Adding Revs to my Mercury 150 4 Stroke while Swapping 3 blade for 4 blade?

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  • Adding Revs to my Mercury 150 4 Stroke while Swapping 3 blade for 4 blade?

    I've got a new to me 2015 Crestliner Super Hawk 1850 - Approx 3000lbs boat & motor.

    Loaded with approx 750lbs of people & gear, getting only 4760 RPM WOT per Mercury VesselView app, and about 37-38mph per GPS.

    RPM range for motor is 5000-5600. Motor has about 75 hours on it, no faults, running well as far as I can tell.

    Current prop (came with boat) is Mercury Tempest Plus (SS) 14 5/8" x 19P

    As currently propped and an average load, the boat is a real dog getting on plane, besides not getting into the suggested WOT RPM band.

    Q1: if i want to go to 4 blade, either aluminum or Stainless, should I drop 4 sizes in Pitch, adding a 4th blade, to theoretically add 700-750 WOT RPM?

    My Calculation: (down 4 sizes in pitch * ~200 RPM / pitch size = ~ +800 RPM minus 50-100 RPM for that 4th blade = +700-750 RPM

    4760 RPM + 700/750 RPM = 5460/5510 RPM

    Q2: Am I being too naive about my pitch/prop/RPM size calculations?

    My Goal: Get motor in proper RPM range, Better low speed planing, Better power for pulling the kids on tubes at 20 mph, Keeping on plane in rougher water, etc.

    Was considering Quiksilver Nemesis 4 blade prop, but sizes available are 14 X 14P and 14 x 17P. 14 seems too small of P, and 17 seems like it will only get me part way to 5600 RPM.

    Thoughts? Thanks!

  • #2
    I love your outboard, I also have no answers for you, my humble opinion, a few more RPM as in 300.
    Ken will answer when he checks in, I'm just cheering for you!.


    • #3

      I haven't personally ran that boat. but Here is the Mercury bulletin.
      They ran 5500 RPM and 48 MPH with your same prop.

      When your 10 MPH and 1000 RPM short on an outboard, I will always tell you to check the motor.
      Your average outboard motor running down a cylinder will be about 1000 RPM low, and 10 MPH slower then expected.
      You could have a bad coil, and that motor will run smooth as silk, idle beautifully, no vibration, just sluggish.
      That would be my first bet.
      Also, throttle postion. You "should" be burning 13.8 GPH at wide open. If you find your only burning 9 GPH at wide open, then your linkage isn't set correctly.

      I understand that its a new motor. I'm not saying it has a catastrophic failure. just saying you may have a bad plug, plug wire, coil, where its only running on 3 cylinders.

      Ken Reeves
      [email protected]


      • #4
        Thanks a bunch for that feedback Ken. I just got a couple more data points today from others running the same boat and motor and they were reporting numbers closer to the bulletin you posted ie 48mph. I will look into plugs, wires, and coils and report back.


        • #5
          let us know what you find

          Ken Reeves
          [email protected]