Limit on lugging a diesel....

I've been thinking about this lots.
I want to cruise at low speed most of the time, but want a lot of power in an emergency etc. Just the ability to pick up the pace if needed.
Lets say boat is 38' long, 8'6" wide, and weighs about 12,000 lbs.

Solution 1.
3 prop shafts, 2 outer powered by big diesels, center powered by 28/2 literoid 800 rpm diesel.

Solution 2.
1 prop shaft, and gearing and clutches to use either a big diesel, or listeroid on the same prop shaft.

I find solution 1 stupid, when for a smaller boat that seems like a ton of equipment, failure points, etc.

Solution 2 is also stupid, because the engines can't sit centered or balanced.



Solution 3.
Use something like a dt 466 diesel, and cruise at 800 RPM or so. Raise to 2500 RPM if you need to.

Does anyone know if propellor drag is linear to RPM??
If a propellor spinning at 1000 RPM's using 100 HP, take 200 hp to spin at 2000 RPM's ?

I ask because I know 800 RPM should be a high enough speed for an engine to stay running good, but I think the load factor would be an issue.

 

Most diesels specs allow any load at any rpm above the torque peak and somewhat less than 100% load at rpms less than that.

IMO, a listeroid is an old design that isn't best for anything.

But see other threads for discussions of slow and fast operation.

 

I will try to find some other threads.

Listeroid design might be old, but it would be nice to find something similar that could put out 30-50 hp at 600-1000 rpm's and sip fuel.

 

I used to import these,and Listeroids are not made for transportation especially boats rocking around.

I do like the idea of the chugging- but if you take off the 600 pounds of flywheels I don't think it'll run.
Even if you can get it going it'll shake your boat to pieces-literally. Making up for it with a heavy prop shaft....being small diameter you'd need a ton or more of shaft and prop.

And seeing as a 12hp weighs in at 1400 pounds...I imported a few 24/2 and I think they were 1900 lbs IIRC...how much you think a 40-50 hp Listeroid will weigh?

Get a 2 into one transmission-Cummins 4Bt on one side and 6BT on the other..

 

Do you have any examples of a 2 into 1 trans?

 

Don't know what you mean by lugging. You do not mean over propped do you? If you run at lower half of rated rpm you can be over propped and not hurt motor. If you try to run at 80% load on a new deisel and are over propped watch out. If you mean running a correctly propped motor at low rpm(under load) no harm done so long as you bring it up to high load for some time toward end of days run. This issue has been discussed much at Boatdiesel.com and the consensus of experts is never seen a deisel go south from light running and do see them explode all the time from over propping and running at to high an rpm. The dock talk about a deisel haveing to run hard all the time to be happy does not apply to modern motors if it even applyed to older iron.

 

Couple years ago:
-I saw reman. US Navy units for sale.
-A couple Taiwanese co.s made them
-many industrial gearboxes in many configurations out there.

Why not a clutched toothed belt? I've seen them in mine settings handling several hundred hp for quite long periods of time.

 

This your biggest problem.

Remove this design feature and your problems are over.

 

No, it is more or less a logarithmic curve, with some strange twists at the high end.

 

There are 2 methods of loading a high hp diesel at low RPM, to keep it alive.

A CPP will do if the spread in HP required is not too large, but may require a complete retrofit to install.

Simpler is the ZF 2 speed tranny.

For low speed operation the shaft would be sped up to raise the load on the loafing engine.

 

It sounds like you're solution is to lug the engine even harder?

I am under the impression that at low RPM, the TQ output should be far less than peak TQ RPM.

 

Run a "small" turbo diesel at an operating point where you can propel the boat in a loafing fashion without turbo boost. Prop it such that it can just barely spool up into max boost/power point. For example, run at 1000RPM off boost, with enough pitch to move at at a reasonable speed. Maybe a 15" pitch prop with a 2:1 speed reducer?

Assuming the turbo can make near full boost by 1600RPM and very little at 1200RPM, then if the difference between 30-40% throttle at 1000 RPM and 100% throttle is enough to accelerate the boat and slip the prop to get the turbo to spool up, then that should work. That should give you about a 3:1 torque difference and ~6-8:1 power difference between 30% throttle at 1000RPM and 100% throttle, plus turbo boost at 1600RPM. Assuming the on boost torque is higher than the normally aspirated max torque at low speed.

 

I think that "lug" is so poorly defined that only makes communication harder.

 

Two engines coupled to a single transmission could do what you propose.

 

"It sounds like you're solution is to lug the engine even harder?"

NO WORK the engine on its power curve.

Every engine comes with a HP curve.

Running the engine with a gear set that does not overload the engine at reduced RPM is not "Lugging" it is proper loading.

Many gensets are set up for 1800 for emergency use, 1500 for Euro 50 CPS use and 1200 for "prime power" 24/7/365.

The load is reduced as the RPM is reduced , but the RPM/HP graph is followed.

Running a 300hp engine at low RPM for 50 hp does no harm if the Mfg specks are respected,and is not "lugging".

As a matter of fact the harder working engine will last loads longer and be more efficient in fuel use.