Looking for help with Cummins 6CTA (8.3 liter medium duty diesel)

[Skillet]

Engine is going into a friend's boat, supposed to launch him on Monday.  New engine from Cummins (factory recon), that was test run at their Tennessee facility.  He's doing a diesel-electric hybrid pilot project, pretty interesting stuff if you want to check it out (https://alaskapublic.org/news/environment/2025-12-23/were-really-breaking-new-ground-hybrid-fishing-boat-prepares-for-maiden-voyage-in-sitka). 

The crux of the matter is when we went to fire up the engine, the bendix gear slammed into the ring gear on the flywheel and wouldn't engage.  Based on the marks left behind, it appears this starter is not compatible with the ring gear (ring gear too "tall" to mate with the starter bendix, or the bendix gear is too large of a diameter)?  Cummins NW has said that they "sometimes" use a crank-nose starting tool to test run the engine before all the accessories like the starter, alternator, engine guards, raw water cooling systems, etc., are installed.  So there is a chance that the incorrect starter was installed after the test run...

Curious if anybody here knows these engines inside and out (or knows a pro that could identify and sell me the correct starter and fly it up tomorrow).  We have an SAE 3 bellhousing and flywheel, 159 tooth ring gear.  The starter is (I believe) a delco-remy 39MT.  It has a 12 tooth bendix gear on it, but I think we need an 11 tooth to reduce the OD of the bendix gear and get it to engage the ring gear?

So how about it folks - any of you medium-duty diesel engine guru's, or know of one in a truck parts shop I could talk to?

Thanks in advance-

Chris

[Fastass350]

I ran into the opposite when I put the last motor in my dump truck, it’s a big cam, 855. The bell housing I had took a small ring gear/flywheel setup so I had to change that out for the larger. Fast forward to installing a new trans with new clutch and new flywheel, they sold me a smaller diameter flywheel, when bolted up it was clear that the starter wouldn’t come close to connecting. Maybe it’s something as simple? Knowing what you’ll have to go through to change that all out I hope it is. Mine has the 39mt starter.

Good luck!

[Skillet]

Thanks for the info Chris...  I'm sure hoping it's something simple like that.  Decoupling this engine/gear combo inside the engine room is going to be low on my "that would be fun" list.

The Cummins NW rep has been getting me in touch with his guys this evening, so he's going above and beyond to make it right.  Hopefully we get to a solution soon.

[Fastass350]

I sure hope for your sake it’s something simple. Sounds like you got a good rep so that’s a huge win.
Something I just thought of, is it possible it’s the wrong rotation starter?

However it plays out keep us posted

[Skillet]

I sure hope for your sake it’s something simple. Sounds like you got a good rep so that’s a huge win.
Something I just thought of, is it possible it’s the wrong rotation starter?

However it plays out keep us posted

You know what, that is a possibility I didn't think of.  I'll check as soon as I get to the boat in the am.

[Skillet]

Update-

Our working theory is that someone swapped the flywheel out after it was test run somewhere along the way, but before it shipped to us... There is no starter available for this bellhousing/flywheel/ring gear combo.  I have to give Cummins NW props on their response, however.  They are airfreighting the correct starter/flywheel combo, and they are sending two techs (all on their dime) to help get this done tomorrow.

Boat is in the water now getting the electric drive stuff commissioned. As soon a they're done,  we tear it apart again...

Lots of valuable lessons learned on this project.

[pd]

Watching from the sidelines here.  I must say that Cummins NW impresses me with their response.

[EnglishSetter]

Sounds like you have a direction.

The only Cummins guru I know is Tony Athens at Seaboard Marine in SoCal.

www.sbmar.com.

Having pulled a 72C off a 6BT in the bilge of my 26 Shamrock, yeh, not fun.

What are you using for a flex plate?  I didn't want to ever do this job again, so went with a Suredrive hub.

https://www.suredrivemarine.com/suredrive

[Skillet]

They're using something similar to the suredrive, not sure if the brand.   It came from Italy.  That setup is pretty standard up here when you get into the 200+ hp continuous duty ratings.

As an aside, I put a suredrive in my own boat when Iput in a new gearbox for exactly the same reason you describe. Original setup was a damper plate, springs were broken. Had to have the folks at Suredrive custom build my hub center to match my oddball engine and gear combo (definitely not an of the shelf part). Expensive, but worth it.

[EnglishSetter]

They're using something similar to the suredrive, not sure if the brand.   It came from Italy.  That setup is pretty standard up here when you get into the 200+ hp continuous duty ratings.

As an aside, I put a suredrive in my own boat when Iput in a new gearbox for exactly the same reason you describe. Original setup was a damper plate, springs were broken. Had to have the folks at Suredrive custom build my hub center to match my oddball engine and gear combo (definitely not an of the shelf part). Expensive, but worth it.

Mine was 6BT to a Velvet drive 72C, so standard hub. Friends called it the flux capacitor damper.

Damper cam apart getting the splines out.

I'm curious on the hybrid set-up.  I've seen "get-home" set ups using a (petrol) pony motor driving a belt driven sheave on the driveshaft.  Wouldn't work for me as I have very little exposed shaft, but........

