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New motor, what prop?

MozartMozart Posts: 3 Greenhorn
I just re-powered my 1998 189 CF with a new 175 Yamaha SHO. I'm not getting the same performance out of the prop that came off the old 2 stroke 175 VMAX. What prop are you guys running on that same hull and motor? 
I kept spinning my old Yamaha prop with the pressed hub so a few years back I threw on a Michigan Merc prop with the square hub assembly. No problems since with spinning. 


  • demersalangelerdemersalangeler SE FLORIDA Posts: 179 Deckhand
      We need some more information. Including number of blades, prop diamator, blade pitch, current engine max rpm with prop & model designation. Also what is your complaint/concern? Loss of top speed, fuel economy, slow plane time? 

     Some people won't like this, but you did a great thing by removing the yamadog prop & replacing with the merc prop. Those Yamaha props are a death trap. The engines are great, but the prop can separate from the inner hub. Leaving you propless. 
  • FishingpervertFishingpervert Deltona, FloridaPosts: 815 Officer
    edited May 21 #3
    no offense intended.
  • MozartMozart Posts: 3 Greenhorn
    On the old motor I was running a ss 4 blade 14 x 22. That was slightly different from the yami prop I had previously. I'll have to look for that old prop tonight and get the numbers. Top end with a light load for the old motor was 57 mph.
    On the new motor I'm seeing around 4700 RPM's wide open and trimmed. I recall I was hitting around 50 mph. 
    I'm not as concerned about top end as I am getting out of the hole quickly. Right now it's much slower getting on plane. I expect the 4 stroke will simply be that way. I am getting some bad vibration just coming out of neutral at a low rpm. I'd like to get that smoothed out. I run fairly heavy with fuel and gear most of the time. Thanks for the replies!
  • demersalangelerdemersalangeler SE FLORIDA Posts: 179 Deckhand
     From what you're saying it sounds like 22 pitch might be just a tad bit much for the new motor. I would expect you to be closer to 5k rpm max or slightly above. I think your vibration may be due to a little to much load at lower rpm's. You might try to go down to 19 pitch 4 blade. Going 19x4 will give you more power out of the hole & you will probably end up with around the same max speed because you will gain a few hundred rpm.
     As mentioned in the above post before he erased it. Sometimes you can find a prop shop that will swap/rent props to try. 

     I love 4 blades. Most people dont talk about this, but a 4 blade is usally a stern lifting prop. A 3 is usally a bow lifter. (This is something that should be taken into consideration when the running attitude of your hull/engine combo is observed.) Just the nature of the 4 should help you pop up out of the hole. I also believe that the 4's have unmatched maneuverability. Especially at lower rpm's like around the dock. 

     This last trip out for me we swapped on a test prop. It was a 4 blade 21 pitch. It was a bit to much pitch for our engine/hull combo. We achieved very similar results to what you describe. The prop that we usually run is a 4 blade 19 pitch. In my opinion its the perfect set up for this engine/hull cimbo. We've had everything from 3 blade 17 pitch to the 4x21 & I  think the 4x19 is the right set up for me. 
     On this particular hull/engine combo a 3 will take longer to plane & breaks water far back on the hull when leveled out, but has a very stern down attitude. The 4 is faster to plane archives a lower cruise speed & allows for more control over the running attitude. 

  • CageyCagey Central FloridaPosts: 917 Officer
    edited May 21 #6
    You should do a prop pitch test. Take your engine specs and take note of maximum RPM's where full horse power is developed. Your choice of prop should be determined by measuring RPM's at full throttle. If the RPM's exceed maximum specifications then you gotta up pitch the prop. If you do not reach factory specs for full throttle maximum RPM's then you gotta down pitch until you are within 150 RPM's wide open throttle.

    A properly pitched motor will be within 150 RPM's of maximum without going over maximum factory specs.

    This is how I was taught to properly adjust a motor's output to the size and weight of any boat. And it is best to do this testing under full load.

    For example, I usually fish with 2 men and gear in my boat. So when I prop pitch tested I chose my prop pitch based on my full loading of the boat.

    The goal is to dial in the pitch of the prop to near maximum RPM's and maximum horse power at wide open throttle without going over factory specified maximum RPM's for that motor which can cause damage. Slightly under is ideal if within 150 RPM's or about the amount one pitch change on the prop will deliver.
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