 88 at 2340

In my case there is a slight benefit to running at 2000 RPM although this is 83% load and some would say to run at no more than 80% load for longest engine life.
 
Sorry, I tried to post a table with actual performance results but it was illegible in this message format.
 
This is far too difficult to answer in a posting here. But let's start with this:

"So my hypothesis is this... theoretically you may use 100% Load to get up out of the water," is an incorrect starting point. 100% load is typically achieved at WOT or darned near WOT. At lesser RPM, the load on the engine is far less than 100%. To make it confusing, it's 100% of the load that the engine can support at that RPM and fuel flow amount. Which, if you think about it for a second, is obvious, because otherwise the prop would be turning faster.

Generally speaking propeller load in a free hull form is nearly linear. It's that the hull form is not free in the water that introduce non-linearities into the system. That is, when the hull is not moving, the load is a bit higher at a given RPM than when the hull is moving. The problem, therefore, it matching the non-linearities of the engine's torque curve with the non-linearity of the propeller. Unfortunately, the period of greatest demand does not correspond to the period of greatest availability. That is, if you look at your torque curve charts the engines provide less torque at low speeds when the propellers need more torque to push the hull up onto plane.

The task of the marine engineers therefore, is to find the correct propellers that will allow the engine to turn up far enough into the engine's torque curve such that the boat easily climbs onto plane while still providing enough load to not allow the engines to overspeed at WOT. This is the point that Gary, Frank, and Jeff have been making.

Best regards,
Frank C
 
You MAN owners that have the manuals in front of you can thrash this out, but I am afraid there are too many variables in boatingfool's hypothesis.

Diesel engines are rated at some horsepower and all manufacturers that I know of state the rpms that they are using for the rating. This is at 100% load and 100% power output. If you haven't done it, get your bottom cleaned, load your boat with full fuel and water, normal waste and normal crew and gear then go run the boat in a place where WOT can be achieved. If your engines will turn to the rated WOT rpm's and no more, your engines are properly loaded. If you cannot turn WOT then your engines are overloaded..if you turn more than rated rpm's, then you are underloaded.

If your engines turn up to rated rpm @ WOT, their performance is at the optimum and it cannot get any better. However, if you allow the bottom to foul and put more weight on the boat than normal to the extent that you cannot turn up rated WOT rpm's then your cruise rpms must be reduced to normal load. If MAN's rated WOT is 2300 rpm and sustained cruise rpms is 2100 but you can only turn up 2200 rpm's at WOT, then you must reduce toyr cruise rpm's ..... 2200/2300= 95.6%X 2100=2007....and if you don't, you are overloading the engines and are burning more fuel.

If the enginea are peoperly loaded, the only variables that have an effect on fuel economy then are how much load (weight) you put on the boat, how fast you run the engines, and how aggressively you drive the boat. As far as computing the most economical cruise, it is a matter of doing the math and determining at what speed you get the best miles traveled per $ spent on fuel.

But honestly, what we find in our area with out boat, unless we are on a very long trip, is that it is the journey, not the destination, that is the most fun and for that reason, we pay more attention to gph than $/mile.
 
I topped the tank off last month, traveled 110 miles north non stop averaging 25 mph @ 2100 rpms, 75% of red line (2800). I then put on 131 gallons..... 110 divided by 131 gallons = .84 miles to the gallon..... 4 adults, gear, full fuel tanks, 1/2 tank of water and a dog !!
 
I can't really complain about my fuel efficiency. I generally get about .6 to .65 statute miles per gallon if the seas are good (1 foot chop - flat calm is worse). My 480 DB is cheaper to go from point A to point B than my 380 DA (gas) was.
 
Now if a train traveling West leaves New York @ 6am and another train leaves Los Angeles heading East @ 9am........?????
 
Boatingfool said:
I unfortunatley do not have a load meter on my electronic diesels.
Click to expand...

What model MAN engines do you have? Are you sure they are electronic?

Boatingfool said:
So, the question for those of you who do have fuel flow & load meters, have you compared what your Fuel Consumption the gauge tells you vs. the Fuel Consumption listed on the diesel manufacturers data?
Click to expand...

