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Prop Selection Help

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  • Prop Selection Help

    1994 Mariah Z222
    7.4l engine
    Bravo Outdrive
    Captains Call Exhaust
    SX trim Tabs
    Current Prop Setup: Mercury Mirage Plus (No Plugs)

    The boat gets out of the hole really well, but lacks the top speed I would expect. The engine was freshly rebuilt back in May of this year. I just noticed the prop cavitates very badly when the exhaust is switched from captains call to the prop. Is there any chance that having the holes open on the Mirage plus prop would be causing the loss of RPM. Please see the prop calculator below. Any help would be greatly appreciated!

  • #2
    What is your max RPM? What is your RPM redline? What is your top speed.

    Does your prop hub fit tight (within 1/16") to the outdrive hub? If it is the right prop, it should.

    Comment


    • #3
      Try plugging the holes and see what happens
      2004 Caravelle 187

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      • #4
        To answer your question, yes, the PVS holes being open can cause a lot of cavitation. I donít think theyíre necessary in most applications. As a comparison, when I use a 21P mirage plus on my 19í wellcraft w/4.3 and 1.84 ratio, I get 7% slip and get 49mph at 4800rpm with smart tabs in the 3rd hole. I keep the PVS holes plugged and itís fine

        You have a lower pitch but a taller ratio which makes your prop turn faster than mine in addition to the extra 400 rpm youíve got to work with. In other words you should easily be moving more water than me and despite your extra weight m, youíve got a significant advantage in power. You should be going as fast as me, or faster. And your problem is clearly excess prop slip.

        Plug the pvs holes, and consider removing the smart tabs. Test it and report your findings.

        Comment


        • #5
          Originally posted by Juttmk View Post
          1994 Mariah Z222
          7.4l engine
          Bravo Outdrive
          Captains Call Exhaust
          SX trim Tabs
          Current Prop Setup: Mercury Mirage Plus (No Plugs)

          The boat gets out of the hole really well, but lacks the top speed I would expect. The engine was freshly rebuilt back in May of this year. I just noticed the prop cavitates very badly when the exhaust is switched from captains call to the prop. Is there any chance that having the holes open on the Mirage plus prop would be causing the loss of RPM. Please see the prop calculator below. Any help would be greatly appreciated!
          What are the RPMs at WOT?
          You say cavitates badly when the exhaust is switch from noisy to tolerable. Is the switching done on the fly, or prior to acceleration? I/Os having a different type of torque curve compared to a 2 stroke outboard respond differently to venting of the prop.
          Those vent holes are very large but there are plugs with a variety of different sized vents. I use one of the two mid sized vent holes. Slips a lot during below planing speeds, but pulls real hard from 25mph thru 55

          "Loss of RPM" a prop that has succumbed to ventilation will lose its grip on the water, and allow RPM to increase

          There are other things that can be causing the 'less than expected performance'. Throttle linkage not adjusted correctly thus preventing complete opening of the throttle plates. 1994, probably a carbed engine, secondaries not opening enough. Ignition timing incorrect or not advancing as engine increases speed

          Comment


          • #6
            Just my tuppence worth....

            Having mainly had pretty powerful boats (none as powerful as your 7.4) I've never found the need for the vent plugs to be removed. In fact, I find them to be a terrible experience, having tried it. It's like the clutch slipping on the car. Not a fan. Understand the idea, but shouldn't be required with boats like yours and most like them.
            A better idea is to get the right and correctly suited prop.
            If the screen shot you posted is what your actual details are, I'd say there are adjustments to be made.
            Firstly, the mirage prop (as far as I know) is just a basic stainless version of a standard black max ally prop. No real benefit in this. Get yourself a decent performance orientated prop more suited.
            Secondly, if you state your revs are 5200....you are way above what the max rpm should be for that motor. You have plenty of pitch increments to play with. I'm not surprised it gets out the hole well, haha.
            You need to up the pitch of your prop. You can at the very least go to a 21", if not a 23". The latter will reduce the holeshot, butt should see her flying.
            Bottom line, get a decent prop like a laser, tempest, enertia, Rev 4 or similar of at least 21" and she will be a different boat.
            You also don't need the plugs out the prop....that's for underpowered boats....yours isn't that.

            Comment


            • #7
              The mirage plus is actually a somewhat high performance prop. By modern standards it a bit dated but thereís a reason why itís still highly used in performance applications. It has a large diameter, moderate rake and high cupping.

              The SS equivalents of the black max aluminum prop would be the vengeance/Silverado.

              After looking, it seems the 7.4 has a range of 4200-4600rpm. If youíre turning 5200, your engine wont last too long.

              Plug the PVS holes. Test. Record speed and RPM. Itís possible your tach could be off. You can turn the adjustment screw back and forth a few times and see if that produces lower numbers. If the engine is still overring then itís time to look at higher pitch props. For now do whatís cheap and easy.

              Comment


              • #8
                The rpm range mentioned above...is that for the MPI ? Is the carb not slightly higher ? Not too sure about the 7.4.
                I'll stand corrected about the mirage perhaps...I always thought it was just a run of the mill prop.

                Comment


                • #9
                  I just googled mercruiser 7.4 rpm range and various sites said 42-4600. Probably accurate as most mid 90s boat engines were based on low speed truck engines from the 80s.

                  The mirage was pretty standard on scarabs, fountains, and cigarettes back in the day. But this is back when 350HP was a lot for a boat, and speeds approaching 70 were considered flying. Itís been surpassed by props like the Maximus, but itís still a heavyweight brawler in its own right.

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                  • #10
                    Very true !

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                    • #11
                      Thanks for all the help!! I purchased the plugs for the prop and was completely blown out of the water. Below is before and after the plugs were installed. I'm sure I have enough trim left to possibly gain even a couple more mph.

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                      • #12
                        Excellent stuff.
                        That said. I always thought that with the plugs out, when up on the plane at speed, the effect of them was non existent ? The way the exhaust gas travels through the prop doesn't mean it is allowed to vent through the holes and cause cavitation. It only is meant to happen at low speeds and when accelerating from displacement speeds.
                        It seems unusual that you have been affected by it.
                        In fact, just thinking on it now...it may well make sense.
                        I have the corsa marine switchable exhausts too. When the exhausts are closed or in silent....some exhaust still comes out the through hull exits. Perhaps because the through hub exit isn't getting the full flow of gas required to have the effect of cancelling out the vents, this is causing the venting and cavitating.
                        I might just have answered my own question, haha.

                        Comment


                        • #13
                          I find those results very strange. The vent holes should only be 5/16" or so, and should not allow that kind of slip differential, at least in my experience. If the vent holes were larger, more slip is possible.

                          On a related point, I do not think his setup will be helped by using the vents, so plugging them is best.

                          Comment


                          • #14
                            Originally posted by Chris1956 View Post
                            I find those results very strange. The vent holes should only be 5/16" or so, and should not allow that kind of slip differential, at least in my experience. If the vent holes were larger, more slip is possible.

                            On a related point, I do not think his setup will be helped by using the vents, so plugging them is best.
                            ​The vent holes on Merc props are half inch. You buy the plugs with different size diameter holes to fine tune them
                            2004 Caravelle 187

                            Comment


                            • #15
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                              Never lost any top speed when I removed the solid plugs on my Enertia, but then I never tried running it without any plugs, as the ones I use are 5/16-3/8. Normally I would say the vents are self sealing when enough water is passing over the vents to basically seal then closed and force the exhaust gases through the hub

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