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2.1 up to a 2.3 gear ratio prop pitch up or down?

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  • 2.1 up to a 2.3 gear ratio prop pitch up or down?

    Mfg boat no idea on weight
    1964 Merc 85hp Tower

    Had to swap for a newer LU with 2.3 ratio as it was really hard to source a same vintage LU 2.1ratio.

    17" pitch prop from the old and newer 1.
    Can't reach wot get to about 3700rpms any higher sounds as if it's not grabbing the water.

    Looking for advice on the prop pitch should I prop up or down ? Any advice would be helpful!

    Thanks
    Chin

  • #2
    Ayuh,..... At 2, 10ths difference, it should plane slightly quicker, have atad more power, 'n spin up to slightly higher rpms,...

    If it's not reachin' the revs it did before, I suspect another problem, but donno what,....

    2.3:1 is a slightly lower gear than 2.1:1,....

    Comment


    • #3
      As mentioned a 2.3:1 ratio will unload the engine a bit compared to a 2:1 ratio. It will require a slightly higher pitch prop to return rpm to previous revs. However, you are certain your old gearcase is 2:1? A 64 85hp is an inline 6 and has a max rev range of 5200 or 5300 rpm. What is the max rpm you get from the engine? You say it seems to lose grip on the water above 3700? Is the engine height correct, is the prop in good shape? Is the engine trim pin set correctly?

      Comment


      • #4
        Originally posted by jimmbo View Post
        As mentioned a 2.3:1 ratio will unload the engine a bit compared to a 2:1 ratio. It will require a slightly higher pitch prop to return rpm to previous revs. However, you are certain your old gearcase is 2:1? A 64 85hp is an inline 6 and has a max rev range of 5200 or 5300 rpm. What is the max rpm you get from the engine? You say it seems to lose grip on the water above 3700? Is the engine height correct, is the prop in good shape? Is the engine trim pin set correctly?
        Everything's the same as B4 the gear case swap only thing that has changed is that the trim pin has gone missing as it was bent out of shape and didn't have the locking end, I do have a spare on the donor motor but haven't pull it out as the engine mount the the previous owner built makes it hard to get to. Prop looks to be in good shape should I bump the pitch up 5 to a 22pitch

        Thanks for your help Jimmbo !

        Comment


        • #5
          Originally posted by Bondo View Post
          Ayuh,..... At 2, 10ths difference, it should plane slightly quicker, have atad more power, 'n spin up to slightly higher rpms,...

          If it's not reachin' the revs it did before, I suspect another problem, but donno what,....

          2.3:1 is a slightly lower gear than 2.1:1,....
          Thank for the tip

          Comment


          • #6
            Originally posted by Chinpoker View Post

            should I bump the pitch up 5 to a 22pitch

            Thanks for your help Jimmbo !
            No no no. I said a slightly higher pitch, like 1 inch, possibly 2 inches. However, you need to know where your are with regards to the 17 inch at WOT. And that means engine trim/tilt pin in correct hole for best operation

            Comment


            • #7
              Originally posted by jimmbo View Post

              No no no. I said a slightly higher pitch, like 1 inch, possibly 2 inches. However, you need to know where your are with regards to the 17 inch at WOT. And that means engine trim/tilt pin in correct hole for best operation
              My bad thanks for correcting me I'll get that pin out some how B4 the weekend and take another spin .
              As far as wot is it ok to run that high because it looks and feels like it's not grabbing at all ?

              Comment


              • #8
                Check for a spun hub on the prop.
                Bob, Seneca Lake NY
                '88 Bayliner 1700 Capri bowrider, 85 HP Force O/B, "Sea Weasel"
                '94 Grumman Fish'N Fun pontoon, 40 HP Merc
                Want a vessel safety check? Click here. Want to join the Coast Guard Auxiliary? Click here.
                Disclaimer: Although I am a member of the USCG Auxiliary, the opinions and advice in my replies are my own and do not necessarily reflect CG or CG Auxiliary policy or regulations unless so specified.

                Comment


                • #9
                  Hub isn't striped but thanks for your input!

                  Comment


                  • #10
                    Trim pin missing? If you don't have power trim and tilt and are running without it, your engine is tilted way back, prop is tucked way in, near the transom and you are PLOWING. That is the reason for your low rpms. As others have stated, the 2.3 unloads the engine and rpms will run higher with the same prop by about 10% everything else being equal.

                    You can make a pin out of most anything. I have used allthread with a couple of locking nuts in the past. Get something in there and set to the mid hole and maybe farther out to get the bow (front) up out of the water and then look at where you are. Push the lower unit out until the boat starts to porpus (front bounces up and down) then hole back down till it just stops and that's your sweet spot.
                    If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

                    Comment


                    • #11
                      Hey Texasmark thanks for your help! Pin in done and I do have pwr t&t. Boat is up on plain 37.8km/hr at about 3800rpms when I push it further to get up on the rpms it sounds as if the prop is not grabbing the water

                      Comment


                      • #12
                        Sign up today
                        Sorry this got old. I was busy. let's see clicks are 60mph vs 100K (speed limit sign in Toronto I saw on a business trip) = 0.6x 37.8 = 23 mph at 3800 rpms is ok, not stellar. Taking that number to 5500 would put you at 1.5x 23 = 33 mph, not all that great.

                        You are running a higher ratio LU so your rpms should be higher at a lower speed. You should easily be able to run your rpms up to 5500 unless you have problems elsewhere. Your prop hub must be slipping. You ought to be able to get up to 5500 but your speed would be slower, like the 33 I calculated. You didn't mention what happens to your rpms when you say the prop doesn't feel like it grips the water. It's either ventilating (sucking in air) or the prop is not locked to the prop shaft and the engine races as you attempt to go above 3800.....does it? Course you said you have PTT. If you trim out too far you could loose grip without having a prop problem.

                        Need some more data from you.
                        If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

                        Comment

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