Fuel Usage Curves

[Riverguy]

I don't think it is necessary to go to wot for 30 minutes. Tony Athens in the years before common rail recommended running a high output diesel up to 60% load which is typically 80% of wot rpm for about 20 minutes at the end of a long day at low speed to blow out accumulated carbon.


As noted above, I think that the advent of common rail has eliminated this requirement as it provides much more precise fuel injection.


Also I think my Yanmar 370 needed it because I often ran it for long days with only 15% load. Your Lugger is probably routinely loaded much higher which means the fuel injectors are being operated closer to maximum than mine, so it burns cleaner and produces less unburned fuel and soot.


David

Exactly right. I typically run both my single Yamnar 6LYA and my (twin) Cummins 6BTAs very gently, 1200-1400 rpm for long periods. Once every 4 hours of so, I spin them up to max rpm for 30 seconds, then back down to 2,400 or so for 5-10 minutes. I've seen no need to run at WOT for more than a few seconds, and no need to run at 60% loads for more than 10 minutes.


FYI, Tony still recommends spinning up the newer common-rail engines, but not as much or as long.


https://www.sbmar.com/community/topic/running-at-wot/

 

[Rufus]

"The prop curve is a simplistic representation of what the power-RPM curve might look like if the propeller were sized exactly for rated power and RPM with no margins. In other words, the prop curve is a completely fictional, idealized curve that does not represent any particular boat, but is intended to generically represent all boats.

It is a reasonably close approximation for slow speed displacement hulls with conventional propellers. However, it is completely unsuitable if you have:

a. semi-displacment boats, planing hulls or catamarans
b. boats under high thrust conditions, such as tugs or trawlers
c. dynamic loading such as acceleration
d. a propeller sized with a power margin (e.g., 85% MCR)
e. waterjets, surface drives, or highly-cavitating propellers

It's my understanding that a different exponent is used in the calculation for boats that are designed to get beyond hull speed. So the shape of the curve is different at higher speeds. Caterpillar used 3.0 for the above chart. Cummins uses 2.7 and the curve bends upward more steeply at higher speeds. Here's the Cummins 6bt chart. View attachment cummins 6bta.pdf

 

[TTrep]

This may help. If I loaded things right....

Thanks for the post. Appreciate the quick response.
Ted

 

[Riverguy]

It's my understanding that a different exponent is used in the calculation for boats that are designed to get beyond hull speed. So the shape of the curve is different at higher speeds. Caterpillar used 3.0 for the above chart. Cummins uses 2.7 and the curve bends upward more steeply at higher speeds. Here's the Cummins 6bt chart. View attachment 85070

No, that's not how I read it. The 'cubic demand curve' is a simple exponential curve that does not work for any hull except a displacement hull. The exponent used was typically 3.0 (hence a "cubic" demand curve), but changing the exponent a little does not solve the problem. Propeller demand becomes a messy and complicated thing as soon as you get away from a pure displacement hull, so MacPherson's whole point is that a simple exponential curve is useless (whether the exponent is 3.0 or 2.7), outside the neat and tidy world of a full-displacement hull. The longer story is in MacPherson's white paper here:



https://hydrocompinc.com/wp-content/uploads/documents/PerfWithECEngines.pdf

 

[Rufus]

I didn't read any of the links, but I know the prop chart for our semi-planing boat (which is a 2.7 exponent calculation) is shaped like the Cummins curve and tracks real world fuel burn very closely.

 

[Delfin]

Tractor engines and engines designed for work versus speed are designed for high torque at lower HP. The torque curve desired is one that hits the torque curve fast and that curve is somewhat flat at the top.

Meaning the torque comes on very soon off idle and stays there as RPM is increased.

>>>>>>>>>>>Action

Kind of like this....

 

[Action]

Yeah that torque curve isn't. It is pretty flat across the 700 ft lb line for the engine RPMs shown. Exactly what one wants in doing work or moving a load.

Notice the torque drops off a little at the 1900 RPM line. Keeping the RPM around 1700 works best for this particular engine.

 

[garycasey]

All interesting discussions about engines and ratings. I did a little research and found that at the typical engine in trawler usage like mine might have a "recreational" rating. The manufacturer says that they "allow" it to be run at full rated power for something like 1 hour out of 6. There is usually no guidance as to why they only allow it to be run for the 1 hour. I would bet that it is temperature related, probably oil. They allow it to be operated continuously at 200 rpm below rated rpm, and that would be at full load. Driving a propeller, the load would be significantly less than full load with a 200 rpm reduction. Bottom line is that if one is worried about harming the engine at any normal cruise rpm, don't. The engine is operating at a speed and load far below any possible maximum. And engines don't have a memory - whether it runs continuously at a cruise setting for 2 hours or 40, 800 rpm or 1500, it doesn't care.
I have a question, though. Some say that the engine should be run at high load for a few minutes every few hours. Of what benefit is this? Some say that it "burns off" any carbon buildup within the engine, and I suppose this could be true. However, I wonder if the visible exhaust carbon isn't simply the soot buildup in the muffler that isn't blown out at low engine speed. The white "smoke" then is simply the condensation of the water in the exhaust. After a minute or two the exhaust system heats up and the condensate disappears. I have never "blown the carbon out" and I don't know if there is a need to do that or not. Any comments?

 

[OldDan1943]

Final (I think question) for you engine nuts. I have been told that it is healthier to kick an engine onto WOT for a while to "remove carbon deposits" that may have built up when cruising for long times at low RPMs. We spend three months cruising the Bahamas where we very rarely go above the mentioned 1400-1500 RPMs. Should we really kick these engines up for a half hour or so every so often to keep them clear of deposits?

