3 blade or 4 blade prop?

Discussion in 'Props' started by Dave T, Jan 17, 2012.

  1. Dave T
    Joined: Sep 2011
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    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    I am trying to find some info and opinions on which is better a larger diameter 3 blade prop or a smaller diameter 4 blade prop. I have built an outboard using an 11 horse Briggs [see my thread 11 horse Briggs outboard conversion under do it yourself marinizing]. I ran some rough calculations with the prop calc program from Castle marine and I will need either a larger diameter 3 blade prop or an 8 1/2 diameter 4 blade. To use the 3 blade required I'll need to cut off the cavitation plate and raise it about an inch this I can do if needed or I could go with a 4 blade 8 1/2" that would clear the plate. Is there any advantage to either solution?

    Dave T
     
  2. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    I assume you are looking for high thrust, not high speed?

    I asked a Honda dealer that question just last week

    He said go for the 4 blader. But then Honda is one of the few outboards that come with a 4 blade option.

    So I hope someone else answers your question to give an unbiased view

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. tuantom
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    tuantom Senior Member

    You're using an 11hp low rpm engine - without getting lost in the details, and if costs are equal, I'd think not having to cut off or modify anything with the 4 blade prop has some weighty advantages right away.
     
  4. Submarine Tom

    Submarine Tom Previous Member

    For the degrees of magnitude involved here (small) go with the four blade and skip the modification.

    -Tom
     
  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If the diameter is not restricted, three blades will be more efficient.
     
  6. tunnels

    tunnels Previous Member

    But 4 blades will be smoother running and has a smaller wake !!! :p

    Personally having a outboard and used both 3 and 4 bladed props the 4 blades was the better option and just kept the fully reconditioned 3 bladed prop for a spare under the seat !!;)
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Given that 95% of outboards carry 3-bladed propellors, they must have an inherent advantage. There are 2,4,and 5 bladed options in some cases, but it seems experience has led to 3 being the norm, except in specialty applications.
     
  8. gonzo
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    gonzo Senior Member

    Outboards have a limited space so four blades often work well. The anti-ventilation plate is the limiting factor.
     
  9. sabahcat
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    sabahcat Senior Member

    cheaper to make than 4 or 5 bladers?
     
  10. gonzo
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    gonzo Senior Member

    They are cheaper and more efficient. A two blade would be even more efficient, but have a bigger diameter and not as smooth running.
     
  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    A 4 bladed prop is only useful where there is insufficient total blade area when sizing a 3 bladed to avoid cavitation. The disadvantage of a 4 bladed is that they are slightly less efficient, you would experience slightly lower rev's at full rack. 3 bladed props also help to avoid resonant frequencies.

    A rule of thumb is when changing from 3 to 4 bladed take about 1 inch off the size of pitch + diameter.
     
  12. Dave T
    Joined: Sep 2011
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    Dave T Senior Member

    Thanks for the replies guys. It will probably be about a month before I can get back to my motor projects I want to get my boat and trailer finished first and then I'll run some better calculations.

    Dave T
     
  13. whitepointer23

    whitepointer23 Previous Member

    2nd that
     
  14. tunnels

    tunnels Previous Member

    My own exsperiance of 3 and 4 blade props is ,4 blade any time over a 3 blade no doubt about it . The wash out the back of the boat is smaller and smoother , theres no vibration and its noticably quieter , gets out of a hole quicker and top speed in my case was almost the same but who travels at flat out every where they go? Just as a matter of interest it was a composite prop i changed to and 100% pleased with its all round performance . I got the 3 blade reconditioned like brand new and kept it as a spare !! :D:p:p

    Forgot to mention reverse is much better and more positive and it goes instantly in the dirrection the motor is pointed !!;)
     

  15. tunnels

    tunnels Previous Member

    But 4 blades are smoother running and has a smaller wake !!! :p

    Personally having a outboard and used both 3 and 4 bladed props the 4 blades was the better option and just kept the fully reconditioned 3 bladed prop for a spare under the seat :p
     
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