Transmission for Cummins 6BT

What transmission would you guys recommend to put behind a 5.9L Cummins 6BT in the 260-280HP range. We have yet to confirm that HP number. We will be having it tuned to that power level if possible. The RPM limit on the tach shows 2600rpm. We're planning to hopefully cap that at 2200-2400rpm. The power and rpm may come down anyway because we are planning to install water cooled exhaust. It was dry stacked with a watercooler turbo. I can only assume the bell housing pattern is I/C because it's round.

The questions you will need to know are:

What ratio (1:1, 1:1.5, 1:2, ect.).
Do you need a straight output or a down angle output.
Left or right rotation. Not all transmissions are happy running in reverse to push the boat forward.

Ted

I have 6bt's at 210 hp using Twin Disc 5050a gears with troll valves, which have given me good service. Not sure if that particular model fits your higher-horsepower application but a dealer could tell you.

I have had Twin Discs behind three different 6bta's with no issues. Can't think what the ratios are (were) but can get that for you if needed.

Twin Disc MG-5050A Marine Gear Transmission

I recommend:

Twin Disc MG-5050A Marine Gear Transmission, it is THE way to go.

--10 degree output flange down angle is the most popular one to get.-- However, need to look at the application to be sure for you.

Bell housings are available for all the standard SAE one's - so for a Cummings I think they use a SAE # 3 bell housing.


Cummings are usually standard with right hand engine rotation, but check that too - to be sure.

You need to measure your propeller shaft angle to see where that is at as that is not easily adjustable since built into the hull & the engine mounting bed relationship to that.

NOTE: the transmission output flange needs to be set up by adjusting the engine mounting angle of installation so the propeller shaft flange & trans output flange meet at exactly same angle to bolt up.

Look at how much room there is to do modifications with the bed stringers & adjust the motor mounts as you want the engine to sit close to level, plus or minus a few degrees.

Rule of thumb is Engine needs to be under 10 degrees angle maximum from level. The closer to level the better. You usage may very with internals set up layout in your Engine room.

These transmissions input to output Ratios vary from 3 .0 to 1 for a very heavy & deep draft full displacement style hull & large big bite pitch slow turning prop and extend to down to 1.0 to 1, so need to do some figuring out what way to go there.

We will need more info to guide you as to which ratio would be best for your application. What else is needed to know is prop diameter & pitch, vessel size & displacement or Semi-Displacement style hull, type of usage, etc.

So probably expect your needs to be some thing in the middle like a 1.8 : 1 ratio maybe ?

These units are Super reliable, Work great & Last a long time.

Can't go wrong with one of these.

I will get the conservative HP ratings for this trans & post it soon below.

I have run this style trans in multiple boats over many decades of years of boating all over US waters with lots of over 300 HP running through them with never an issue.

Good Luck.

Alfa Mike

Twin Disc MG-5050A transmission

Here is more info on these HD commercial quality Marine Transmissions.

Twin Disc MG-5050A - Marine Transmission
units weigh about = 200 Lbs.
10-degrees down angle output flange is what most applications use.
these all have an interchangeable & easily replaceable SAE J617 Bell Housing
Design is a Vertical Offset, & is in an Aluminum Case for these units.
Mechanical control vale suitable for cable control – “F - & -N- & -R”
This is a very strong HD unit & conservatively Rated HP for usage as follows:
M-1 – Continuous Duty – 156 HP @ 2100 RPM
M-2 – Medium Duty – 172 HP @ 2100 RPM
M-3 – Intermediate Duty – 251 HP @ 2500 RPM
M-4 – Light Duty – 341 HP @ 2800 RPM
M-5 – Pleasure Craft – 365 HP @ 2800 RPM
Maximum Allowable Rated Engine speed 3300 RPM
Rating shown are for use with standard Right-Hand Rotation Engines

good luck.

Alfa Mike

ben2go said:

What transmission would you guys recommend to put behind a 5.9L Cummins 6BT in the 260-280HP range. We have yet to confirm that HP number. We will be having it tuned to that power level if possible. The RPM limit on the tach shows 2600rpm. We're planning to hopefully cap that at 2200-2400rpm. The power and rpm may come down anyway because we are planning to install water cooled exhaust. It was dry stacked with a watercooler turbo. I can only assume the bell housing pattern is I/C because it's round.

