3 blade propeller vs 4 blade propeller

Discussion in 'Props' started by Michelle Elliott, Jun 27, 2019.

  1. Michelle Elliott
    Joined: Jun 2019
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    Location: Pinehurst, TX

    Michelle Elliott New Member

    I heard a four blade prop will pull better at low speeds but it will have more drag and be marginally less efficient at higher speeds,is that ture ??
    I had 3 blade propeller on my outboard which was bought on hexautoparts this spring,got me back on the water and it makes my boat faster.
    What's your opinions about the advantange and disadvantage of 3 blade and 4 blade prop?
     
  2. tpenfield
    Joined: Dec 2016
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    tpenfield Senior Member

    In addition to the number of blades, there is also the matter of pitch. Typically when you add blades, you get rid of some of the pitch. An example would be going from a 21" pitch 3 blade prop to a 19" pitch 4 blade prop.

    Yes the 4 ( and 5 ) blade props will get you a lower planing speed and is great for water sports, like pulling a skier or wake boarder up out of the water.

    As you mentioned, your trade off is at the higher end of the speed and RPM scale where a 3 blade prop will out perform a 4 blade prop. You can expect to loose about 5% off the top speed with a 4 blade prop. Regardless of the number of blades, you will want a prop that is well matched to the power and size of the boat in question.

    The 'best' prop is really the best one for your needs, be it 3, 4, or even 5 blades.
     
  3. gonzo
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    gonzo Senior Member

    Propellers with more blades are used to get more blade area when the maximum diameter can't be increased. Are you using the boat for water-skiing?
     
  4. TANSL
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    TANSL Senior Member

    What area are you talking about, the area of the blades or the projected area of the blades ?. In any case, with 3 blades you can get as much projected area as with 4 blades. As has already been said, one must also take into account the pitch of the blades and, probably, several other factors that have nothing to do with the number of blades. To determine the correct propeller for a boat, depending on the service that the boat is going to give, not only the number of blades must be counted. If I could give an answer to the OP, I would do that, so I think the best thing I can say is that he consults with an expert, forgetting advice that does not contribute anything and that, sure, will only confuse him.
     
  5. tpenfield
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    tpenfield Senior Member

    The OP's name appears to be Michelle. That would be 'her' not 'him'. :D
     
  6. TANSL
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    TANSL Senior Member

    There is nothing like having traveled and speaking languages. That, yes, that is the answer that will solve the problems of the OP. And "OP (original poster)" is a generic noun that the same applies to him as to her.
     
  7. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Generally, yes, a 4- blade will be less efficient than a 3-blade for the simple reason that the leading edge of the prop loses efficiency as it encounters the disturbed water left by the blade ‘in front’ of it. Typically a 3-bladed prop will have more space between the blades to reduce that effect.
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Presumably less vibration with a three bladed. I haven't seen too many 4 blades, there are a few fives around though, that suit aerated water better than three blades.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    More blades equals more wetted area, which doesn't make them spin any faster !
     
  10. gonzo
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    gonzo Senior Member

    Blade area and number of blades are not a direct relationship. The blade area depends on diameter and shape of the blades. More blades reduce vibration, like more cylinders on an engine. The highest theoretical efficiency is a one blade propeller, but it would be unbalanced. You can think of blade area as the width of you car's tires. Larger tires will give you more traction and acceleration. They will also have more friction and reduce mileage.
     
  11. TANSL
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    TANSL Senior Member

     
  12. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    The OP’s post is severely lacking in detail, the only way to truly determine the best prop for a given application is to examine all known factors, which are nicely listed on any propeller shops worksheet.
    For starters, we need to know the type of boat, the target speed, hull speed, motor type, hp available, weight, transmission ratio, current prop and it’s performance graph, motors torque curve and probably a whole lot more.
    There is no magic number of blades that will make a boat faster or more efficient, each vessel must be considered based on its own peculiar set of specifications.
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    There is a distinct preference for three blades with outboard and sterndrive makers, I have been told that an odd number of blades is preferable because of vibration issues, don't know the truth of that, but there are also a few five blades for those same engines, which reinforces the idea. Years ago you'd see two blades on small engines, maybe still that way with tiny ones.
     
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