Quick answer, NO. Run it a hull speed for greatest fuel efficiency .......
I have a AT34 with a Cummins 380 electronic. I run it at hull speed, the fuel efficiency is good and sort of follows the builder provided gph. The reserve power and speed..... that's reserved for unusual events such as getting to a port at a particular time or perhaps staying ahead of the weather.
The speed (RPM) vs GPH is created when the boat is running on flat water, has 1/2 of fuel, 1/2 of the water and the fridge running.

I took my AT 34 to the yard.... speed vs RPM and GPH was terrible ..... then, I passed under a relatively narrow bridge. I could actually see the very small waves (3-4 inches approx.) created by an incoming tide, traveling into the wind. The narrowness of the bridge accented the tide and wind effect. I went to the yard for an insurance haul out. Sort of 'out and in' the water, never leaving the slings. Coming back, the GPH was pretty close to the builder provided chart making allowances for a boat filled with all the stuff associated with being a 2 person live aboard 3/4 fuel and full water tank. (300 gal of fuel remaining and 150 gal of water)
My point is, builder supplied chart, the speed vs RPM should be viewed as a suggestion. The best chart/graph is the one you create.

 

[FF]

"Run it a hull speed for greatest fuel efficiency ."

Hull speed is NEVER the greatest speed for fuel efficiency.

It is the point where the bow wave is so large the boat can not go faster , or climb up on top.

To cruise with lowest fuel burn for the miles traveled , the SQ RT of the LWL, times .9 to about 1.15 (depending on the boats hull and loading ) is the most common.

For many its about one K below theoretical "hull speed".

Hull speed 1.34 is mostly of use for displacement wind sailors with free propulsion to understand when the boat is being pressed enough to consider sail addition as unproductive and perhaps dangerous.

 

[Action]

To cruise with lowest fuel burn for the miles traveled , the SQ RT of the LWL, times .9 to about 1.15 (depending on the boats hull and loading ) is the most common.

For many its about one K below theoretical "hull speed".
.

Thanks for this! Since I don't have a trawler, I would like to ask a question about this formula.

Say a trawler has a 36 foot LWL. The square root would be 6 feet. Times .9 to 1.15 would still be in the less than 6 to just over 6 range. Or about 6.

Would that speed be miles per hour? Knts per hour or some other measurement. I assume the variable of .9 to 1.15 would be hull design of semi-D hull versus full D hull. Or maybe weight or ??

Am I tracking at least some what correctly???


>>>>>>>>>>>>>>>Action

 

[Wxx3]

"Run it a hull speed for greatest fuel efficiency ."

Hull speed is NEVER the greatest speed for fuel efficiency.

It is the point where the bow wave is so large the boat can not go faster , or climb up on top.

To cruise with lowest fuel burn for the miles traveled , the SQ RT of the LWL, times .9 to about 1.15 (depending on the boats hull and loading ) is the most common.

For many its about one K below theoretical "hull speed".

Hull speed 1.34 is mostly of use for displacement wind sailors with free propulsion to understand when the boat is being pressed enough to consider sail addition as unproductive and perhaps dangerous.

This assumes your prop is sized correctly, right?

I have another way I look at it.
Max torque for my SP 135 is between 1600 and 1700 rpms.
Thus, that's why I run away that rpm setting, as the engine should be most efficient at max torque.

 

[sunchaser]

That's a ton of HP for that boat!

A few months ago I was looking at a a 64 GA that had the original Cat engines (current owner didn't know model) removed and JD 6068s put in. Unfortunately with several GAs the first owner specified engines that in retrospect proved wrong.

BTW, at genset speeds - 1800 RPM - the JD 6068 is designed to run 24/7. Tens of thousands doing it. I was recently on two different vessels that had NL gensets with the 6068s. One with about 8,000 hours and the other new. Great reports from the engineers.

 

[Delfin]

Thanks for this! Since I don't have a trawler, I would like to ask a question about this formula.

Say a trawler has a 36 foot LWL. The square root would be 6 feet. Times .9 to 1.15 would still be in the less than 6 to just over 6 range. Or about 6.

Would that speed be miles per hour? Knts per hour or some other measurement. I assume the variable of .9 to 1.15 would be hull design of semi-D hull versus full D hull. Or maybe weight or ??

Am I tracking at least some what correctly???


>>>>>>>>>>>>>>>Action

Knots are the correct unit of measure using the hull speed formula. The .9 to 1.15 of hull speed as a "sweet" spot applies to FD boats since SD or planing hulls have other options on defining what is a sweet spot and what is not. My son's planing Grady White has greatest efficiency defined as max distance traveled per mile of fuel consumed at around 28 knots, which would be 4x its hull speed.

 

[FF]

The formula is in Knots and varies as some boats are fat and heavy some are leaner and lighter.

5K = about 6 statute MPH .Inland charts are usually in miles , so folks prefer talking in statute.

On a boat with a tach and even a hand held GPS , simply create a graph of RPM to boat speed .

You will find every RPM increase gives a modest speed increase , until a certain point where much larger RPM increases are needed for more speed .

Looking over the side there will be a bow wave and a stern wave with a hollow amidships.

As you increase RPM the boat will begin to climb the bow wave , and the deck will angle up.

The boat may begin to leave the stern wave and the transom will sink below the water line.

That's when your mostly just warming the ocean and making waves, fun perhaps but expensive .

A Knot slower may cost 1/2 the fuel per hour.

 

[OldDan1943]

True or false, years ago, you can determine the hull speed by watching your stern wake. When the wake comes together, this is supposed to be the hull speed. I dont know if it is true. Gotta be close.