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I am more curious about your engine. What exact model did it start life as? If you get the CPL off of the block any Cummins dealer can look it up and tell you exactly.



The 6BT is not aftercooled and is about 220 hp. If it is a 6BTA then these are 270 if coolant cooled aftercooler and 315 and 370 if seawater cooled.


You can cap the rpm my playing with the throttle stop, but why would you do that. If you do you can never run them up to rated rpms to blow out soot or check propping at wot.



David

Thanks, everyone. I knew I would need more info but I wanted to get an idea of what transmission.



The engine is a 6BTA Cummins remanufactured engine with 350 hours on it and 2600rpm governed speed. We will leave it that way. The aftercooler was removed. Dunno why. We have the water cooled marine turbo, and exhaust manifold with a riser. The engine was in a barge and ran either a generator head or large alternator along with two fairly substantial hydraulic pumps. The engine was fresh water cooled using a keel cooler and the exhaust was converted to dry stacked. We plan to go with fresh water cooling using raw water heat exchangers.



We are hoping to get a 22 to 24-inch prop under the boat. We will need a 10 degree angled transmission. The boat hull length is 55' and 14.5' wide and approximate weight is 40,000lbs loaded. Water line length is 49.8'. She has generous flare so I'll say about 13.5 wide at the water line. The draft should come in at around 33 to 36 inches.



We are keeping it light. She is flat bottom with slight fore and aft rocker with a short keel in the rear for the shaft exit. While the boat is a planing design, she will be run mostly at displacement speeds. Cruise speed is roughly 12k. Displacement speed should be around the 7 to 8-knot range.


All of this is to get to an estimated total to get a signed contract to build the boat.

ben2go said:

The engine is a 6BTA Cummins remanufactured engine with 350 hours on it and 2600rpm governed speed. We will leave it that way. The aftercooler was removed. Dunno why. We have the water cooled marine turbo, and exhaust manifold with a riser. The engine was in a barge and ran either a generator head or large alternator along with two fairly substantial hydraulic pumps. The engine was fresh water cooled using a keel cooler and the exhaust was converted to dry stacked. We plan to go with fresh water cooling using raw water heat exchangers.

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I think you should set the throttle so the engine can't go above maybe 2,200 rpm. Why?

The aftercooler cools the incoming air and keeps the combustion temps lower than it would otherwise be. At high loads/rpms with no aftercooler your exhaust gas temps will be higher which could melt valves or pistons. Reducing the rpms to about 2,200 will help avoid this ever happening.


Another way if you don't want an aftercooler is to set the injection pump to the settings for the 220 hp engine, if it is the same pump otherwise replace it with one for the 220 hp engine so you won't ever get too much fuel delivered.


Also that engine in its previous life probably never ran above 1,800 rpm, the speed required for a generator. That is why it got by without an aftercooler.

I would talk to a Cummins pro about what you are doing with that engine. Tony Athens at Seaboard Marine in California is such a pro and he might be able to help on a paid consulting basis.

David

djmarchand said:

I think you should set the throttle so the engine can't go above maybe 2,200 rpm. Why?

The aftercooler cools the incoming air and keeps the combustion temps lower than it would otherwise be. At high loads/rpms with no aftercooler your exhaust gas temps will be higher which could melt valves or pistons. Reducing the rpms to about 2,200 will help avoid this ever happening.


Another way if you don't want an aftercooler is to set the injection pump to the settings for the 220 hp engine, if it is the same pump otherwise replace it with one for the 220 hp engine so you won't ever get too much fuel delivered.


Also that engine in its previous life probably never ran above 1,800 rpm, the speed required for a generator. That is why it got by without an aftercooler.

I would talk to a Cummins pro about what you are doing with that engine. Tony Athens at Seaboard Marine in California is such a pro and he might be able to help on a paid consulting basis.

David

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I never thought about the lower 1800rpm with the genny head. I think you're right about that. The aftercooler cooler was disposed of by the PO during the dry stack conversion. We plan to go back with full marine dress as we think this engine was intended to be. I'm familiar with exhaust temps and melting stuff. Another hard learned lesson but on a turbo gasser.



