Announcement

Collapse
No announcement yet.

Bravo III Drive.

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Bravo III Drive.

    This is the search for the perfect prop, is the such a thing? My issue I switched from 350 Chev. 260 HP to a 6.5 turbo Diesel, the drive was changed from a Bravo II to a new Bravo IIIX the Eng. Builder recommended a Ratio of 2.20. I have searched now for that perfect prop, I went with 4 blade Signature 2020 that was a joke jumped up to a 24P and a slight improvement but no cigar. Sent them back and on pins and needles waiting for a set of 28 Pitch, if that's not good the gear ration will need changing very well explained by Rick in the owners Colum. Not looking forward to tarring in to a new drive, not need to look that one up.
    Slightly modified 2859 6.5 Diesel Bravo III X drive
    96 Dodge 5.9 5 speed Gear vender OD.

    #2
    This is what I posted to his thread in the other forum area.

    Originally posted by Rick
    Dave sent me a PM linking me to this thread.
    I may have seen it had it been posted in the Tech Section forum!
    Originally posted by fritzman
    Hi Dave the drive was changed to Bravo IIIX ratio,--Ratio by ENG. builder, recommendation to 220, so that is what I have, yes it may need to go to 2.00 don't know jet. My next move is send them in for exchange, pulled them of and Monday there of to HILL marine for a set of 28P I am thinking. P/N my old Eng. was a 350 260HP. Have not had the Drive apart what gears would need changing? I
    Originally posted by Rick
    With what I have learned over the years, when changing from a higher RPM engine to a more torque oriented engine (with less RPM capability), the propeller must be spun at a higher revolution.

    But I digress for the moment.

    When you refer to 220, are you meaning a reduction of 2.20 : 1?
    In other words...... an engine rotation of 2.2 to a propeller rotation of 1?
    If so, a reduction of 2.20 : 1 would typically be used for a low torque but high RPM engine.

    Since I am not familiar with the different BIII ratios, I can only give you a few examples using Volvo Penta:

    BBC or BBF w/ Duo Prop...... the final reduction would be 1.78 : 1
    SBC or SBF w/ Duo Prop....... the final reduction would be 1.95 : 1
    Small high RPM engine w/ Duo Prop....... the final reduction would be 2.30 :1

    As you can see, the numeric value becomes higher as the torque is reduced.
    In other words, the reduction is greater as we go to a smaller/less torque producing engine.

    The small but high RPM engine needs the lower reduction in order to transfer the torque to the propeller.
    The smaller engine's higher RPM makes up the difference (of the greater reduction), by spinning the propeller faster once the boat is moving.


    I also know for certain, that we can never "prop" ourselves out of an incorrect drive reduction. It just doesn't work like that!


    If I'm understanding that your reduction is now 2.20 : 1, I would think that you need to go to a lessor reduction, and then drop down on prop pitch!

    Rick E. Gresham, Oregon
    2850 Bounty Sedan Flybridge model
    Twin 280 HP 5.7's w/ Closed Cooling
    Volvo Penta DuoProp Drives
    Kohler 4 CZ Gen Set

    Comment


      #3
      I hope you get the necessary results with your 28 pitch props Fritz. If not I’d be all over the guy that “recommend “ the potential incorrect 2.20:1 ratio.
      🥴
      Dave
      Edmonds, WA
      "THE FIX"
      '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
      (.030 over-Vortec top end-part closed cooled)
      The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
      Misc. projects thread
      https://www.baylinerownersclub.org/f...56-gctid789773

      Comment


        #4
        Let's say that the 6.5L turbo diesel's WOT RPM is 3,400, and that a cruising RPM would be more like 3,000.

        A reduction of 2.2 : 1 would limit the prop shaft to 1,364 RPM at a cruising engine RPM of 3,000.

        However, a reduction of 1.5 : 1 would allow the prop shaft to spin at 2,000 RPM at a cruising engine RPM of 3,000.

        Perhaps you need to make a change to the over-all gear reduction, and then calculate for a prop set that would be effective at 2,000 prop shaft RPM.

        Also, the 2.2 : 1 gear set has a smaller diameter lower gear unit "drive" gear, whereas the 1.5 : 1 gear set has a larger diameter lower unit "drive" gear.
        The larger diameter drive gear alone creates more gear tooth contact at any given time, thus making this a more stout gear set.

        D S and I had a disagreement over this years ago. I was finally able to convince him as to why this was so!
        Rick E. Gresham, Oregon
        2850 Bounty Sedan Flybridge model
        Twin 280 HP 5.7's w/ Closed Cooling
        Volvo Penta DuoProp Drives
        Kohler 4 CZ Gen Set

        Comment


          #5
          Thank you Rick, yes I think as you indicated the Ratio will need to be changed and the pitch will then be reduced, are the gear change on top or in the bottom of the drive? Dave yes I am not a happy camper. Question to Rick Power torque required to turn lets say a 24 pitch set of props at 3600 RPM - The red line of that Eng. is there a simple way to calculate that?
          Slightly modified 2859 6.5 Diesel Bravo III X drive
          96 Dodge 5.9 5 speed Gear vender OD.

