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  • Prop question for the pros here.

    I have a 19' chaparral runabout with the 220hp alpha one mpi. The boat came with a three blade 21p aluminum propeller and I can run mid 40s when I'm solo and the boat will Rev to 5200rpm. I purchased an aluminum dual geometry 4 blade 21p and it slowed my rpms down to a straining 4200, so I returned it and got a 4 blade 19p. It runs a bit slower but is definitely better when running heavy loads but again my rpms get to about 5200. I want something that'll give me just a little bit more top end and keep my rpms below 5k at WOT. will a 3 blade 21p stainless lower my rpms because it grabs better or will it potentially increase rpms because it's thinner? Or should I get a 23p? All help is welcomed, thank you.
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  • #2
    One prop will not fit all conditions.

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    • #3
      Thank you, my primary question is will an SS propeller increase or reduce my WOT if it is the same pitch and conditions. My 3 blade 21p aluminum turns 5200 rpm. Would an SS prop of same design and pitch turn faster or slower under the same conditions as the aforementioned aluminum propeller. I understand the traits of each and researched propellers pretty heavily when I landed on the 4 blade aluminum. After reading I expected it to pull my rpms down because everything I read said that the turning point 4 blades will act like a higher pitch due to less flex and the additional blade can increase drag, however in actual usage this was not the case. It did perform better when the boat was loaded down but most of the time its my wife and two kids under 3yo. I know SS grabs better due to less blade flex so I was curious if it affects the WOT for the same reason. If I turn 5200 rpms with aluminum would the same prop in SS turn around 4900? I typically cruise around 4k but if I could get a few more MPH and lower wot without stressing the engine and drive it would be better. I kept aluminum for the last few years because I was a beginner at the helm but I think I can graduate now.

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      • #4
        The MPI 5.0 should be 260 hp. The carb version is 220.

        How fast is that thing supposed to spin? It sounds like it is hitting the rev limiter. The rpm range is something like 4600-5000, but I guess it could be 4800-5200
        .Four blade props just give you more blade surface and less slippage. Think of it as four wheel drive for a boat. The penalty is drag. You lose top end speed, but pick up a better 0-30 holeshot, among other benefits, which makes towing sports more enjoyable.

        If you are towing, a 4 blade 19 pitch is probably perfect. Nobody goes top speed except to check to see if you are propped right.

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        • #5
          Sorry I thought I included that, it's the 4.3lx motor with the 220hp. I always use the 4 blade when towing or carrying heavy loads but I'm looking for a new cruising prop. If I can get a few more knots out of cruising speed why not. My 3 blade aluminum prop needs to be retired. I'm just curious if SS affects the WOT in a similar set up, since it has less slippage.

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          • #6
            2 things for you...

            1. If your numbers are correct...Get yourself a tempest plus 3 blade 21”...all will be well. You will only be doing about 4950-5000 rpm max and on the limiter at that. Ideally your 4.3 mpi wants to be about 4800 for best all round. Going by your numbers, a large diameter stainless 21” like the tempest should see you about good as it gets.

            2. Only thing I would say is that if it’s the assumed usual 1.62 gearbox on these types of set ups..and you are getting to 5000+ rpm and only early 40’s...that’s likely a lot of slip.

            Perhaps best you check your tachometer out or get a vessel view mobile to verify it. Very useful when propping a boat...amongst lots of other things.

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            • #7
              Also. A 4.3 lx is a carb. Is it an lx or an mpi ? Very different things really.
              All the best.

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              • #8
                It is the mpi, thank you. The prop you mentioned is almost double to cost of some other stainless props I have seen. I'm not trying to cheap out but is there really that much of a difference between that prop and say a Solas or turning point?

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                • #9
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                  Originally posted by Jgcrock99 View Post
                  It is the mpi, thank you. The prop you mentioned is almost double to cost of some other stainless props I have seen. I'm not trying to cheap out but is there really that much of a difference between that prop and say a Solas or turning point?
                  In my opinion...yeah. No doubt. If you aren’t that bothered, I’d just stick with a black max ally prop. They will outperform all but the well known mercury stainless props of a quality nature. I’d certainly rather run a black max over a Solas stainless.

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