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37 Bertram 6v92 550HP shaft size

Discussion in 'Bertram Yacht' started by Breckster, Aug 17, 2018.

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  1. Breckster

    Breckster Member

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    I have a 1986 37 Bertram w/ 6v71’s @ 435 and 2” drive shafts. I’m getting ready to to repower with 6v92’s (550’s) and would like to know if they ran the same 2” shafts and same twin disc transmission(s) as the 6v71’s (435) and 6v92’s (450)? I realize I’ll have to re-prop the boat as well but just wondering if much was changed between the 450’s and 550’s. From what I’ve gleaned from my manual and one I’ve downloaded from the internet, the 6v71 and 6v92 @ 450 are identical as far as transmissions and propping....

    Other info: (always a) fresh water boat gets used about 70-100 hrs a year, Detroit’s and people to work on them are easy to come by in Michigan. ...and yes I would love to do a Cummins C conversion but figured a swap to 92’s would be the easiest and least expensive route for the limited use the boat gets 1350TT since new (1986)!
  2. PacBlue

    PacBlue Senior Member

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    A swap to rebuilt 6V-92's would not be advised. Too much weight and will negate the extra 100hp. I would never consider it.

    A Cummins ReCon engine is the real way to go. Either an 8.3 or a 6C. The weight savings alone will give you a bump that is not available with the Detroit's.

    After looking at their ReCon site, I would probably lean towards a ReCon QSB5.9 at 480hp.

    https://cumminsengines.com/showcase-item.aspx?id=330&title=QSB5.9+ReCon+for+Marine&#overview
    Last edited: Aug 17, 2018
  3. Breckster

    Breckster Member

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    The 37’s came with 92’s from 87-93 so I know the weight won’t be an issue and I don’t think the QSB’s will have enough torque without some serious gear reducing??

    I have to rebuild my 6v71’s and have set of 92’s avail and figured they would be the “easiest” route to take being the they are almost identical to my 71’s.... transmission, eng instr, etc etc..
    Last edited: Aug 17, 2018
  4. MBevins

    MBevins Senior Member

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    Yes b

    Yes..but..if you're putting the 92's in because you can get them and not because of the bump. Wouldn't it be a lot easier to just rebuild the 71's while in place?
    Swapping any engine for another type is never straight forward in a boat.
    The labor I'm betting is about the same.
    So if replacing make it worthwhile from an upgrade perspective for resale and fuel savings.
    You'll get neither of those with a 92 swap.
    Just my opinion.
  5. PacBlue

    PacBlue Senior Member

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    The 92's in the 37 were way too much iron and killed the engine room space. It was a bad engine match.

    We had a 38' Blackfin SF with 6V92-TA's (550hp) and it was a terrible match as well - loud, sucked too much fuel, overwhelmed the engine room , and had trim issues as the aft fuel tank was consumed, etc. It is just not the right engine for a 38' SF, no one today would select it if it were still available. There are blogs about the Blackfin's having success with Cummins repowers and note that the Blackfin President at that time came from Bertram.

    A 6V-92TA weighs at least 3,000 lbs. without gear. A Cummins QSC8.3 500hp weights 1.975 lbs. without a gear. A Cummins 6B 430hp weighs 1,570 lbs., a Cummins QSB5.9 480hp weighs 1.470 lbs.

    Weight is your 38 Bertram's enemy, not its friend. If you want to rejuvenate the boat and have more than what you started with, the Cummins is the way to go. If you have a budget that allows for only a rebuild, keep the costs down and go with the existing 6V-71 rebuild and save all the "minor" issues you will encounter with changing to a 6V-92 that will eventually run into much more money than you anticipated.

    As far as the shaft question goes, what gear ratio do you have?
  6. Breckster

    Breckster Member

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    ^^^ very true. I feel the 71’s are a tad “small” for the boat and thought 92’s, with the extra torque and power would do the trick?

    I’m just try to do my “due diligence” before anything is started in motion... :)

    ...and appreciate ALL the insight
  7. Breckster

    Breckster Member

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    Twin Disc m507’s (?) 1.5’s turning 25x28 wheels..
    Last edited: Aug 17, 2018
  8. Pascal

    Pascal Senior Member

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    The lack of use indeed makes it hard to justify a full repower.

    A prop shop can do the shaft safety factor calculations for you, you may be able to use the same 2” but may have to get stronger grade material. That was the case with the repower of my 53 My, had to use AQ22HS to stay with 2”

    Another consideration is engine mounts. Can you reuse the engine beds with the the 92s? If not new beds and mounts aren’t cheap... don’t ask how I know

    And of course What conditions are these 92s in? Which needs to be balanced with their cost. Saving a few bucks now but having to rebuild them in a couple of years wouldn’t be a good solution

    Finally you have resale values. More and more buyers don’t want Detroits and if you re not sure you ll keep the boat for many years, the money spent in upgrading to 92s will be wasted.
  9. Breckster

    Breckster Member

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    the 92’s would be rebuilt, and my 71’s are already being planned(sold) for another boat... (if I start this crazy project/idea).

    I do believe my 2” shafts are AQ22’s, for when we changed out my cutlass bearings I believe I looked into that.