I know diesel electric is commonplace in ships and trains, but that's largely to let engines run where they're most efficient and let the electrics buffer the ups and downs.  So I'm not sure where this technology is supposed to (as in the article) cut fuel costs by 50-60% 

Now a nice Mr. Fusion on the other hand. 

[Skillet]

I know diesel electric is commonplace in ships and trains, but that's largely to let engines run where they're most efficient and let the electrics buffer the ups and downs.  So I'm not sure where this technology is supposed to (as in the article) cut fuel costs by 50-60% 

Now a nice Mr. Fusion on the other hand. 

Good observation, and I share your skepticism.  The main benefit that I keep hearing about is the ability to run under electric power during extended low power consumption activities - ie., slow speed trolling and hauling back most longline gear.  To be fair, this low power operation accounts for roughly 80% of my own boat's annual run time. However, I think the fuel saved by periodically running a 30kw genset at max efficiency to charge the battery bank (this is the main idea - run a smaller diesel more efficiently, less often) vs just idling along with a traditional main engine will be negligible.  The thing most often forgotten in these schemes is the power loss due to multiple energy conversions.  Going from rotational power to electricity, then back again to rotational power is accepting two large conversion losses.

I may be wrong about the overall viability.  Fishing styles, gear types, relative efficiency of the main vs genset at low power outputs, all come into effect.  There may be a situation where this makes a lot of sense.  I can't yet see it for my operation, or Jeff's.

However, if nobody ever experimented with technology, we'd still be sharing pics of our hunts on cave walls.

[EnglishSetter]

I know diesel electric is commonplace in ships and trains, but that's largely to let engines run where they're most efficient and let the electrics buffer the ups and downs.  So I'm not sure where this technology is supposed to (as in the article) cut fuel costs by 50-60% 

Now a nice Mr. Fusion on the other hand. 

Good observation, and I share your skepticism.  The main benefit that I keep hearing about is the ability to run under electric power during extended low power consumption activities - ie., slow speed trolling and hauling back most longline gear.  To be fair, this low power operation accounts for roughly 80% of my own boat's annual run time. However, I think the fuel saved by periodically running a 30kw genset at max efficiency to charge the battery bank (this is the main idea - run a smaller diesel more efficiently, less often) vs just idling along with a traditional main engine will be negligible.  The thing most often forgotten in these schemes is the power loss due to multiple energy conversions.  Going from rotational power to electricity, then back again to rotational power is accepting two large conversion losses.

I may be wrong about the overall viability.  Fishing styles, gear types, relative efficiency of the main vs genset at low power outputs, all come into effect.  There may be a situation where this makes a lot of sense.  I can't yet see it for my operation, or Jeff's.

However, if nobody ever experimented with technology, we'd still be sharing pics of our hunts on cave walls.

30kw = 40 hp with no losses.  I have to believe the losses will be substantial.  Electric motors can be torquey, but I'm expecting 10% loss at the motor and another 10% for charging + any rotational losses so we're down to say 30 hp at the prop.

I dunno what it takes to troll a 50' boat with gear set and of course that will be influenced by current and wind.  Just my WAG, it will take the genset running full tilt to keep up, which kinda gets me back to the "get home" pony motor to perhaps a clutch driven chain drive on the main shaft.  But nobody granted me 6 figures to come up with something better.

Keep us up on the project though.  It's interesting.

[Skillet]

*edited some efficiency numbers because... math is hard.


You're on the right track.  There's a few more things to consider.  Fair warning, I'm about to nerd out here....


This is a series hybrid, meaning the electric motors are connected to the gearbox via what would be considered both a "PTO" and a "PTI" (power take in).  There are 2 ea 20KW motors attached.  The running modes for the boat are- 

~Non-electric:  Traditional running from main directly coupled to prop shaft via gear.  PTO disengaged.  His typical running speed is around 7 knots with the ICE engine.  Full throttle speed is about 9knots. 

~Fully electric:  Main decoupled from gear, PTO engaged.  Maximum of 40KW of power output.  In testing this week, we found this will push the Mirage to 5.7 knots in calm water, no gear in the water. 

~Hybrid Propulsion: Both main and electric motors are coupled together, with the electrics are adding to the propulsion power.  In a boat, this is virtually unnecessary, since we don't have hills to climb and displacement hulls only go as fast as they can go.

~Hybrid Charging:  Both main and electric motors are coupled together - but the electric motors are driven by the main and become charging units.

The dynamic charging capabilities along with the massive 250 KW battery bank is the secret sauce.  The battery management system will, based on projected usage that it will learn over a period of time, know that when Jeff is trolling for coho he's consuming roughly 30 KWH of power (WAG guess).  He'll start every morning topped off at 250KW of juice, and the rest of the 16 hour day will be managed to end up at the optimal amount of KWH of stored power (based on available charge time overnight, time running to and from the achorage, etc.).  To simplify, lets say he drifted overnight and wakes up on the fish ready to put in 16 hours of trolling and battery management wants to take the bank down to zero.  Since he starts with 250KW and he'll burn 480KWH of power, he'll need to add 230KWH of power at some point throughout the day.  The battery management system will decide if that's either via genset alone, running the main in hybrid mode to charge with the electric motors, or both, in order to optimize fuel efficiency. 

