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Different Gear Ratios in Outdrives-gctid357022

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    Different Gear Ratios in Outdrives-gctid357022

    So the PO of my 305 sent me some service records finally, and I ran across something odd.

    'PORT Engine -- While testing it was found that outdrive is a 2.0 to 1 gear ratio and the starboard drive is a 2.2 to 1 gear ratio. The port drive is a newer drive and was replaced with this lower gear ratio. At this point the customer has opted to run a 17 pitch on this drive for now to allow the different ratios and not overload the port engine.'

    This strike you as a odd way to fix this? This something I should be concerned about? It seems like kind of a wonky fix to me.....

    #2
    I've heard of I/B's with slightly different ratios to offset prop hand characteristics....., but I don't recall hearing of I/O's with different ratios.

    If the net propulsion is the same, and if like RPM can be maintained per speed (without torquing the hull in one direction), I don't know if it would cause concern.

    Interesting!

    I'd call a Merc Tech representative and ask the question of them.

    Perhaps call a second Tech and get two different opinions.

    .
    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

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      #3
      I wonder if the manufacturer changed ratios slightly from years - like what Merc did on their OD's - old ratio was 1.50 now they are 1.47.

      Might explain it?

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        #4
        This would bother me. I don't know if you could accurately handle the difference with prop pitch because it would be all total trial and error due to prop slip and the lack of fine adjustment in 1" pitch changes. I think one of your drives will be working harder. I don't know if it would harm it , but it would be in the back of my mind all the time. If it's Bravo 2 or three the reduction is in the top of the drive. Perhaps you could swap them out.

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          #5
          So here's the existing prop setup:

          Port 2.0 ratio BRV2 18.75X17 LH

          Starboard 2.2 ratio BRV2 18.25X19 RH

          The guy at the prop shop is saying they've actually got the math wrong and the props should in theory be swapped around. I'm also only getting about 4200 RPM port and 4000 RPM starboard. One engine only has ~50 hours on it, and his math is leading him to beleive the boat is way overpropped. He's reccomending the following:

          Port 17.25x15 Stainless 4 blade

          Starboard 17.25x17 Stainless 4 blade

          Any opinions? It seems like a pretty big change to me. What props are other folks running on there 305's?

          Comment


            #6
            Forgot to mention I have the 5.0 carb'd engines.

            Comment


              #7
              aa2300 wrote:
              So here's the existing prop setup:

              Port 2.0 ratio BRV2 18.75X17 LH

              Stbd 2.2 ratio BRV2 18.25X19 RH

              The guy at the prop shop is saying they've actually got the math wrong and the props should in theory be swapped around.

              I'm also only getting about 4200 RPM port and 4000 RPM starboard. One engine only has ~50 hours on it, and his math is leading him to beleive the boat is way overpropped.

              He's reccomending the following:

              Port 17.25x15 Stainless 4 blade

              Stbd 17.25x17 Stainless 4 blade
              I think that the guy at the prop shop, who says these should be swapped around, has his thinking reversed.

              Do the math:

              The Port engine is turning a prop shaft with less mechanical reduction (2 engine revolutions to 1 prop rotation)... and at the same time is applying more speed to the prop shaft.

              This means reduced pitch in order to be Apples/Apples on propulsion.

              The Stbd engine is turning a prop shaft with more reduction (2.2 engine revolutions to 1 prop rotation)........ and at the same time is applying less speed to the prop shaft.

              This means increased pitch in order to be Apples/Apples on propulsion.

              You can see that someone made an attemtp to correct this with a change in prop pitch.

              While I cannot attest to this being absolutely correct........

              [COLOR]"#0000FF" wrote:
              Port 2.0 ratio BRV2 18.75X17 LH

              Stbd 2.2 ratio BRV2 18.25X19 RH[/COLOR]

              .... it's certainly more in line with what you want to achieve, IMO.

              To do what this Prop Shop guy is recommending, may over-burden the Stbd engine, and if not, would decrease the load on the Port engine.

              Your WOT RPM (between the two engines) would be even more out of whack.


              .
              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

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