    Bertram, I think, only put the 6v71’s in the 37 for one year, 1986? Then put the 92’s @450 from 87-9?? with 1992-3 being the 550’s.

    As mention before, would love a set of Cummins QSC’s at 500-600 but I know the associated cost of motor mounts, transmission etc etc will be considerably more. Easier to justify if I could use the boat more than 2.5 months out of the year :(
    Last edited: Aug 17, 2018
  10. PacBlue

    PacBlue Senior Member

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    The shaft calculation is easy and can be found at Western Branch Metals website: http://www.wbmetals.com/shafting.asp

    The calculation is:

    upload_2018-8-17_10-37-48.png

    You want to have a Safety Factor of at least 5.0 for Diesel Recreational boats. the other properties are in their catalog, with the yield in torsion for AQ 22 Shafts at 70,000 psi for 2" diameter.

    Your Safety Factor for the 6V-71TA rated 435bhp @ 2300rpm (1.50/1 gear ratio) works out to 6.34 for 2" diameter AQ22.

    Your Safety Factor for the 6V-92TA rated 550bhp @ 2300rpm (1.50/1 gear ratio) works out to 5.01 for 2" diameter AQ22.

    Good news for you since you would not need to change shaft diameter (as long as it is AQ 22).
  11. Breckster

    Breckster Member

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    Thank you PacBlue :)

    figured I would have about 100k into a QCS conversion and less than 30k for a 92 swap?

    I had heard that the 550hp 37 Bertram’s where “rocket ships” in their day; 27knot cruise?
  12. PacBlue

    PacBlue Senior Member

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    $30K/12 cylinders is $2.5K per cylinder for rebuilt 6V-92TA's. Quire possible but I would only use a DD Distributor, with DD parts, maybe W.W. Williams in your area? There will be some unseen costs, but that will be discovered during the swap.

    For $30K you most likely will not see a re-sale bump down the road. For the $100K investment you will have a rejuvenated boat, quieter, more fuel efficient, smoother, faster and a better overall "feel". This will translate to some kind of re-sale bump down the road, call it $30K, so the difference is coming down a bit.

    I have to admit though, for 1350 total hours since 1986, it would be hard to justify the extra $50K - $70K. Assuming that you are the original owner, glad to see you keeping up on her. Walker Airseps will go a long way towards keeping the bilge clean from blow-by oil mist.
  13. PacBlue

    PacBlue Senior Member

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    As far as speed goes, here is the 1986 Boating magazine published test with 6V-71TA's:

    upload_2018-8-17_13-39-30.png
    You should be able to compare your boat numbers to the chart above, but I would say a 27 knot high cruise at 2100 rpm is possible with the 6V-92TA's, although I like to cruise these Detroit's no higher than 1950 rpm myself for better longevity. Keep in mind that fresh water (Great Lakes) speeds are less as you displace more than boats in saltwater.
  14. Breckster

    Breckster Member

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    Perhaps I have “light” boat, scales say it is 25k? At 1900 I’m at 24-25 knots and 2300 + I’ve seen 32 knots.

    In all honesty, it probably would be best (economically) to do a rebuild on current engines and/or buy a newer faster boat :) 531D3082-A5DF-41E6-8997-3558A6C12139.jpeg
  15. PacBlue

    PacBlue Senior Member

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    That's a really nice looking 1986 Bertram, your pride of ownership really shows well. Freshwater and Winter storage are easy on FRP boats when compared to all that saltwater and UV robbing Florida sun!
  16. Breckster

    Breckster Member

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    Thank you... it was sold new here in Michigan (Harbor Springs) and has stayed here ever since (probably equivalent to 3 Florida summers ;-). We are the third owners and love how she rides. I don't mind the noise, fuel burn and general "things" that come along with DD ownership and thought a bit more HP (200'ish) would make us like it that much more? Being the motors are getting near the end of their usefulness I presumed that perhaps converting to 92's instead might be an easy viable and economical answer for our short summer boating needs. I'm able to rebuild them (mostly) by myself and already have someone wanting my 71's, it's just a matter of the yard doing the "grunt work" of lifting those hunks of IRON out and in... and I'm sure a multitude of "other" things!
  17. Breckster

    Breckster Member

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    86BDF860-E254-4854-A259-F652B90D67A6.jpeg
  18. Pascal

    Pascal Senior Member

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    Repowering with QSCs factory recon is going to be a lot more than 100k by the time you redo the engine beds, shafts etc. but you will get more HP and shed 2500 lbs or so

    I went thru this and ended up replacing my 8V71s and installing 430s C series recons. Huge difference but not cheap. I just didn’t want to spend money on rebuilding the DEtroits and Alisons. That would have been about $50k when for another $25k I could get “new” engines and gears. Plus about $15/20 for the beds and shafts
  19. Breckster

    Breckster Member

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    ....still cheaper then a 40 Cabo convertible, I'm trying to convince my CEO/CFO/wife we "need" :)
  20. PacBlue

    PacBlue Senior Member

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    Here is the torque comparison for conversation sake:

    upload_2018-8-17_16-47-27.png