A simple propeller question

Discussion in 'Inboards' started by CDK, Sep 21, 2007.

  1. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Can anybody tell me what the hub diameter is for the average 15" to 17" 3-blade bronze prop ? Is there a standard for hub dimensions or a fixed relation to the prop diameter? Even the extensive Michigan catalog does not mention this at all.
     
  2. jehardiman
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    For standard B, Ka, and Kd props, the hub is 0.167 D. In general, ship/workboat sized hubs are sized to be stronger than the shafts even after the press on. On smaller wheels, the hubs are usually sized to ensure no damage to the hub when the shear key goes. Older wheels vary depending on how they attached to the shaft (keyed, pinned, pressed, etc.).
     
  3. CDK
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    CDK retired engineer

    Simple question, simple answer. Thanks very much!
     
  4. jehardiman
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    jehardiman Senior Member

    Now you've embarassed me. :eek:

    That hub diameter is at the line of the blade face only.

    On the back end (fwd side of the hub) the hub is a little larger, on the face side (the aft side of the hub) it is a little smaller, so it has a taper to it. That taper generally follows the specified shaft taper, normally 1:12 for shafts over 6", less for smaller shafts.
     
  5. CDK
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    CDK retired engineer

    No need for embarassement. I wanted to know the hub dia at the blade face. The prop shaft will rotate in a tube with the same dia as the hub, so the waterflow will be as undisturbed as possible.
     
  6. redtech
    Joined: Feb 2007
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    redtech Senior Member

    CDK,
    what would seem to be a simple question may not always have simple answer. lets ask some question to make this simpler. horse power? prop shaft diameter? drive type? prop material? thru-exhaust? over the hub exhaust? this information i think will help answer your question.
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Hello Redtech,
    Here are the answers: 2 diesel engines, 70-80 HP each, 25mm prop shafts, approx 2 ft long, down angle 5 degrees, encased in oil-filled stainless steel tubes. A tunnel drive, like Saro, but incorporated in the bottom, not bolted on. Bronze props, probably 15", larger if necessary, one turning left, the other right if there is any advantage, both turning right if it makes no difference. Four small rudders alongside the props. Exhausts are thru-transom, just above the waterline now.
    This may all sound a bit weird but has to do with Berkeley jets that will be removed after two years of service. They leave gaping holes in bottom and transom that I will turn into half circular tunnels.
     
  8. Çemberci
    Joined: Apr 2007
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    Çemberci Senior Member

    Propeller HUb Diameter

    We are finding Propeller Hub Diameter according to Shaft Diameter.
    See Attached File.
    Oktay Çemberci
    www.cembercidenizcilik.com
    İstanbul/Turkey
     

    Attached Files:

    • hub.doc
      hub.doc
      File size:
      48.5 KB
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      1,145
  9. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Decision point

    The decision point for the props is getting nearer. I've made a mold for the tunnels that has a width of 16,5" and 2 ft. lenght. Allowing 1,5" clearance, the prop diameter must be 15". But I cannot decide what pitch to use.
    The original boat was powered by two 140HP Mercs, had 30 kn. top speed and could steadily plane at 22 kn with 3200 rpm. At the time they were also available with twin 120 HP Volvo Penta's, which had approx. the same performance but took a bit longer to climb out, so most owners added trim flaps.
    I've added 1 ft waterline lenght, there is no drag from 2 Alpha-One stern legs and nearly 500 lbs less weight. But I only have two 75 HP diesels.....Is 150 HP enough to obtain planing speed? Don't ask me about weight or displacement, I am not in a position to find that out.
    It is my (well informed) estimate that the torque of the VW turbodiesels is about the same as that of the Mercs at 3000 rpm, the diesels just don't rev as high as the gasoline engines, hence the difference in output..
    Could I set my speed target at say 24 kn. or would that be too high?
    I've read the discussion about comparing a 50 HP outboard and a diesel powered Levi drive in 25 ft Asian fishing boats, but I have no idea about the construction. They surely do not have two cabins, a bathroom and a kitchen to carry around.
    And what sort of props do I need?
    My starting point was that I would make surface drives, but when I look at it now, I doubt there will be anything but water in the tunnels. They are fully submerged while at rest and will come up in plane, where they may draw in some air from the sides if I leave gaps there.
     
  10. Frosty

    Frosty Previous Member

    I like your idea but your venturing on dangerous ground there. I wish you well. You may get success and you might well, but i'le guarantee you will need to play with a lot of props there.

    Your idea sounds ok but how long are the tunnels, they will need to start as far forward as possible.

    Im not a naval architect or anything but the hub of a prop depends on the stress of the root of the blade. Im not too sure why you are concerning yourself with this.
     
  11. Çemberci
    Joined: Apr 2007
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    Location: Istanbul/Turkey

    Çemberci Senior Member

    Pitch

    Dear CDK
    Some Hull parameters we need to calculate hull resistance and also Tunnel effect..
    Hull waterline Lwl
    Hull Chine Lenght Lpx ....
    Hull Chine Beam Bpx
    Deadrise Mid ship
    Projected Chine Square
    Center of Gravity
    .............
    Engine Horsepower
    Engine Rpm
    Reduction Ratio
    .....................
    Tunnel Dimension

    Oktay Çemberci
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Attached are sketches of the boat's transom and hull, everything measured in millimeters. These are not the shipbuilder's drawings, but adaptations from earlier sketches I made while the boat was out of the water.
    Center of gravity was at approx. 1/3 boat length with sterndrives, it will have shifted a fraction now because there is no stern load anymore.
    Engines are 2x 75 HP at 4000 rpm, gearbox reduction 2:1.
    Tunnel width is 425 mm, length 620 mm plus a faired part from the original hull on both sides of the stern tube.

    (From Frosty:) "Im not a naval architect or anything but the hub of a prop depends on the stress of the root of the blade. Im not too sure why you are concerning yourself with this."
    It seemed like a good idea to make the tube the same dia. as the hub to reduce turbulence....
     

    Attached Files:

  13. Çemberci
    Joined: Apr 2007
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    Location: Istanbul/Turkey

    Çemberci Senior Member

    Finding Size

    Dear Mister
    I find these sizes for your boat.
    Hull Speed 24 knot
    Hull weight 2.8 ton

    Diameter = 14.00 inç
    Pitch = 17.33 inç
    Dar = % 59.05
    3 Bladed
    OR
    Diameter = 13.00 inç
    Pitch = 18.50 inç
    Dar = % 50.00 .. Michigan Dyna
    3 Bladed

    Oktay Çemberci
     
  14. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Hello Çemberci,

    Many thanks for the excellent service.

    Just to satisfy my curiosity: I made the tunnels 16,5" so they can accommodate 15" props, yet you suggest 13 or 14 inches. Is there a philosophy behind that?
    Would you recommend left and right hand rotation or will the difference not be noticable?

    Regards,
    Cornelis Koger
     

  15. Çemberci
    Joined: Apr 2007
    Posts: 127
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    Location: Istanbul/Turkey

    Çemberci Senior Member

    Propeller

    Dear Mister
    Left and Right propeller is recommended.Three weeks later some one
    is coming to New York/USA.If you like to order our propeller.Here is
    Our Price for ManganBronze 13 inç ***211 Euro .14 inç ***290 Euro
    each one
    OKtay Çemberci
    İstanbul/Turkey
     
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