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Bertram 42 prop

Discussion in 'Bertram Yacht' started by Mpetrel, Jun 21, 2016.

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  1. Mpetrel

    Mpetrel New Member

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    Guadeloupe
    hello
    I have a bertram 1981 42ft in Guadeloupe frensh west indies
    We repower the boat with 6v92 550hp each
    Today we have 3 blades prop 30x30 i cruise at 2000 rpm at 17knots
    We want to know what is the best prop for this boat
    We realy want to Cruise at 20knts
    Thnks
  2. Capt J

    Capt J Senior Member

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    Gear reduction? WOT rpms? Tower?
  3. Mpetrel

    Mpetrel New Member

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    Jun 21, 2016
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    Location:
    Guadeloupe
    2:1 gear ration
    WOT RPM 2350
    No tower
    Weight 38000 lbs
  4. Capt J

    Capt J Senior Member

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    What speed are you getting WOT. You might get more speed with 4 blades because they'll give you more stern lift. I just don't know. I would email a few good propellor shops and see what they think.
  5. Mpetrel

    Mpetrel New Member

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    Location:
    Guadeloupe
    Now top speed 20/21knots
    I have 2 prop 4 blades in stock next week i will try them
    We will see what happen
  6. Bahma

    Bahma Member

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    The WOT revs should be the same when the throttle is fully wide open at the dock, as when running in calm water, fully loaded with full tanks, crew and a clean bottom.
    Ideally the revs should be measured with Laser rev counter aimed at the engines shaft. (Rev counter instruments are notoriously incorrect).
    If the revs while running cannot reach those at the dock.....the props should have their pitch reduced enough to allow the revs to increase.
    Not doing so will overload the engines and cause early major engine failure.
  7. PacBlue

    PacBlue Senior Member

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    Agree that it helps to confirm your Tachometer readings by measuring the actual shaft rpms.

    But, the No Load rpms taken at the dock in neutral will NOT match the WOT rpm taken under load. I believe No Load rpms will be around 2450 while Loaded rpm underway should be targeted at 2350 rpm, as these engines are rated at 2300 rpms.
  8. PacBlue

    PacBlue Senior Member

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    Correction, No Load rpm is 2500 rpm.

    That is for an engine in a perfect state of tune, perfect rack settings, perfect throttle linkage, no play, etc.

    Attached Files:

  9. Bahma

    Bahma Member

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    As the dockside WOT is a speed controlled by the governor, then the revs should equal those for which the engine is rated.

    If it does not, the control linkages etc sbould be checked to make sure the throttle is being fully opened.....in any event this test will not be accurate unless the engine freely revs to its rated max.

    Likewise, with a fully laden boat,..... fluids......crew.....supplies,....clean bottom.....undamaged props, etc...it must also rev to it's rated speed at WOT, on the open water.

    If it does not, then the engine is being overloaded, not only at WOT, but at all revs throughout it's range.....this will dramatically reduce it's useful service life.

    This did not matter too much with the older Jimmys and Cats, which were massively over built and inder stressed......however, as these engines became more powerful by turbo charging etc they too did start to suffer early failure.
    Of course, with the modern high speed, high powered engines, it is even more important that they are not overloaded by turning a wheel with too much pitch.
    This problem is misunderstood and many have suffered the expensive consequences as a result.

    Boatdiesel.com discusses this at length.......I'm sorry if I do not explain it properly.
  10. PacBlue

    PacBlue Senior Member

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    Check page 2 under the Configuration section - Maximum No Load Speed is 2500 rpm. Have tested this a few times on our previous boats powered by the same 6V-92TA's. The Overspeed Limit Protection for the Mechanical Governor is set at 2500 rpm. This is not the same as WOT under load. The maximum fuel delivery for WOT is set at 2300 rpm. Hence the difference

    Misinformation abounds on the internet, boatdiesel.com or an interpretation thereof.
  11. RER

    RER Senior Member

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    How many times are you going to post the same incorrect misinformation on this site?
  12. Bahma

    Bahma Member

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    Largely because it is not incorrect....

    The OP asked how he after repoplwering his Bertram could get another 3 kts by changing his props. I confess to having no personal knowledge of the 6V92, however, i do know it has very high bearing loadings and must be treated with great care if the OP wishes it a long life.
    As am also island man, I am well aware of the very high premium we pay on all items, paricularly marine diesel engines.

    All I am doing is to try to make him fully aware of the implications of over loadung his engines.

    We gather our knowledge by years of experience, from knowalls at the dock, ethereal individuals on the net, and goodness knows where else.......it is up to us as individuals to separate the chaff from the seed.

    My 42ft Bertram Flush Deck was repowered with Yanmar 6LY3s 470 hp,
    They reach their WOT speed at both dock and at sea. That speed is governed by the engines themselves. She maxes out at 27 kts, runs comfortably at 22kts at 3000 rpms. I don't make a habit of running that fast, however, I know that my engines are not being overloaded at any revs, because it meets the criteria I list, and which i believe, from my experience of over 60 years of owning and servicing large power boats, is correct.

    That you dissmiss my opinion as wrong, smacks more than being somewhat timerous.

    I sincerely hope to OP will let us know the results of his sea trail with the four bladed props, and that they will allow the engines to turn at 2300 revs with his boat fully loaded.

    All i can say is if they cannot reach that figure..... IMO...his engines are over worked and will, more than likely, fail much sooner than he would like.

    Incidentally, i do not understand your ....comment... check page 2 under the configuration section?.. where.???
  13. PacBlue

    PacBlue Senior Member

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    There is a 2 page attachment in Post #8 - the Marine Engine Specification Data for the 6V-92TA, rated 550 bhp @ 2300 rpm
  14. PacBlue

    PacBlue Senior Member

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    Doing a quick search on 42 Bertram's and it seems the 6V-92TA's in your model year 1981 were the 475/435 hp version, not the 550/535hp ratings that are typically the highest mechanical injected ratings before they went to the 635hp DDEC 6V-92TA's.

    So this leads to the question, are you sure you have the 550hp 6V-92TA's version, do they have the 7125 Injectors and the TW83 Turbo?

    I have seen some stats of the boats in your era appear to be making up to 24 knots WOT, although it is always questionable if they are actually reporting knots or mph. 24 mph is right in the range of your 20/21 knot WOT. The 55ohp version should do at least 27 knots WOT.

    At the factory, Bertram usually took the path of least resistance and went to a 3-Blade prop as this gave them the highest top speed for your model. They usually did not test at Full Load either, but is was really all about marketing at the time. Some of the 42 Bertram's came with Cummins 903VTA's as well as 6-71TI's.

    A 6V-92 rated at 435hp is a lot of iron (weight) for not a lot of hp, so it is a bit of a handicap. I do not think you will see an improvement on WOT speed by going to a 4-blade. If you have the original, or near original propellers, circa 1981, Michigan Wheel (and others) have done some improvement on the blade design since the past 35 years, so a new propeller may help a little bit. You can talk / email directly with Michigan Wheel to get their feedback.

    Since you reported a WOT rpm of 2350, it would appear that you have the right size for your current 3-Blade propeller.