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    #61
    Originally posted by smitty477 View Post

    - 2,850 on both? Exactly?
    - are your tachs strobed?
    - What do you get at high idle? (no load WOT)
    They are both almost exactly 2850. I just bought a photo tach and I'll be checking them next time to the boat in a couple of weeks. But... the lower helm shows the same rpm, so I feel like they're likely accurate. I'll check no load WOT next time I go to the boat as well. I might start another thread when I collect more info so as not to hog this one... but looking forward to your thoughts (such as repropping and/or investing in pyro/egts.

    By the way you mentioned $500 for both pyro/egt gauges. Sbmar has them for $295 each. Is there a better/more inexpensive source you can recommend?
    1995 Bayliner 2452 Mercruiser 5.7L Alpha 1 Gen 2

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      #62
      Originally posted by Fishtank View Post

      They are both almost exactly 2850. I just bought a photo tach and I'll be checking them next time to the boat in a couple of weeks. But... the lower helm shows the same rpm, so I feel like they're likely accurate. I'll check no load WOT next time I go to the boat as well. I might start another thread when I collect more info so as not to hog this one... but looking forward to your thoughts (such as repropping and/or investing in pyro/egts.

      By the way you mentioned $500 for both pyro/egt gauges. Sbmar has them for $295 each. Is there a better/more inexpensive source you can recommend?
      Check with the photo tach both at lower rom and at near the top - they can vary along the spread.
      I do not know about the prices for Cummins gauges - the engines we had were Hino's and we bought the boost gauges from thegaugestore(dot)com the pyros came from a common diesel engine store.
      I believe your engines are rated at 2,800 and if so you would be looking for +3-5% so 2,885 to 2,940 (higher preferred)
      The goal for those min rpm would be on a hot a humid day , fully loaded, with the hull/props in the condition you will cruise with.
      another way to say this is achieving 2,900 as you launch early in the season does not count.
      Your high idle may determine your WOT loaded speeds due to governor droop- that is why you need to know it.
      I believe your boat cam with 800 series transmission and if so they have matched gear ratios (A & B).
      FWIW - I do not know many folks that have their props balanced as well as ours and they were still out like 1.5% - so at 3,000 the rpm would then be 'out' 45 rpm for equal loads.

      When we cruised we would 'balance' the engine loads with the boost gauges which would put the rpm about 40 off.
      We travel a bit now and will be offline for a couple of weeks or so.
      Northport NY

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        #63
        Late again to the post. Smitty I don't have pyrometers as I've had other things to deal first on the boat, but I'd like to ask you a question. I've run boats all my life but my 3288 is my first twin engine setup. My tachs are not calibrated, I can tell because they read differently from fly bridge to wheel house. They are close but not exact. The way I have learned to run this boat is by ear (both of which still work by the way) I set my engines by harmonics. When I hit the "spot" everything is smooth as silk. When I set by tach it just doesn't feel right hence setting them by ear. The boat just purrs when I hit that sweet spot. The sound doesn't pulse it's even. Do you think with this method I may still be working one engine harder than the other or am I good to go? Thanks. Chip

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          #64
          Originally posted by Smitten View Post
          Late again to the post. Smitty I don't have pyrometers as I've had other things to deal first on the boat, but I'd like to ask you a question. I've run boats all my life but my 3288 is my first twin engine setup. My tachs are not calibrated, I can tell because they read differently from fly bridge to wheel house. They are close but not exact. The way I have learned to run this boat is by ear (both of which still work by the way) I set my engines by harmonics. When I hit the "spot" everything is smooth as silk. When I set by tach it just doesn't feel right hence setting them by ear. The boat just purrs when I hit that sweet spot. The sound doesn't pulse it's even. Do you think with this method I may still be working one engine harder than the other or am I good to go? Thanks. Chip

          Using noise to synchronize engines is a way of matching the RPM. When engines are running at different RPMs one hears a drumming, beat, or a varying sound. When that sound disappears the RPMs of the engines are matched.
          1999 3788, Cummins 270 "Freedom"
          2013 Boston Whaler 130 SS
          Anacortes, WA

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            #65
            Trivia...

            Many multi-engine airplanes have engine synchronizers installed. These work by adjusting the RPM of one or more "slave" engines to a "master" engine. Some airplanes have a more sophisticated system called a synchrophaser. These systems not only match RPM, but they also match the relative phase between propeller blades. Optimizing the phase relationship between propeller blades can reduce noise more than by matching rotational speed alone. Some synchrophasers are not adjustable; they always synch to the same phasing. Others are adjustable and allow a pilot to vary the phasing until the noise is minimized.

            Jet engines also can have synchronizers. I used to fly a Lear-36 that had a synchronizer; it allowed a pilot to sync the fan or the turbine on the engines.

            It is normal to see small variations in the EGT on synchronized airplane engines. These are attributable to variations in engine build, wear, etc.
            1999 3788, Cummins 270 "Freedom"
            2013 Boston Whaler 130 SS
            Anacortes, WA

            Comment


              #66
              Originally posted by Norton_Rider View Post


              Using noise to synchronize engines is a way of matching the RPM. When engines are running at different RPMs one hears a drumming, beat, or a varying sound. When that sound disappears the RPMs of the engines are matched.
              The drumming is called beats. The frequency of beats is exactly the difference in rpm between the engines.
              1988- 3288, 1994- 3288, now Soul Asset: 2001- 3788 w/330s

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                #67
                Originally posted by Smitten View Post
                Late again to the post. Smitty I don't have pyrometers as I've had other things to deal first on the boat, but I'd like to ask you a question. I've run boats all my life but my 3288 is my first twin engine setup. My tachs are not calibrated, I can tell because they read differently from fly bridge to wheel house. They are close but not exact. The way I have learned to run this boat is by ear (both of which still work by the way) I set my engines by harmonics. When I hit the "spot" everything is smooth as silk. When I set by tach it just doesn't feel right hence setting them by ear. The boat just purrs when I hit that sweet spot. The sound doesn't pulse it's even. Do you think with this method I may still be working one engine harder than the other or am I good to go? Thanks. Chip
                As mentioned above when you sync the engines for sound you are attempting to match tie rpm as exact as possible.
                As a result of that goal you will have engines running at the same rpm - they will not be matched for engine loading.
                You do not say what year 3288 you have or which transmissions you have.
                Trans - Look on the trans plates and see the ratios stamped "A" and "B" - are they the same? Likely not.
                So right away when you engines are at the same rpm your props are not spinning at the same rate.
                Props - Are they known balanced to some spec (S1 maybe?) Even with an S1 spec they can easily have a variance in the loads when at the same prop rpm.
                Is each prop exactly the same cleanliness, any small dings since last prop balance? etc

                So there are many reasons why engine rpm almost never equals engine load balancing - that is why I have never understood the desire to match engine rpm.
                Northport NY

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                  #68
                  The Hurth 630A transmissions in my boat have a 2.04 reduction ratio starboard, and 2.10 reduction on the port, I run about 50-75 rpm higher on port to load the engines the same at cruise rpm of 1800 and sync by ear to the load to take into account any slight differences in the propellers.

                  James
                  1989 Bayliner 3888, 175 Hinos,
                  Hurth 630's Onan 8kw MDKD
                  Lowrance Electronics!
                  MMSI
                  316030379
                  VHF
                  CFA 2587
                  Boating on Georgian Bay
                  Currently on the Great Loop

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