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Bertram 42 Detroit 6v-92TA Repower with Yanmar or Cummins

Discussion in 'Bertram Yacht' started by etang789, Jul 5, 2017.

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  1. etang789

    etang789 New Member

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    I have a 1986 Australian Bertram 42 with original Detroit 6v-92TA 475hp engines. The boat has been with the family since new, and for sentimental reasons we decided to keep and repower it.

    The boat is currently in Hong Kong, a place that repower is not common practice here and information is limited and inaccurate locally.

    After a few rounds of price quotes of different brands, I am down to Yanmar and Cummins. Yanmar being slightly cheaper.

    My main question is which gearbox should I go with either Yanmar or Cummins engine? Here ppl suggest to go with ZF286A gearbox with 2.2:1 gear ratio, I doubt it is the optimal since I had a 2.0:1 originally?

    Yanmar 6LY3-ETP (480hp/3300rpm)
    http://www.yanmarmarine.com/Products/Light-Duty-Commercial-SOLAS-Engines/6LY3-ETP-348/

    or

    Cummins QSC 8.3 (493hp/2600rpm)
    https://cumminsengines.com/showcase-item.aspx?id=70&title=QSC8.3+for+Marine overview
  2. rcrapps

    rcrapps Senior Member

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    The Cummins and down angle 2.2:1.

    I have worked on those 8.3s. I believe they will offer more service life over the smaller Yanmars.
    Yanmar 5.8L vs Cummins 8.3L replacing Detroit 9L.
    I'd almost prefer the de-tuned QSM11 over all.

    2.0: vs 2.2:1 is hardly a change and not worth the text t0 worry about it. Of anything, it will help.
    Just your weight reduction in the engines will require prop adjustment any way.

    Have the shafts examined very carefully, Repeat...

    The down 7 degree angle needs close attention. I luv down angle clutches. Just not to much. If its a Bertram, I'm pretty sure it's over 7 degrees but make sure and this will make for a great and roomy install.

    Where is your gen-set? Old Oh-No? Aft center or forward stb? You may need to think about replacing it also and where. Your removing a lot of weight, the ships trim will change.
    Last edited: Jul 5, 2017
  3. rcrapps

    rcrapps Senior Member

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    We have a YF contact and Bertram owner in Hong Kong. Hopefully he will pick up this thread and may be helpful in the area.
  4. PacBlue

    PacBlue Senior Member

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    What is the current gear ratio on the DD 6V-92?
    It is a 2300 rpm and you should try and match the propeller rpm with the new engine in order to use the same running gear. You also want too make sure to size the new shafts correctly for Diesel engines, with the right shaft material and safety factor of at least 5.0.
    Propellers get a bit trickier with different engine power curves, but you should be ball park on diameter by keeping the prop roms the same, for the relatively same bhp of the three, and some minor adjustments on prop pitch and/or cup.
  5. Wl3175

    Wl3175 Member

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    Can't comment on Cummings engines but we repowered a Huckins with yammer 480 hp and they have been flawless aince new .just did the 1000'hr service and they were perfect . We did also change shaft,p bracket ,props etc .
  6. etang789

    etang789 New Member

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    The current gear ratio is 2.0:1. The current shaft should be no problem, but will service it for sure.

    The Twin Disc dealer here suggests MG-5065A to go with the Cummins. What do you guys think? And they also mentioned about getting some flexible coupling between gearbox and shaft.
    Last edited: Jul 6, 2017
  7. PacBlue

    PacBlue Senior Member

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    2300/2.0 = 1150 shaft rpm.
    For the Cummins, 2600/1150 = 2.26 gear ratio.
    So continue with a gear as close to 2.26 as possible. ZF will be lighter. Not sure if you have enough room for a flexible coupling and you certainly do not need it, but make sure to have the shafts checked for straightness and the ends checked for run out during the process.
  8. etang789

    etang789 New Member

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    Yea most likely getting the Cummins now.

    The generator is at forward stb now. But I want to relocate to aft center if I get a new smaller generator. right now it is a 12kw

    Thanks for the input seems like the ZF 2.2:1 gearbox makes sense here. Let me double check my old gearbox ratio again.
    Last edited: Jul 6, 2017
  9. PacBlue

    PacBlue Senior Member

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    I am surprised that the 6V-92 has a gear ratio of 2.0/1, would have expected more like 1.75 or even 1.5.

    I would double check just to be on the safe side.
  10. etang789

    etang789 New Member

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    Engine Model 80627401 SPEC
    475HP at 2300RPM
    Max RPM NL 2500

    Just went to check the Bertram today it has

    Allison Transmission
    Part No. 23015127
    Model No. M20L
  11. PacBlue

    PacBlue Senior Member

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    Do you have a picture of the Allison gearbox name plate - it should have the gear ratio stamped in place?
  12. RER

    RER Senior Member

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    Every Allison M20 I've ever seen is 2:1 ...M15 1.5:1
  13. etang789

    etang789 New Member

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    Many thanks!

    Does these Allison Transmission have any down angle degree?
  14. PacBlue

    PacBlue Senior Member

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    I do not think it is a down-angle gear but there is a horizontal offset, as the input shaft centerline from the diesel end is higher than the output side of the gearbox flange centerline.

    It is best to make a jig or mock-up of the original versus the new engine to understand if there will be any engine stringer or engine mount modifications necessary, and there usually is.
  15. rcrapps

    rcrapps Senior Member

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    The Alison gears rely on a special flywheel. I can't see it working on a newer motor. Alison gears are serviceable but the original company is long gone.
    IMO, get a new clutch.
  16. PacBlue

    PacBlue Senior Member

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    Yes, the assumption is that the OP will be replacing his 475hp DD 6V-92TA and Allison M20 gearboxes with a 493hp Cummins QSC 8.3 and either a new Twin Disc MG-5065A or possibly new ZF gearbox.
  17. etang789

    etang789 New Member

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    Yes exactly. I want to see if the newer Twin Disc or ZF gearbox with down angle can get some extra room vs the old Allison (not knowing if it already has down angle or not)
  18. rcrapps

    rcrapps Senior Member

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    Allison gears did not have down angles . They used drop cases when needed on much larger power options. Berts and Hatts usally used stright clutches in line with the crank.
  19. PacBlue

    PacBlue Senior Member

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    Are you looking for extra room as in height? As stated, the twin disc gear will be a "drop down" or vertical offset from engine crank centerline to output flange centerline.
    The down angle you are talking about may allow for a more level engine installation measure for an aft if you need it. There are technical requirements for all these parameters, and a lot is dependent on stringer height, shaft angle and the centerline of the shaft coupling distance as measured from the interior hull bottom. That is why I like a mock-up before final selection of all your gear, especially to understand the new relationship of your Cummins engine mount brackets, gearbox mounting brackets and their resilient mounts. You have some geometry homework
  20. etang789

    etang789 New Member

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    Yes I am trying to understand the height relationship when down angle is used as well as if the new gearbox if have the drop down offset vs the old one. The current engine room height is already very tight, but knowing any new engine choice I have already smaller than the current Detroit 6v-92TA, only questions I have is gearbox offset and stringer height.

    How do you usually do a mockup?

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