Thanks for the Seaboard tip. We may need the advice.

Mike you don't mention lube oil coolers. Will one be required?

The PO probably canned the aftercoolers because they were sea water cooled and he didn't need a sea water pump with his dry stack exhaust and radiator cooling.


If so, that means it was originally a 315 or 370 hp sea water aftercooled engine that was cobbled together for genset or marine pump use. Also I am surprised by a 2,600 rpm wot rating. I thought that the after cooled engines were 2,800 or 3,000.


You really do need someone like Tony. It won't be cheap to buy the missing parts: after cooler, r/w pump, exhaust gas injection elbow, etc to put it back into original condition. Promise to buy them from Seaboard (and they won't be any cheaper from any other Cummins dealer) then Tony may do the consulting for free. Don't think about buying Chinese parts.


David

Transmission

The engine you have definitely was run at 1800 rpm as that is the standard generator run speed for 60 Hertz A/C power here in the USA.

As I see more of what your doing, & I run some numbers over here, you definitely need to go with the 10 degree down angle Twin Disc MG-5050A transmission.

It will handle the engine HP as is now or when you up grade it to higher 365 HP neighborhood as well & it is rated for up to 3300 engine rpm.

For your displacement & design & engine I would think a transmission ratio of 1.8 to 1 ratio in the trans would be good for 12 knots at 2600 rpm with some propeller pitch tweaking depending on what you do with the engine & HP available.

Obviously you may go with the engine as it is now, which if not after cooled is actually about 220 HP & later up grade to the higher HP options you may want, at a later date.

And then you could also run all day - 24 / 7 - at 8 knots running in the range of 1600 to 1800 rpm and get great fuel economy.

Let me know if I can help.

Sounds like a fun boat.

Good luck.

Alfa Mike

djmarchand said:

The PO probably canned the aftercoolers because they were sea water cooled and he didn't need a sea water pump with his dry stack exhaust and radiator cooling.


If so, that means it was originally a 315 or 370 hp sea water aftercooled engine that was cobbled together for genset or marine pump use. Also I am surprised by a 2,600 rpm wot rating. I thought that the after cooled engines were 2,800 or 3,000.


You really do need someone like Tony. It won't be cheap to buy the missing parts: after cooler, r/w pump, exhaust gas injection elbow, etc to put it back into original condition. Promise to buy them from Seaboard (and they won't be any cheaper from any other Cummins dealer) then Tony may do the consulting for free. Don't think about buying Chinese parts.


David

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alfamike said:

The engine you have definitely was run at 1800 rpm as that is the standard generator run speed for 60 Hertz A/C power here in the USA.

As I see more of what your doing, & I run some numbers over here, you definitely need to go with the 10 degree down angle Twin Disc MG-5050A transmission.

It will handle the engine HP as is now or when you up grade it to higher 365 HP neighborhood as well & it is rated for up to 3300 engine rpm.

For your displacement & design & engine I would think a transmission ratio of 1.8 to 1 ratio in the trans would be good for 12 knots at 2600 rpm with some propeller pitch tweaking depending on what you do with the engine & HP available.

Obviously you may go with the engine as it is now, which if not after cooled is actually about 220 HP & later up grade to the higher HP options you may want, at a later date.

And then you could also run all day - 24 / 7 - at 8 knots running in the range of 1600 to 1800 rpm and get great fuel economy.

Let me know if I can help.

Sounds like a fun boat.

Good luck.

Alfa Mike

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Thank you both. As soon as we nail down a start date for the build, I will dive deeper into the specifics of the engine and setting it up. Whatever the power rating was as the engine was originally spec'd is how we plan to set it up. The six-cylinder turbo Cummins is the recommended engine. A transmission wasn't mentioned. Now we have that nailed down, I can get some prices and keep rolling with the estimated build cost.

Many of the Cummins 6BTA's are fitted with the ZF 220 or ZF 220A (down angle, I think 7deg).

Ski in NC said:

Many of the Cummins 6BTA's are fitted with the ZF 220 or ZF 220A (down angle, I think 7deg).