          Comment


            #6
            .....................
            Originally posted by fritzman View Post
            Thank you Rick, yes I think as you indicated the Ratio will need to be changed and the pitch will then be reduced,
            Yes, I believe so.

            are the gear change on top or in the bottom of the drive?
            I cannot help you with that!
            I do think that Merc has used a combination of gear sets in both the upper gear unit and the lower gear unit in order to obtain the various over-all reductions.


            Dave yes I am not a happy camper.

            Question to Rick Power torque required to turn lets say a 24 pitch set of props at 3600 RPM - The red line of that Eng.
            Red Line and WOT RPM may not be the same.
            I would check to see at what RPM the engine is intended to be operated at for cruising.
            You will still want to perform the WOT RPM test just as you would with any engine/drive/prop-set!


            is there a simple way to calculate that?
            Either search on line for this data, or find a Merc technician who knows his stuff!

            Here is an example:


            Please read the text in this one very carefully.
            In the text you'll find a hyperlink to their "prop calculator"


            https://www.go-fast.com/sterndrive_gear_ratios.htm





            Rick E. Gresham, Oregon
            2850 Bounty Sedan Flybridge model
            Twin 280 HP 5.7's w/ Closed Cooling
            Volvo Penta DuoProp Drives
            Kohler 4 CZ Gen Set

            Comment


              #7
              Thank you Rick,
              28 P props will be delivered Fry. - Hope , as it turns out I ha the link info in my book, so we will see, have a great day.
              Slightly modified 2859 6.5 Diesel Bravo III X drive
              96 Dodge 5.9 5 speed Gear vender OD.

              Comment


                #8
                As much as we hope, we will not be able to prop ourselves out of an incorrect gear reduction. Many have tried and the same amount have failed.
                Rick E. Gresham, Oregon
                2850 Bounty Sedan Flybridge model
                Twin 280 HP 5.7's w/ Closed Cooling
                Volvo Penta DuoProp Drives
                Kohler 4 CZ Gen Set

                Comment


                  #9
                  Fritzman
                  if you do the gear change I might be in the market for your 2.2 set. I don't know if the bravo x gears will fit my leg. I have a set of 2 43 : 1 in it now and I'm sure it's not going to work with my boat at sea level. The drive came of ski boat that lived in lake Tahoe 5000ft altitude.
                  1992 2452
                  5.0 alpha1 gen2

                  Comment


                  • builderdude
                    builderdude commented
                    Editing a comment
                    Id guess a 2.0:1 at the most for ya 2452 with a 383 & bravo 3. Possibly a 1.81:1

                  #10
                  builderdude
                  I'm still thinking about that one.
                  You might be right. I haven't been able to find much good info on drive ratios.
                  1992 2452
                  5.0 alpha1 gen2

                  Comment


                    #11
                    Originally posted by Lineman1 View Post
                    builderdude
                    I'm still thinking about that one.
                    You might be right. I haven't been able to find much good info on drive ratios.

                    Based on engine RPM, not necessarily HP or Torque.

                    https://www.go-fast.com/sterndrive_gear_ratios.htm

                    https://www.go-fast.com/Prop_Slip_Calculator.htm

                    Rick E. Gresham, Oregon
                    2850 Bounty Sedan Flybridge model
                    Twin 280 HP 5.7's w/ Closed Cooling
                    Volvo Penta DuoProp Drives
                    Kohler 4 CZ Gen Set

                    Comment


                      #12
                      Hope this helps Click image for larger version

Name:	1609.gif
Views:	201
Size:	31.2 KB
ID:	530640
                      1992 2452
                      5.0 alpha1 gen2

                      Comment


                      • builderdude
                        builderdude commented
                        Editing a comment
                        Line, can you post up a high res of that chart? Pretty please😁

                      #13
                      If line items #9 through #15 corresponded to the HP and Torque output of the different Diesel engines, it would be very helpful.
                      Rick E. Gresham, Oregon
                      2850 Bounty Sedan Flybridge model
                      Twin 280 HP 5.7's w/ Closed Cooling
                      Volvo Penta DuoProp Drives
                      Kohler 4 CZ Gen Set

                      Comment


                        #14
                        Hi Rick, yes that would, in my set up right now I have too much power but its not working for the RPM, so more then likely there is a gear change in the works.
                        Slightly modified 2859 6.5 Diesel Bravo III X drive
                        96 Dodge 5.9 5 speed Gear vender OD.

                        Comment


                          #15
                          Basically it's just a list of what's out there from Merc. I think HP and torque curves are up to us to figure out.
                          I wonder what Whipple would charge to dyno my motor? I have to get the ecm reflashed from them anyway.
                          It would seem to me a 1.65 or 1.81 to 1 would be the right range for a diesel.
                          Fritz
                          how much torque do you think your making ? V8 turbo you might be able to turn even a 1.50:1 with a fair pitched prop. If I understand what I have been reading the higher pitched props are more efficient.
                          1992 2452
                          5.0 alpha1 gen2

                          Comment

                          Working...
                          X