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4788, 370HP, props revisited, gear ratios-gctid805778

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    4788, 370HP, props revisited, gear ratios-gctid805778

    It seems clear that we need to repitch from our current 24 x 23.5. The last hangup has been RPM at WOT with faulty original gauges. For the last 2 and a half season since we bought Surprise we've been 99% of the time a hull speed. This Spring, at WOT with full load except for half full gas, it looked like the starboard engine got to 3050, but the port tach only read 2800, though the sync showed us maybe 50-100 RPM off (we need to bleed our Hynautics and tune them up - another subject). We were doing 22.1 knots. I didn't stay there long enough for EGT to catch up, but at 2400 RPM, which is what I'd want in rougher weather or emergency, the starboard EGT was about 75 degrees higher than port, boost about the same maybe slighty higher in port. We're putting new impellers in both engines and I'm about to buy two Noland RS-11s to digitize our instruments.

    The one unknown for me is the gear ratios on our Hurth transmissions. I can't find a plate on either of them or anything clear in the literature. Am I correct that one engine has a reverse gear which will make their ratios different, and perhaps trying to balance RPM by ear (always in line with sync) is causing the starboard engine to work harder? That was suggested by our prop shop, who originally advised us not to repitch any lower than 23 until we got everything worked out for proper WOT testing.

    Comments welcome.
    Steve & Cary Sober
    Woodstock, VA
    SURPRISE
    2000 Bayliner 4788
    Cummins 370's
    Achilles 350DX with Eurohelm and 20HP Tohatsu
    Kent Island, MD

    #2
    " I can't find a plate on either of them or anything clear in the literature"

    The plate is there maybe painted over by the OP. I will try and find a diagram on the tarns plates and post back here.

    "This Spring, at WOT with full load except for half full gas, it looked like the starboard engine got to 3050, but the port tach only read 2800, though the sync showed us maybe 50-100 RPM off"

    It is unclear if you have accurate tach readings or not - are you sure these rpm's are correct? Have you strobed the engines as a confirmation?

    "It seems clear that we need to repitch from our current 24 x 23.5"

    Are you sure this is what the current props are- have they been scanned? H are they 4 blade NiBrals? Have they been balanced" Do you have the printouts of that?

    "We're putting new impellers in both engines"

    That is good but it will not affect EGT at all.

    "That was suggested by our prop shop, who originally advised us not to repitch any lower than 23 until we got everything worked out for proper WOT testing."

    Two comments here....

    1. That is fine but if you run at higher rpm and the engine(s) are overloaded the prop shop will not be paying for the repair. The Cummins 6b is sensitive to overload conditions and needs the rpm to be at WOT +3-5%.

    2. Please state your exact concern about going to 'low' in pitch. What exactly is the problem with going too far below 23"?
    Northport NY

    Comment


      #3
      Plate is on the top of the trans (looking down) typically attached by rivets about 1-1/2" by 3" in size.....

      "A" and "B" data denote the ratio in each rotation direction.

      Take a good digital picture of each so your prop shop can read the data in both "A" and "B" for each.


      Northport NY

      Comment


        #4
        I don't believe the tachs are accurate. That's why I plan to digitize them and the rest of our gauges.

        Prop people recommended this 23.5 to start with, then work down with WOT data. I've been slow to follow up getting ready for proper WOT testing. (Each engine attained 3000 RPM with the original 24 x 24 props at survey but the boat wasn't loaded.) They are 4 blade nibrals, reworked and repitched in Spring 2015 by Black Dog Props. I'm considering new props, use what I have as spares, they're old and may not be reworkable beyond a certain point.

        I'm not concerned about under propping, I just want them to be as right as they can be. Our boat, according to boot stripe, tends to be a bit low in the stern (2" maybe) with a slight list to port maybe 1". We want to do some serious cruising this year now that most kinks have been worked out, upgraded, or replaced.

        We've spent by far most of our time living aboard on and off for around two weeks at a time, a couple of weeks at anchor and some day cruises, all at hull speed. Now, knowing the boat is more comfortable on plane at around 14-16 knots when the Bay kicks up, we are ready to start burning more fuel going farther in rougher weather. So, it's time to do the props right.
        Steve & Cary Sober
        Woodstock, VA
        SURPRISE
        2000 Bayliner 4788
        Cummins 370's
        Achilles 350DX with Eurohelm and 20HP Tohatsu
        Kent Island, MD

        Comment


          #5
          "Now, knowing the boat is more comfortable on plane at around 14-16 knots when the Bay kicks up, we are ready to start burning more fuel going farther in rougher weather. So, it's time to do the props right."

          Without the exact rpm and confirmation of the trans ratios the rest of this will be just a decent guess.

          Assume the Black Dog brought them down to 23.5" do you have the scan sheets of the results?

          We cannot asses the port to stb rpm discrepancies if we are not sure the rpm is correct.

          Just for clarity - lets do a comparison.....

          Our last boat was a 1995 4788 with 310Hp Hino's / heavily loaded with a 3 person jet ski / 2:1 trans ratio / 24 X 20.5" 4 blade Nibrals

          (20% less hp , 15% less pitch). We covered 90% of 1,000 miles+ per season at 16-17 knots over 12 years with this boat and before that did the same thing with 2 previous 4588's (at 14-16 knots). Measuring Boost and EGT made sure that we never (never) had any issues with the engines over 20+ years and 30,000+ miles.

          YMMV

          In salt water / with 6-9 POB / full water / half fuel / loaded for a weeks vacation.

          We would get about 22.5 knts at WOT of 3,100 / about 17 knts at 2,700 rpm.

          With the same setup we would get just under 22 knots WOT while pulling a #3,500 (24') dinghy.

          The big key here is - do not try and pull too much hp out of these engines at too low an rpm.

          Overpropping a diesel if not good , overpropping a Cummins is really not good.

          Underpropping a diesel in moderation will not affect your life in any way and protect the engines.

          You are going to need accurate rpm, accurate prop scans and accurate trans ratios to help the prop shop achieve your goal(s).
          Northport NY

          Comment


            #6
            Do you have the prop scan / balance sheets from Black Dog?

            PS - these are not my props these are sample sheets from a scan outfit as examples only....




            Northport NY

            Comment


              #7
              I found a picture (from 2009) that shows the trans plate on the transmission of an 800 series Hurth in a 4788 - should be the same as yours

              Look on the center of the picture the plate is silver and blue below the tube going to the intercooler on top of the transmission.


              Northport NY

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