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Changing prop pitch-gctid354919

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    Changing prop pitch-gctid354919

    We have a 2000 2855 with a factory rebuilt 7.4 Mercruiser w/ Bravo III outdrive. We have a 24" pitch on there now with 3 blades. Our RPMs will go over 6,000. We were told to go up a size in pitch and we would lower our RPMs into a safe range. How many pitches should I change and what should I expect? Is a 4 blade prop an option and would it be the same pitch relation as the 3 blade?

    #2
    you can't go any higher....24" pitch on the b3 drive is the largest you can get..check again to make sure it reads 24 on the side of the hub....for a 7.4ltr engine ....it sounds more like your running a 20" pitch...

    :arr arr

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      #3
      Verify your tach.
      Be good, be happy, for tomorrow is promised to no man !

      1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

      '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

      Manalapan N.J

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        #4
        What gear ratio is your drive?
        2007 Discovery 246
        300mpi BIII
        Welcome island Lake Superior

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          #5
          The general rule of thumb is that a 4 blade prop of the same diameter is equivalent to an extra 1" of pitch, so a 3 blade with 24" pitch would perform similar to a 4 blade with a 23" pitch.

          The 4 blade will improve hole shot and likely run smoother but would add more drag at high speed so you'd loose some top end.

          I agree with the other comments though, you need to confirm that you really do have a 24" pitch prop and check that your drive ratio is correct for that engine and prop and also get your tach readings verified. What's the GPS speed like at WOT? that would also be useful to know.

          Comment


            #6
            I've checked the prop and it says, 24P, which I assume is a 24 pitch. Is that correct? I have seen 26, 28, and 30 for sale for the Bravo III.

            As far as the RPMS, I cannot check, as the boat is in storage. I was hoping to change the prop situation before we put her back in the water. When the RPMS were over 6,000, the motor had a screaming/racing noise, like it was way over-reving. We had to put a new motor in in the summer (other blew), and I'm thinking this was due to the RPMS.

            What size pitch are you using on a 2855 with the Bravo III?

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              #7
              I have the same boat ,my pitch is also 24 but only get 5000 rpm wide open.

              Comment


                #8
                Dohio wrote:
                I've checked the prop and it says, 24P, which I assume is a 24 pitch. Is that correct? I have seen 26, 28, and 30 for sale for the Bravo III.

                As far as the RPMS, I cannot check, as the boat is in storage. I was hoping to change the prop situation before we put her back in the water. When the RPMS were over 6,000, the motor had a screaming/racing noise, like it was way over-reving. We had to put a new motor in in the summer (other blew), and I'm thinking this was due to the RPMS.

                What size pitch are you using on a 2855 with the Bravo III?
                OK, first off you do not need to actually run your engine at 6,000 RPM. Back off on the throttle some.

                Your gear ratio is irrelevant, it is what it is. Changing it is not as easy as changing the pitch of your props, so why bother even looking at gear ratio. The outdrive might have been replaced, or reworked. What others have for a gear ratio is what they have, what you have is what you have. Its the gear ratio and the prop pitch that combined make for "proper" WOT performance.

                You have two symptoms that seem to correlate. You feel that your engine is sounding like its over-revving and your tach is indicating the same thing.

                IF your tach is correct then your boat is under propped. There is REALLY nothing wrong with that unless you do not like the performance of your boat. Just watch your RPM's and do not ever operate the 7.4L engine abouve the mfg rated RPM which I believe is 4500-4700 RPM.

                If, you want to change your props, then buy new props. You can get props up to 30 pitch in the B3 according to the Mercury Marine web site. If I had to make a guess as to how much to go up in pitch I'd venture a rough guess of 3" or so.

                KEVIN SANDERS
                4788 LISAS WAY - SEWARD ALASKA
                www.transferswitch4less.com

                where are we right now?

                https://maps.findmespot.com/s/36S4

                Comment


                  #9
                  Chief Alen wrote:
                  Verify your tach.
                  do this first
                  Captharv 2001 2452
                  "When the draft of your boat exceeds the depth of water, you are aground"

                  Comment


                    #10
                    Has that prop been rebuilt? I,m wondering if it is undersized or has a bad hub in it? I had one that used to slip in the hub!
                    Ernie
                    1986 3270
                    Volvo 305s


                    MMSI 338130047
                    Lake Michigan

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                      #11
                      Come on guys, WTF? The B3 props go to at least 28" and I thought I had seen a 30" in the past but maybe not. And changing number of blades? There is a special B3 set with more blades, but it's for special purposes and the price is astronomical.

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                        #12
                        CaptUgly wrote:
                        Has that prop been rebuilt? I,m wondering if it is undersized or has a bad hub in it? I had one that used to slip in the hub!
                        Yes, they were rebuilt.

                        Thanks for all the great advice. Just wanted to let you know that I ended up ordering a set of 26P 4 blade props from Hill Marine. I talked to Chad who thoroughly explained everything to me and he suggested using the 26P 4 blade and if I was not happy with them, I could exchange them. The boats performance with the new props gets on plane quickly and it feels smoother/more balanced. It now planes at lower RPMS. I may have lost a little bit of top end, but the gps was showing 35MPH, with the RPMS at 5,200. I cannot thank Chad enough for his recommendation, follow-up and willingness to make sure all worked right. And, a great price compared to others we found!

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