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Prop Help 4.3 Merc

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  • Prop Help 4.3 Merc

    Hello! I'm looking for some guidance on a new prop for my old mans boat. It's a 19' Maxum (approx 2400#) with a fresh rebuilt 4 bbl 4.3 Merc and a 1.84 ratio Alpha One. It currently has a 4 blade 17 pitch Trophy Plus Stainless prop. The boat runs good but over revs, going 39 MPH at 5300 RPM. This works out to about 16% slip. I'm thinking that going to a 19P 4 blade should get it back into it's power band, but any advice would be appreciated.
    Last edited by ttrip; June 10th, 2019, 07:27 AM.

  • #2
    I run 21p props on my 19’ wellcraft with a 4.3 2barrel and the same gear ratio as you.
    1990 Wellcraft Eclipse 196
    Mercruiser Vortec 4.3 0M322285
    Alpha gen 1

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    • #3
      Welcome to Iboats,

      What's the min-max wot rpm range factory stated for that motor ? You like boating lightly, medium light or fully loaded ? Load if extreme is a natural wot rpm killer.

      Happy Boating


      Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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      • #4
        The WOT range is 4400-4800. The boat might see 4 adults on occasion, but is typically loaded less than that. The boat is definitely under propped right now

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        • #5
          Yeah I agree that 17p is way too low, a 19p SS 4 blade should drop your RPMs by around 4-500 which would get you right at max RPMs.
          1979 V5 SS140 HP Merc ...... 1970 18' Chieftain IO restoration
          1978 16' SS160 SeaNymph .. 1978 14' Seafarer

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          • #6
            I think we'll go with a 19P Quicksilver Nemesis and see how that goes. Thanks for the replies!

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            • #7
              I would try a 21p three blade. You will loss about 700rpms and your slip will go down compared to the four blade.

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              • #8

                If like 4 blades props try same 4 blade with 19 pitch, definitely will lower wot rpm a lot but only a wot run on flat calm water cond will determine so. It isn't know if going 3+ pitches will render -700 wot revs as the rule of thumb not always works spot on all combos will depend entirely if being, light, medium light or heavy ones.

                Happy Boating
                Last edited by Sea Rider; June 11th, 2019, 12:19 PM.


                Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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                • #9
                  Sign up today
                  I’d be at a 21” 3 blade good quality stainless like a enertia, mirage + or laser 2, for sure. I’d have a 23” for cruising and too end too, haha.

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