Announcement

Collapse
No announcement yet.

38xx - Prop / tunnel clearance...?

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

    #16
    If those numbers are true then that says from 2000RPM to the WOT it only makes 10 more HP and torque is falling off at a good clip. The boost must be about 18-20psi at that speed as well given the displacement of the engines is nearly the same.

    It also says that the Hino is loafing at 1800 RPM only making about 100hp.

    Prop makes a bit more sense but with that - it seems like a 1.5:1 transmission might be a better solution.
    1989 3888
    Nobody gets out alive.

    Comment


      #17
      I got the numbers from the internet so they have to be right.
      Actually, the Hino numbers came from the service manual
      https://www.marinepartssupply.com/me...vicemanual.pdf
      and this for the Cummins
      https://www.sbmar.com/docs/performan...M-90761%5D.pdf
      P/C Pete
      Edmonds Yacht Club (Commodore 1993)
      1988 3818 "GLAUBEN”
      Hino EH700 175 Onan MDKD Genset
      1980 Encounter Sunbridge "Misty Blue" (Sold)
      MMSI 367770440
      1972 Chevrolet Nova Frame off Resto-mod in the garage
      Boating on the Salish Sea since 1948

      Comment


        #18
        WOW...great discussion...thank you all...lots of good questions to answer...

        Couple of things...I still need to verify the 26 pitch. 21x26 is what is in a work order from the PO but he also told me they tried several props during the repower. So I won't know for sure until I haul the boat out for the winter. I'm thinking it's right as my cruising rpm (2300)is lower than the Hino's so something must be going on to keep speed up in the 16k area assuming the same 2:1 RATIO...gotta be the torque pushing that much pitch.

        I am aware of the "standard" of 15-20%...but even applying standard rules for prop/hull clearance it would be impossible even for a 20" prop. There just is not 4" clearance from the edge of a 20" prop... So Bayliner must have gotten "special dispensation"

        kwb My trannies are ZF63A's...The 26 makes sense if the Cummins are to move the same amount of water at lower rpm's than the Hino's... I do not have a boost gauge so can't tell what the turbo's are doing...


        So...(1) anybody else out there sporting 21's...? and, (2) do your tunnels have any "turbulence" damage like I described...?



        1988 3888 "Liberty"
        Twin Cummins 6BT's 210hp
        Onan 8.0
        Boating Raritan Bay

        Comment


          #19
          Our 1986 Bayliner 38 with 175 Hp Hinos had 4 blade 20" x 21" props that were scanned and balanced.
          The boat may have similar 2:1 (1.97:1) ratios as the ZF63A offers those similar ratios.
          The Hino has a rated rpm of 3,000 vs the 2,600 on this Cummins. (delta = 15%+)
          The assumed prop differences are 21" vs 26" (about 23% less).
          The engines put out about 210 hp vs 175 ( (about 20% more)

          So conceivably they can have nearly the same gear ratios if the shafts/cutlass/couplings and prop clearances can tolerate the changes.

          Is this RBMitchel's old 38 by any chance?
          Last edited by smitty477; 08-27-2018, 01:42 PM. Reason: Correct a guess
          Northport NY

          Comment


          • Nickp
            Nickp commented
            Editing a comment
            Thanks, Smitty...yours and kwb explanation makes sense of it...

            Boat was out of Baltimore...not Mitchell's...

          #20
          That is what I was getting at - with the lower operating RPM, higher torque output that boat probably should be closer to 1.5:1, spin the props a bit faster and end up with a closer to "square" prop which should be more efficient than that.

          With the tunnels on these you are kind of painted into a corner on props much like the OB and I/O where you often times can't get diameter to match the application and have to use pitch to use the torque.

          For slower applications (like a 38xx) I will take diameter over pitch for a more effective prop, since the tunnels won't let you do that... then you might need to spin them faster.
          1989 3888
          Nobody gets out alive.

          Comment


          • Nickp
            Nickp commented
            Editing a comment
            Thanks for thinking it out with me...gratefully the Cummins have the torque to push the bigger pitch. When the boat comes out for the winter I'll be able to verify the size.

          #21
          Looks like regardless of prop size, they're doomed to a 1" wacking for some additional clearance. Unfortunately, it won't get to 15-20%...
          1988 3888 "Liberty"
          Twin Cummins 6BT's 210hp
          Onan 8.0
          Boating Raritan Bay

          Comment


            #22
            Originally posted by Pflat View Post
            I don't think damage to the tunnels is that uncommon, here is a pic of mine after I had the bottom soda blasted last year
            BTW, I'm running with 20x22's with the 210 hp Hino's and WOT is 3000rpms.
            1989 3888
            Hino 210's

            Comment


              #23
              For boats with tunnels, the clearance can be as little as 5%.

              https://www.researchgate.net/publica...ower_Vibration

              I have 18" props on my 32. There is about 1" clearance with no damage to the tunnels. The bottom paint stays intact.
              100T MMC 2307794

              Comment


              • Nickp
                Nickp commented
                Editing a comment
                Thank you...I really appreciated the link...very informative. Boat comes out in a few weeks and will be able to judge if damage got worse or not. If not, will glass repair current damage and move on till next season. "Doing nothing is always an option"...

                Thanks for taking the time to send me the article...Nick

              #24
              21X21.on my 3870 900hr last year with no problems
              1984 3870 bayliner

              Comment


                #25
                oldcrow

                Good to hear someone else is running 21's...thanks for replying...

                Time to figure out what's going on with my clearances...about 1/2" on outboard, about 1 1/2" on the inboard...

                Winter project...
                1988 3888 "Liberty"
                Twin Cummins 6BT's 210hp
                Onan 8.0
                Boating Raritan Bay

                Comment

                Working...
                X