Limit on lugging a diesel....

We need to determine --lugging!!!

 

Here are some ways to run a diesel that are abnormally bad for it:

Running rich
Running at max power output for many hours
Running cold (or too hot)
High RPM
Running with loads so low that the rings don't seat well

 

http://forums.tdiclub.com/showthread.php?t=151647

 

I had thought of this, but laying the engines out would be weird.

 

I had thought of this, but laying the engines out would be weird.

OTS,, Off The Shelf , two engines side by side with manual lever to disconnect either engine underway.Must shut down to reset , dog clutches.

Look on any landing craft built since WWII.

 

Yes but one engine is way bigger haha.

 

For the purpose of this discussion,
I would label "lugging" as:

"Operating a diesel engine at a power output that is not suitable to the RPM it is operating at, possibly causing engine damage or reducing the engine lifespan. "

 

This is still subject to interpretation.

 

"Not suitable" is vague enough that the arguments may never end. I suggest referring to specific RPMs and loads - for example, "100% load at an RPM less than the RPM at the torque peak" is clear. Speaking of which, I don't know why engine manufacturers say that this is not recommended. For example:

CAUTION: Operating the
engine at full throttle below
peak torque will shorten
engine life to overhaul,
can cause serious engine
damage, and is considered
engine abuse. Do not
operate the engine at full
throttle operation below peak
torque rpm for more than 30
seconds.

 

Where is that from,--- Perkins P6 1975

Problem is there is many engines now different as chalk and cheese.

 

Cummins for sure, perhaps others too.

I did some more research and indeed, some engines show a large increase in wear at high loads and low rpms. Ie, 1000 rpm at 100% load can exceed the wear at max output/max rpm. Probably a good idea to not exceed 50% load at rpms less than the torque peak rpm. Mid rpms and ~80% load typically gives best economy and lowest wear.

 

Perfect--- it means don't do it.

The design requirement are not realistic. Do you know how much a twin input gearbox will be yet no space to put the engine x 2 anyway.

Isnt that enough to consider the design un practicable.

The design is a contradiction of engineering possibilities.

 

The people who sell motors often demand sea trails before awarding a warrenty. They are looking for overproping and thus overloading, and if pressent no warrrenty since this is a sure way to ruin a motor. This particularly applies to new motors with high HP/displacement. True if overepropped and run at low load the motor will not be harmed to any great extent, but why would you do that when you can prop right and have full power available when needed? If you want to learn more about this go to Seaboard marine on net and look at articles under Tony's tips about this issue.

 

I have no idea what kind of boat (displacement, semi displacement or planing) we are talking about, but generally the power taken by a propeller increases to 2-3 power of rpm. 3rd power is the maximum and that would happen in bollard pull or with high slip (overpowering a displacement boat). Thus 200 hp at 2000 rpm translates to 12-50 hp at 1000 rpm.

If by lugging you mean taking too much power at low rpm, that is very unlikely to happen with a propeller that is somewhat properly dimensioned for running at full power. A typical diesel will have at quite low rpm the same or higher torque than at full power rpm. Thus maximum power will be over the linear line drawn from zero rpm to full power rpm. The only exception to this might be a high speed planing boat running between planing and displacement speeds. Then you may need rather high power at very low rpm.

Take a look at the specs for DT 466: http://www.astleford.com/pdf/dt466dt530brochure.pdf

For the 195 hp model you have the same torque at peak power (2300 rpm) and at 1150 rpm. Even at 800 rpm you still have about 85% of that. Now if you dimension your propeller so that you can reach 2300 rpm and take all 195 hp, you will only need 8-24 hp at 800 rpm while the engine could produce about 60 hp. Thus you are well below 50%.

The problem is more likely that you are not taking enough power from the engine. Some say this will cause problems through glazing etc. http://www.boatdesign.net/forums/diesel-engines/underloading-diesel-39966.html

 

True if overepropped and run at low load the motor will not be harmed to any great extent, but why would you do that when you can prop right and have full power available when needed?

The usual first reason for overproping (a cruise prop) is the engine was selected by the marketing folks , not the boat designer.

Second is some larger lawn implement or pickup truck engines are cheaper than a proper sized HD marine built engine.

AS the boat assembler must please the eng mfg in terms of full throttle rpm, the result can be really inefficient operation at cruise , less than WOT.