blade number & type talk

Discussion in 'Props' started by aitkenkingfish, Oct 20, 2010.

  1. aitkenkingfish
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    aitkenkingfish Junior Member

    Im interested in understanding the differance between 3 & 4 blade props as well as blade shape. whats it all about?:confused:
     
  2. jehardiman
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    jehardiman Senior Member

    Blades are added and blade shapes are changed to play with the Blade Area Ratio (BAR) and the Center of Pressure (CP) of a propeller to adjust the thrust produced and torque absorbed by the propeller at a given rpm and speed. Generally speaking, the optimum propeller would have a single blade, an infinite diameter, and an infinitely small BAR. That being unrealistic, propellers are designed to produce a given thrust, at a given advance ratio (J = velocity/diameter*RPS because pitch is proportional to diameter ) and diameter.

    Usually in design, there is a maximum diameter that can be installed on the hull and the best efficency is found by using the maximum diameter. Once this diameter is reached, thrust can only be increased by decreasing the advance ratio (i.e. increasing the RPM for a given speed) and/or increasing the blade width (i.e. blade area). Decreasing the advance ratio and increasing the blade area reduce effeciency, especially if the blade area is increased towards the tip because it moves the CP outward increasing the torque needed to rotate the blade. Also, increasing the blade area increases the pressure gradent on the blade back (i.e. the "forward" side of the blade) which leads to cavitation which reduces thrust.

    It can be shown that there is a break point, based upon the BAR and RPM necessary for the given thrust at a fixed diameter, where it is more efficient to have more, narrower blades, than fewer, wider, blades. Also, we have to consider the prime mover. Many ICE produce thier maximum HP at high RPM which means low torque. It is therefore necessary to move the CP of the blades in towards the hub to reduce the torque absorbed. This is contrasted to LSD or steam engines where abundent torque can be produced a low RPM and blade area can be moved out to the more efficient, faster moving, blade tips.

    Propeller design and optimization is a fairly specialized field in Naval Architecture. Depending on how deep you want to get into the design theory and tradeoffs, I can recommend some texts. For the average boater or mariner, it is better to leave propeller design up to the experts or expect to spend a lot of time and money playing with blade shapes. There are some propeller calculators out there, that will get you in the ballpark and allow you to see the differences changes make, but they all have assumptions and limitations. As I normally tell my sponsors, you will spend 90% of your time and money trying to get the last 10% of the answer.

    Edit to add; Hopefully Don MacPherson will see this thread, it is his specialty.
     
  3. gonzo
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    gonzo Senior Member

    Theoretically, the most efficient propeller has a single blade. Because of balancing problems, a two blade works better. Diameter constrains forces the use of more blades at less efficiency.
     
  4. aitkenkingfish
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    aitkenkingfish Junior Member

    im interested in a prop with a high lift for getting the transom up
     
  5. jehardiman
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    jehardiman Senior Member

    Props don't do that; as a matter of fact they do the opposite. It is the aft hull shape, shaft angle, and trim tabs if any that control transom squat.
     
  6. gonzo
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    gonzo Senior Member

    The only way a propeller will provide lift, is with a very high shaft angle. However, it will have little forward thrust. Do you want to go up or forward?
     
  7. powerabout
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    powerabout Senior Member

    Cleaver props are transom lifters, no bow lift at all assuming very low rake angle.
    Probably even says that in the Merc prop guide
     
  8. yipster
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    yipster designer

  9. jehardiman
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    jehardiman Senior Member

    Propellers do not generate an "up" force perpendicular to the shaft unless the flow is at a significant angle to the shaft or blade. This means that any "lift" force is more a function of hull shape than propeller blade number or shape (which is the OP question).

    While it is possible to manipulate this force with blade rake, this is very inefficient (which is why an engine manufacturer may recommend it) requiring more HP and larger diameter to generate the same lift as triming the shaft as well as disappearing as soon as you enter a turn. Additionally, this would only work on boats with props just aft of the transom such as I/Os and OBs, otherwise normal prop squat will occur. While it is a small effect, it is another case of throwing HP at a small boat problem rather than designing the problem out. <<Shrug>>.
     
  10. yipster
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    yipster designer

    cleaver props also called surface piercing props if i'm not mistaken operate ventilated and half submerged
    when i once asked an arneson guy on a show how that might sidewalk he said the vertical force was of greater concern
    athough the prop gives no lift itself, its half submerged working would give transom lift than i belive
     
  11. jehardiman
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    jehardiman Senior Member

    Cleaver props don't have to be surface piercing, but they are super cavatating. "Cleaver" just refers to the wedge shaped blade section caused by the truncated front of the blade. It is also important to realize the SP props have twice the number of blades than they need because only half of them are being used at a time (the whole BAR issue). So you could have a super cavatating cleaver prop with 3 blades and a surface piercing cleaver prop with 6 blades for the same HP application.

    Surface piercing drives do give a bow up torque because the center of thrust is below the shaft CL. This is why they are normally trimmed down at speed so the thrust line passes through the cg of the boat. Really, thats all thats going on. It is the manipulation of the thrust line relative to the CG of the boat. Below the CG, bow up...above the CG, bow down.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    When a propeller is working completely submerged, but close to an open surface (ie not under a boat bottom or in a tunnel), the inflow into the propeller disc becomes asymmetric; it varies with depth. As a result, the incidence angles will vary with blade position, and as a consequence of that, the outflow will have a downwards composant, causing a stern lift. Zero (or even negative) rake increases the effect.

    This effect is strong enough to cause "prop-riding", provided the hull and propeller characteristics match.
     
  13. jehardiman
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    jehardiman Senior Member

    Perhaps, but I have seen no near surface high speed prop data that supports that induced flow into the prop generates a up force similiar to prop walk. What I have seen is data that shows wake inflow angle effects bow up moment. This is a time vairant function and one of the major drivers of propeller vibration. I do concur, that in the presence of wake inflow at an angle, that varying the rake will induce a change in this moment generated by the prop thrust.
     
  14. powerabout
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    powerabout Senior Member

    I was assuming an IO or outboard
     

  15. jehardiman
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    jehardiman Senior Member

    LOL, this is what I find so funny some times, that some small fact is grasped and used to explain everything.

    With a IO or OB the prop is going to be far more influenced by the power leg and cavitation plate angle, that any bow up flow effects influnced by blade rake would be masked.
     
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