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Prop and overall ratio help needed

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  • Prop and overall ratio help needed

    Hello all ,

    I could use some help with a project that I am working on . I am converting or upgrading my drive. I am going from an old electric shift OMC stringer to a mechanical stringer. I have aquired all of the parts but now I need to get some props for the new unit and could use some help there.

    My "old" electric shift drive ratios.
    upper - 21/19
    lower - 15/23

    I dont know the overall ratio or how to calculate. I can tell you that I use a 14x14 for 'mountain lakes" and a 14x16 for lower elevations about 3700' or so and those 2 props have always worked great and put the RPMs in an acceptable range

    My "new" drive ratios
    upper 21/16
    lower 14/26
    overall .71 (from my manual)

    So my main question is - How can I calculate or find out what prop(s) will be best for new drive ? basically an equivalent of what the previous props and drive ratio

    any help would be appreciated so I can start buying props for the new setup .

    Thank you much
    1977 Bellboy 18 footer with a 175 HP OMC 302

    "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

  • #2
    Make sure the lower is a 800 series not 400
    When in doubt,Trim it out!!!
    Business Owner & Certifed Technician
    Boats I own & use
    20ft Javlin w/250 Junkrude
    17ft Bullet w/200 Merc (SOLD!!)
    13ft Allison w/150 I-6
    17ft Hydra-Sport 175GT
    15ft Allison w/75 Stinger(project)
    14 x 48 Drifter with 1100SS Merc

    Comment


    • #3
      I have the complete out drive and it's an 800 with the 21/16 ratio in upper
      1977 Bellboy 18 footer with a 175 HP OMC 302

      "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

      Comment


      • #4
        Well your engines WOT range is 3800 - 4200 rpm
        Your old drive, if stock, was listed a 1.79 ratio for a pre 78 175hp, your new unit is in the range of 1.4:1 which is quite a jump
        What rpms were you attaining with the old unit and prop?

        Comment


        • #5
          Shoot , that's a pretty big difference. Thanks for figuring that out

          With my 14 pitch I can hit 4000 and that puts the boat at about 35
          Last edited by REDRIG; January 12th, 2019, 06:18 PM. Reason: Spelling
          1977 Bellboy 18 footer with a 175 HP OMC 302

          "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

          Comment


          • #6
            So knowing the large gap in overall ratio , I may have to change strategy . That may run like a dog.

            That stinks , cuz that 5.7 upper is in great shape and resealed . Oh well , I can just sell it .

            Now the question lies if I can use my electric shift upper with the mechanical lower. It looks identical and the vertical driveshaft is listed as the same part , so thats promising . Will have to pull it and take a better look .
            1977 Bellboy 18 footer with a 175 HP OMC 302

            "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

            Comment


            • #7
              A pre 78 upper housing has a different part#(0980804) than the post 77 upper housing(0982226).
              The upper driveshafts are different too.

              That doesn't mean they won't swap, but...
              Last edited by jimmbo; January 13th, 2019, 12:55 AM.

              Comment


              • #8
                Actually jimmbo , they appear to all be the same from my initial reaserch . The vertical driveshaft is the same for all low profile uppers 909120 , water pump shaft too , so that means splines areaall the same. You may have just picked a tall profile model.

                Also I found this on a site that rebuilds stringers

                "The ratio change is in the upper housing. All 1973 - 1985 upper housings are identical* except for the tooth count on the gears and left or right steering worm."

                ​​​​​​I will pull off my electric shift and verify and measure but at this point it looks somewhat promising .

                I need a favor , can you tell me what my overall ratio will be assuming I can get this to work ? The ratios in the lower are different and that will change my overall ratio if I use the same upper

                So it will be upper of 21/19 and lower of 14/26

                That shouldn't be a huge difference on overall ratio to overcome with a prop change in pitch an inch or two from my current setup

                Thanks again for the help
                Last edited by REDRIG; January 13th, 2019, 02:50 PM.
                1977 Bellboy 18 footer with a 175 HP OMC 302

                "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

                Comment


                • #9
                  I thought the housing where different lengths....
                  When in doubt,Trim it out!!!
                  Business Owner & Certifed Technician
                  Boats I own & use
                  20ft Javlin w/250 Junkrude
                  17ft Bullet w/200 Merc (SOLD!!)
                  13ft Allison w/150 I-6
                  17ft Hydra-Sport 175GT
                  15ft Allison w/75 Stinger(project)
                  14 x 48 Drifter with 1100SS Merc

                  Comment


                  • #10
                    Originally posted by Faztbullet View Post
                    I thought the housing where different lengths....
                    you are right , there were different lengths . The Tall vs short profile . The tall profiles though I think were only in the late 60s early 70s . Both of my units are the short profile .

                    1977 Bellboy 18 footer with a 175 HP OMC 302

                    "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

                    Comment


                    • #11
                      Actually , I did some digging and was able to figure out the calculation for compound gears .

                      My old electric shift was a total Ratio of 1.38 (21/19 upper +15/23 lower)
                      My "new" mechanical shift has a total ratio of 1.41 (21/16 upper + 14/26 lower)

                      So that is only a difference of .03 , that should be fine right ? The prop is basically spinning at the exact same RPM on old vs. new drive .
                      Last edited by REDRIG; January 14th, 2019, 10:31 AM.
                      1977 Bellboy 18 footer with a 175 HP OMC 302

                      "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

                      Comment


                      • #12
                        Should work ratio wise.....
                        When in doubt,Trim it out!!!
                        Business Owner & Certifed Technician
                        Boats I own & use
                        20ft Javlin w/250 Junkrude
                        17ft Bullet w/200 Merc (SOLD!!)
                        13ft Allison w/150 I-6
                        17ft Hydra-Sport 175GT
                        15ft Allison w/75 Stinger(project)
                        14 x 48 Drifter with 1100SS Merc

                        Comment


                        • #13
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                          So one last question on this . My electric shift used 14 inch diameter props , the 800s appear to use 15 inch diameter props .

                          Based on these ratios , and the change in diameter , should I stick with 14 pitch for higher elevations and 16 pitch for lower elevations ( that was basically the strategy before )

                          Will a 1 inch larger diameter prop change that ?
                          1977 Bellboy 18 footer with a 175 HP OMC 302

                          "The trouble with quotes on the internet is that it's difficult to determine whether or not they are genuine." - Abraham Lincoln

                          Comment

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