Longlining is a more likely use case, as you typically spend 3-4 hours hauling back (low power usage case, mostly just hydraulics and no propulsion as the boat is "pulled" along the groundline"), with a 30 minute full-throttle run to either reposition to set it back or to pick up another set of bouys.  Jeff will set the BMS to "hauling groundline" and then the computer will anticipate something like a 1:7 ratio of full speed running to no propulsion needed, and manage accordingly. 

The BMS (battery management system) has full capabilities to start and stop the genset, the main, couple and decouple the main from the gearbox, etc. 

Added to all of this is all of the pumps on board are now "on demand" electric drive.  The hydraulic pump is now a load sensing design instead of the traditional gear pump/pressure relief valve system, steering pump on demand, the deck water pumps have multiple switches around the boat to turn on and off, etc.  All in the name of fuel savings.

But how much fuel can we save?  Work still needs to be done, and it doesn't matter where the source of the power in a fishing boat comes from, it is all generated from burning diesel.  The optimal thermal efficiency for a turbo diesel engine is in the 60% range.  Run a turbo diesel under the boost curve, however, and it goes way down.  Something like 45%.   So ideally, all of this expense will be made up by savings realized in the delta in fuel efficiency between low power requirement and high power requirement operations - a theoretical 33% increase in efficiency (going from 45% efficient to 60% efficient is a 33% increase, for you guys with your socks off right now...). 

As you pointed out though - converting power from one form to another doesn't come free.  Changing rotational power to electricity is commonly cited as 90% efficient, and converting electric power back to electric is only marginally better - but let's just say 90% as well.

If you take that theoretical max thermal efficiency at 60%, convert it two times (90% and 90% again), you're down to 48% efficient... almost exactly what a turbo'd main engine does on its own at low power requirement...  That means a theoretical maximum gain of 6.25% here (going from 45% to 48% efficient).  If everything is running perfectly.

At $5/gal diesel, a 6.25% gain will take about 800,000 gallons of fuel to break even against a $250,000 installation like this.  On my boat, with a seasonal average of 4 gph, I'd have to run about 200,000 hours with no break downs, battery replacement, etc...  That's about 50 years of getting after it with 4k hours per year of run time.

I know the article said 50-60% fuel savings - but they sure didn't interview me!   

As an aside, hybrid systems make a lot of sense in some applications.  Edison Motors concept for a logging truck is one.  They use the stored energy of the load (logs) high up in the mountains and capture energy via regenerative braking coming downhill to charge the batteries, and then go back up the hill light using relatively little power.  Locomotives are another good use case, as the electric motors allow 100% power output at virtually any rpm to get that massive load moving and keep it moving through varied terrain.  And plug-in electric is a relatively very efficient way of powering anything vs onboard ICE engines, but their use cases are very narrowly defined. 

But boats on the ocean... I'm not sold yet.

[EnglishSetter]

Thx much for the time/effort to learn some inside scoop.

Overall, I'm a fan of hybrid.  I have a Hybrid Highlander (non plug in).  Whatever Toyota did (they call it synergy) just plain works in a seamless fashion.  The hybrid battery is small and has very limited travel on electric only and can only do about 25 mph on level ground before the wind load requires the ICE.  But it does give it some more pep off the line while limiting the "city penalty" on mpg.  But...the highway mpg for the hybrid and the straight ICE models is effectively the same.  The stop/start of the ICE is barely noticeable (it ain't no CTA...haha) and with the electric propulsion motor turning the ICE vs a small starter motor, there's no starter wear.  I can "idle" on electric or ICE via the computer,  damn near forever.  Nice when stuck in traffic or say you needed to overnight with the HVAC running.

What's the battery technology?  What's it weigh?  I recently upgraded my RV 12v system to 2.4kwh LifePo4 from AGM.  More compact, 1/2 the weight and at least in theory nearly double the run time coupled with several times the lifespan.  Upgraded the roof solar at the same time.  There's still some questions that need to be answered by application, but goal is to increase my boondock time without using the genset.  Loads have not been changed at all.

I've long felt the perfect "commuter" vehicle (and off-road) was diesel hybrid, but the small automotive diesel has a grim future in USA (VW kinda put the final nail in that coffin).  And while long being a diesel fan, the current emission requirements have soured me.

[Skillet]

No problem, fun to get in the weeds on this stuff when I have a little time. 

I believe it's LiPO4 as well.  Weight doesn't matter in our world, as long as it's relatively low and evenly distributed.  Space is a big issue, however. Jeff had to rebuild his cabin interior to house most of the bank.  I know he's sleeping on about 150KW of them in his main cabin, and almost needs a ladder to get in and out of his bunk now .  I think the other 100KW are under one settee bench and the raised floor of the settee.  He gave up a lot of dry storage for these batteries.