4-Blade Cupping for 38 Bayliner ?

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  • 4-Blade Cupping for 38 Bayliner ?

    Hello…my name is Nick…

    I have a 1988 Bayliner 3870 repowered with Cummins 6BTA’s 5.9M’s, Tranny gear at 2:1, WOT 2600…propellers 21x26x4 Nibral, boat weight 20,000lbs.

    My issue…I have some damage in the corners of the tunnels where the trailing edge of the props have pitted a hole about 3/8″ deep apparently caused by water turbulence/cavitation. The port side is less damaged but the prop is slightly further away from the hull...1.25". Starboard side where there is more damage is about 0.75". I believe the damage may have been created in the first season after repower when the boat was fitted with 22x26's. I can't tell that it has gotten worse since changing the props to 21's but I don't think so as the boat was hauled last year when I bought it and it doesn't seem it has gotten worse. I've put about 110 hrs last year above the original 110 hrs when I bought the boat. Obviously it was not used much.

    Right now I cruise at 2350RPM/16.5kts, WOT 2600RPM/22kts. I don’t want to lose that performance. Based on internet formulas it seems I have somewhere around 24% prop slippage at WOT and 34% at cruising. Not sure if that makes sense. It would be nice to improve that.

    It has been suggested that cupping the props slightly at the trailing edge to move the water better in the tunnel will help minimize cavitation. I’m afraid this will also require a pitch change and am concerned that there would be some guesswork involved with more and more changes later. When the boat was repowered about 6 years ago, the shop that did the work worked with the previous owner and went through several props before settling on the 21x26's. These perform well and give me the proper WOT at 2600 RPM.

    My question…have others cupped their 4-blade props of around the same size and what was the associated pitch change and what effect did it have on cruising and WOT speeds and other boat performance…?

    Thanks in advance…Nick

  • #2
    Welcome Nick..

    Maybe Ken will be able to jump in and help with this.. If it is about props, he knows..

    https://www.bandofboaters.com/forum/...eves-prop-gods

    Comment


    • #3
      I'm not really an authority on the inboard props. I normally suggest GeneralPropeller.com in Bradenton
      They are a Huge shop here in FL, they do great work and know their stuff on the big inboard props.
      I actually sent this link to Pablo at General to see if he can chime in here.

      Ken Reeves
      KR@Propgods.com
      941-735-5808

      Comment


      • #4
        Originally posted by ken2 View Post
        I'm not really an authority on the inboard props. I normally suggest GeneralPropeller.com in Bradenton
        They are a Huge shop here in FL, they do great work and know their stuff on the big inboard props.
        I actually sent this link to Pablo at General to see if he can chime in here.

        Thank you...

        Comment


        • #5
          Hey Nick,
          Pablo is trying to register, can see the thread but can't post yet. I'd just email him directly. Pablos@gpcprop.com

          Ken Reeves
          KR@Propgods.com
          941-735-5808

          Comment

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