Intrepid 390 Sport Yacht Propping

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  • Intrepid 390 Sport Yacht Propping

    I recently acquired a 2008 Intrepid 390 with triple Verado 300's and I completely repainted hull so no bottom paint and serviced all engines. I notice that the center engine however shows a significantly higher GPH than the outer ones. I contacted Intrepid whom supplied me with the "Performance Report" from same hull model and engines and I compared all data which matches mine, ie. engine heights, props etc. At WOT the center engine is about 450 rpms less than the outers however I cant seem to find any mechanical issues with this engine. No trouble codes, and seems to be running fine. In addition once I get to above 4000 rpm on all that's when the center seems to "fall behind". I did swap coils from one of the other engines and no difference.

    I have a few questions, should the boat now be overpropped, why would I see a difference in ONLY the center engine? What would be ideal prop to change to on center engine to have the RPM range match back and hopefully the GPH?

    I have attached a copy of the Original Intrepid performance report and highlighted my current test results for comparison. All props are 14.75 x 21 and center engine as per report is on 5th hole which is the highest.

    Thanks in advance for any help!

  • #2
    got your email, but I figure I'd post my reply on here as well.

    Ok, you said you swapped all the coils, so that is not it.
    That would have been my first thought.

    "Most" triple engine boats turn the center motor higher RPM then the outside. Often they will run a 4 blade or higher pitch prop in the center to keep the RPM down equal to the outsides.
    Your having the opposite problem, I have actually seen that on an intrepid before.

    First thing we need to figure out is which gearcases you have.
    the original Verado 300 had a black gearcase, 4.8 inch diameter and 1.75 gear ratio.
    They updated to the larger "HD" gearcases, and most people got them changed, as mercury was replacing them for a lot of people.
    The updated gearcases are either
    Black, 5.44 diameter and 1.85 gear ratio.
    or Silver 5.44 diameter and 1.75 gear ratio.

    If I am reading your graph correctly, your turning 5730, 5310, 5830 on RPM at 50.3 MPH.

    Motors were originally mounted outside in hole #4, center in #5

    I would say your problem is in the outside motors slipping badly, and your center motor is the only one with any traction and doing all the work.

    I would suggest trying 19 pitch Revolution 4 propellers all the way across.
    Or Quicksilver Q4's (same exact prop)

    But it would be nice to figure out what lower unit you have. If you send me a picture of your lower unit with the propeller on it, I might be able to tell you from looking at it.

    Ken Reeves
    [email protected]
    941-735-5808

    Comment


    • #3
      I'll add to this, "IF" we find out you have a 1.75 gearcase, I'm probably going to tell you to run 17 pitch Revolution 4's instead of 19.
      But it will depend on how you plan to load the boat most of the time.
      They got 57 MPH with a light load.

      Ken Reeves
      [email protected]
      941-735-5808

      Comment


      • #4
        Click image for larger version

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        Lower appears to be the original 4.8 diameter 1.75 gear ratio.

        Ken Reeves
        [email protected]
        941-735-5808

        Comment

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