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Prop pitch question

Discussion in 'Props, Shafts & Seals' started by CSkipR, Feb 14, 2010.

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  1. CSkipR

    CSkipR Member

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    I want to reduce the pitch on my 28x39 4 blade nibral props as my boat is turning 100rpm + less than the recommended 2300 min. Typically speaking will reducing the pitch by 1 degree (to 38) be enough or will I need to do 2 degrees? Thanks
  2. Capt J

    Capt J Senior Member

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    Generally with diesels it is 1" pitch= 50 rpms. I would say 2" of pitch will get you there. I would recommend calling Frank + Jimmy's propellor shop in fort lauderdale and running your scenario by them (they've done a lot of props for Cabo, as has lauderdale propellor) and they'll get you where you need to be.
  3. Bamboo

    Bamboo Senior Member

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    What is the cup of the props now? Both cup and pitch will alter top end RPM. I dropped 2 inches of pitch this summer off my wheels from 42 to 40 and gained another 120-130 rpm, but I had no cup to begin with. Capt J's got it right- call several prop shops to get the info you need to make a good decision.
  4. CSkipR

    CSkipR Member

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    I can't tell you the cup right now although I have it from my last prop scan. You gained 120 - 130rpm which is what I hope to gain. Did the engines load factor & exhaust temp drop? What happened to your top end? Did it slow you down or remain about the same. How about cruise speed?
  5. Capt J

    Capt J Senior Member

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    Load factor and exhaust temp will drop quite a bit, when you turn the proper rpm's. Top end should actually increase a touch because you're spinning the propellors faster and making more HP. Cruise at say 1950 rpm's before and after and you'll lose around 1.5knots.......but you could cruise them at 2000rpms and still have the same load factor as before the pitch change at 1950rpm's.
  6. Bamboo

    Bamboo Senior Member

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    I've got mechanical 16-92's- so no computer to tell me loads. I have no egt gauges at the helm. Top end was a touch more, and I had alot less smoke at all rpm's.
  7. Capt J

    Capt J Senior Member

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    Anytime you reduce pitch (and are overloaded to start with), it will reduce load at a given RPM. Full throttle will still be 100% load. But if before you turned 2200 rpms and now turn 2320rpms at full throttle, at 1950rpms your load percentage will be reduced probably about 10% and reducing load will also lower your EGT. High EGT's relate typically directly to being over-propped.
  8. Marmot

    Marmot Senior Member

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    Inquisitive readers might do well to take a look at some propeller curves.
  9. CSkipR

    CSkipR Member

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    Props

    Capt J,
    Thanks for the info.
    Skip
  10. CSkipR

    CSkipR Member

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    Prop pitch reduction cost

    What should I expect to pay to have the pitch on my props reduced by 2? From 28x39 to 28x37.
    Thanks