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I'll need a bit more than 7* to keep the engine level enough.

Just curious , whats the hull construction?

Fiberglass reinforced marine Douglas fir plywood on white oak frames. Every piece of wood will be CPESS saturated in the hull build that's not fiberglassed.

Nothing wrong with engine being a little nose up. Especially on a trawler where boat running angle is pretty flat.

What is you shaft angle relative to WL? Sounds like it might be pretty steep.

If 10deg, you can use a 7deg gear.

You can also install nose down a few degrees. Just have to make sure the cooling system vent is in the upper most point.

Our 1990 6BTA JWA's are 250HP. They came with Twin Disc MG-506-1 trannies. Seem to work fine.

Have you considered discussing this with a qualified marine engineer?
Might be a bit expensive but, there would be no guessing.

Ski in NC said:

Nothing wrong with engine being a little nose up. Especially on a trawler where boat running angle is pretty flat.

What is you shaft angle relative to WL? Sounds like it might be pretty steep.

If 10deg, you can use a 7deg gear.

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I haven't gotten the plans yet to make those calculations. Running angle should remain flat as we don't plan to overpower the hull to planning speeds.

jleonard said:

You can also install nose down a few degrees. Just have to make sure the cooling system vent is in the upper most point.

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Thanks for mentioning that. I was keeping that idea in my back pocket.

NightCrawler said:

Our 1990 6BTA JWA's are 250HP. They came with Twin Disc MG-506-1 trannies. Seem to work fine.

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Thanks for posting up. That's another option I have run into, during my research.

OldDan1943 said:

Have you considered discussing this with a qualified marine engineer?
Might be a bit expensive but, there would be no guessing.

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Good idea. Already happening. We may ask the designer/NA about some changes on engine location. Also considering a full keel option for prop and rudder protection along with better tracking.

I consider the advantage goes to Twin Disc.

The commercials seem to love them and best of all they have been in service for decades.

This means there are plenty around so a good rebuilt tranny with no core charge should save 1/2 - 2/3 or more .

With their high service life it is doubtful a recreation boat could ever wear one out.

Good hunting! Look in Boats and Harbors for rebuilders.

FF said:

I consider the advantage goes to Twin Disc.

The commercials seem to love them and best of all they have been in service for decades.

This means there are plenty around so a good rebuilt tranny with no core charge should save 1/2 - 2/3 or more .

With their high service life it is doubtful a recreation boat could ever wear one out.

Good hunting! Look in Boats and Harbors for rebuilders.

Click to expand...

BINGO! You have confirmed exactly what I have found through research and calls. These things could be rebuilt in the dirt and last. Kind of like the Detroit Diesels of transmissions.

ben2go said:

What transmission would you guys recommend to put behind a 5.9L Cummins 6BT in the 260-280HP range. We have yet to confirm that HP number. We will be having it tuned to that power level if possible. The RPM limit on the tach shows 2600rpm. We're planning to hopefully cap that at 2200-2400rpm. The power and rpm may come down anyway because we are planning to install water cooled exhaust. It was dry stacked with a watercooler turbo. I can only assume the bell housing pattern is I/C because it's round.

Click to expand...

Not a Trawler guy but i have a 32 Hatteras FB with 370 HP 6BTA Cummins
diesels. I have the ZF 220A Gears and love them. Being a fisherman they get a work out when trying to stay on a spot.

ftmorgan said:

Not a Trawler guy but i have a 32 Hatteras FB with 370 HP 6BTA Cummins
diesels. I have the ZF 220A Gears and love them. Being a fisherman they get a work out when trying to stay on a spot.

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The ZF 220A is 10 Deg

https://www.sbmar.com/docs/ZF/DS-ZF_220_A.pdfwn angle

For the length and weight of your boat you probably will need to look at 2:1 or 2.5:1 ratio and swing a larger diameter prop, larger heavier displacement vessels need torque not HP so much to maneuver properly

ben2go said:

Also considering a full keel option for prop and rudder protection along with better tracking.

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Good idea for the keel, that’s a big boat with a lot of flat bottom, you don’t want it slewing around on you.