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Diagnosing my low WOT RPM, speed and fuel economy issue 2001 4788

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    Diagnosing my low WOT RPM, speed and fuel economy issue 2001 4788

    On my 2001 4788 I have been experiencing problems with speed, WOT RPM, and fuel economy.

    A month ago I took my boat out on a 300NM journey and only got 1.4 NMPG at a slow cruise. I also noticed my speed was down, but I attributed that to bad memory at the time.

    First time this year trying a WOT check I only got 2720 RPM. I’ve got a real problem.

    A bit of background...

    My boat has 330 HP Cummins diesels, about 900 hours since I put them in in 2011. My props are custom cast Nakashimas 24D X 21P.
    When I put the props on with a fresh bottom paint I remember making something well over 2800 RPM but did not document it.

    My last bottom job was March 2016, and I’ve had the boat dived on every year since then, with the last dive and scrub just two months ago.

    Now... How to fix the problem.

    I checked my throttle linkages for engagement of course. My tachomoters are digital Atena units, so they are fine.

    My boost was great, but EGT’s were creeping up to a bit over 900, higher than they used to be.

    No black smoke, so engines seem like they are running right. Both engines were the same general RPM around 2720.

    Yes I was getting some growth at the waterline, but nothing the other boats around me did not get, and I had the boat scrubbed just a couple months ago.

    So...

    I decided to haul the boat and take a look myself.

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    KEVIN SANDERS
    4788 LISAS WAY - SEWARD ALASKA
    where are we right now​​​​​​???​

    https://share.findmespot.com/shared/...j23OquWOj2N3Xe

    #2
    Yuck!!!

    So I decided to have the boat bottom painted. I was out of the water all of 28 hours.

    Click image for larger version

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    KEVIN SANDERS
    4788 LISAS WAY - SEWARD ALASKA
    where are we right now​​​​​​???​

    https://share.findmespot.com/shared/...j23OquWOj2N3Xe

    Comment


      #3
      This is what I was able to get RPM wise after launching an hour ago

      Click image for larger version

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      KEVIN SANDERS
      4788 LISAS WAY - SEWARD ALASKA
      where are we right now​​​​​​???​

      https://share.findmespot.com/shared/...j23OquWOj2N3Xe

      Comment


        #4
        in the PNW, the growth this year is ridiculous. I had 30 lbs of slime, mussels, barnacles, and flickers on my 6' dink when I pulled it out Sunday (I have an old one I use to clean the non-dock side of the boat and such.) The hair at the waterline of Wickus grows so fast. I've had to use the power washer to keep the waterline clean all year. I've still be able to pull full wot, though. I paint every two years, period, and pull the boat every year. I couldn't tell, is that an ablative paint or a hard paint?
        Tally and Vicki
        "Wickus" Meridian 341
        MMSI 338014939

        Comment


          #5
          Originally posted by talman View Post
          in the PNW, the growth this year is ridiculous. I had 30 lbs of slime, mussels, barnacles, and flickers on my 6' dink when I pulled it out Sunday (I have an old one I use to clean the non-dock side of the boat and such.) The hair at the waterline of Wickus grows so fast. I've had to use the power washer to keep the waterline clean all year. I've still be able to pull full wot, though. I paint every two years, period, and pull the boat every year. I couldn't tell, is that an ablative paint or a hard paint?
          Ablative, and I’m switching to a 2 year schedule

          KEVIN SANDERS
          4788 LISAS WAY - SEWARD ALASKA
          where are we right now​​​​​​???​

          https://share.findmespot.com/shared/...j23OquWOj2N3Xe

          Comment


            #6
            I think that is the issue. The diver is getting too much paint off. If you had a good hard paint and had it scrubbed, you'd be good longer. The problem is prep the first time. Ablative paints are so easy to prep, they go on fast and are pretty forgiving, hence the 2 year schedule for Wickus. If you went to hard, you'd have to get it all off, prep the base, and get it on right. It might get you 3 or 4 years if the paint is right. I like to have eyeballs on the bottom once a year. I found a nice chunk out of the bow thruster fairing a few years ago that I would not have like to have gone for two years. I also moved slips because my zincs were being destroyed by a neighbors boat. I would not have noticed that without the boat being out more often. I am in a much better situation, zinc-wise, now.
            Tally and Vicki
            "Wickus" Meridian 341
            MMSI 338014939

            Comment


            • ksanders
              ksanders commented
              Editing a comment
              Maybe, but the folks around here pretty much only put on the ablative paint. They say it does not build up, and there is no worries about bonding of paints. I’ll haul next year to get an idea about what is happening and take it from there. No more divers...I want to see for myself.

            #7
            Kevin,

            wow 100 rpms just due to friction of the bottom. Impressive. Just to be sure you didn't do anything to the running gear?

            Comment


              #8
              Originally posted by rbcooper View Post
              Kevin,

              wow 100 rpms just due to friction of the bottom. Impressive. Just to be sure you didn't do anything to the running gear?
              Nope, just polished the props to get the hard growth off them as well.

              KEVIN SANDERS
              4788 LISAS WAY - SEWARD ALASKA
              where are we right now​​​​​​???​

              https://share.findmespot.com/shared/...j23OquWOj2N3Xe

              Comment


                #9
                Kev, that is the other thing I'm religious about. I burnish the brass and apply pettit prop coat with the bottom paint. Two cans to the props, struts, shafts; two coats. The results are impressive for not much effort. Others have said they just use a zinc primer (same ingredients?) I have used the Pettit product every other year with paint with great results.
                Tally and Vicki
                "Wickus" Meridian 341
                MMSI 338014939

                Comment


                  #10
                  Out of curiosity, what was the speed you got before and after?

                  Comment


                    #11
                    Originally posted by Mag535 View Post
                    Out of curiosity, what was the speed you got before and after?
                    To be honest I didn’t look that hard at the speed yesterday. I was into a stiff afternoon sea breeze, and remember looking at the GPS and seeing 17 knots, but I am certain that I could have trimmed the boat out better and reached close to 20 knots. I will say that my photo of the tach’s was a bit off, as I later saw 2890 RPM.

                    The big tests will come next week when I go on a couple hundred mile trip. I log all my fuel usage, and will have some better info then.

                    Last trip out I ramped up to a 2400 RPM fast cruise and was in the 11-12knot range. That is the big reason I chose to haul out, the slow speeds

                    I am fully expecting that to be 14-16 knots range at 2400 RPM next week.

                    I do not run up on plane much, but my wife went with and she does not like the beam seas that are prevalent in out area, so I get after it for her comfort.

                    KEVIN SANDERS
                    4788 LISAS WAY - SEWARD ALASKA
                    where are we right now​​​​​​???​

                    https://share.findmespot.com/shared/...j23OquWOj2N3Xe

                    Comment


                      #12
                      I’ve had good luck coating the rudders, struts and shafts with Lanocoat prop coating. I just leave the container in the sun for a few hours then scoop it on with a putty knife smoothing with a gloved hand. I’ve also had good luck with rustoleum zinc paint on the props.
                      2000 4788 w Cummins 370's, underhulls, swim step hull extension
                      12' Rendova center console with 40HP Yamaha
                      MV Kia Orana
                      Currently Alameda CA

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