It's close but it does vary depending on how loaded up the boat is. When the boat is fully loaded, I can still get the WOT spec of 2330 but it will burn more fuel over the range of RPM's. As the boat lightens up, the fuel burn vs. RPM curve shifts down. I've actually got real data I take every year over the range of RPMs with the boat fully fueled up to make sure the boat is performing the same every year.

My published engine data only gives me numbers for WOT so I do check those from time to time as well. For example, the published boost pressure for my QSM-11's is 72 inches Hg at 2330 and I get 72 on the starboard and 69-70 on the port... If I open the engine hatch, I get 72 on both.

I personally think it's a good idea to check performance curves every so often to see if something is going wrong... like say some lose bolts on the aftercooler.

Boatingfool said:
And, once you are up on plane does the boat run at 100% load or does it back down to say 70-80% load?
Click to expand...

It backs down to a lower number once I am on plane. Now we are talking the gauge "Load Factor" number and I'm not really sure how that's computed... but... Mine generally varies between 70 and 85%. I try and keep it between 70 and 80%.

However, this is the deal with my boat anyway. I can be running a 1900 RPMs and have a load factor of 80% and going say 22 knots. I can boost the throttles up to 2000 RPMs and the gauge will settle back to 80% and then I'll be going 24 knots (with a higher fuel burn). I can bump it up to 2100 RPMs and the gauge will settle back to around 80% and then I'll be going 26 knots (with an even higher fuel burn). The published "cruise" on mine is 2100 RPMs and anything over that generally pushes the load factor on the gauge up over 80% and at WOT I'm at 100%. So I have no idea how the thing computes "load factor".

As a side, I generally run the boat slower (1900-1950 RPMs) when I'm full of fuel, people, wife's shoes, and water and raise the RPMs a little when the boat lightens up over a run... so I generally cruise at 23-26 knots depending on how heavy I am and the sea conditions... really crappy stuff slows me down but I like to stay over 1500 RPMs to keep the turbos spinning. Also, my boat falls off plane at about 12 knots.

Boatingfool said:
What am I trying to establish? How much load decrease, if any, you see to help me better understand the correlation between the published fuel consumption data.

The only reference point I currently have is that the MAN literature says the engines consume 84gal hr. at Propeller Demand @ 2100rpm (which gives me 30-31knots). Sea Ray says their recommended crusing speed is 2100rpm (they say 35mph) and at 2100rpm they say the engines are burning 68gal hr.
Click to expand...

I don't think there are any absolute numbers. I generally run a little slower than the published cruise.. The engines also get a lot louder over 2050 RPMs so it's better for the people hanging out below to run a little more quite.

But then again... some people just run around with the engines pegged and add antifreeze by the gallon everyday.
 
osd9 said:
25MPH (~22 knots) at 'only' 2100RPM....:wow:....do you have the 420HP 3126 motors? Can you turn 2800 RPM?
Click to expand...


Yes, I have the 3126's and WOT is 2800 and I never run there.... No good can come from it !

Cat mechanic says run 'em 70-80% and they will be around for a long time.....
 
One other thing... I believe my engines anyway are limited to the WOT number by the ECM and not load.
 
THEBILLCOLLECTOR said:
Yes, I have the 3126's and WOT is 2800 and I never run there.... No good can come from it !

Cat mechanic says run 'em 70-80% and they will be around for a long time.....
Click to expand...

Mark:

The reason I asked if they are the 420HP motors (which you didn't say) is because your cruise speed of 22Knots at only 2100 RPM on the 450DA seems out of wack to me unless you do have the 420HP option with bigger wheels. My 410, with 350HP 3126 CATs needs to see 2275 RPMs for a ~22 knot cruise....(as per calibrated/verified digital tachs and GPS).
 
I'm waiting for Frank H. or W. to throw some red balls at me...

tick tick tick...
 
Sorry, yes I have the 420 HP engines.... I thought all 3126's were 420 and the 3116's are 350 HP....?
 
Have fun guys.... I'm off to the water. It's beer thirty and I'm out of here.
 
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