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3 vs 4 blade

Discussion in 'Post Yacht' started by kapoc, Nov 17, 2009.

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  1. kapoc

    kapoc New Member

    Joined:
    Nov 14, 2009
    Messages:
    14
    Location:
    Virginia beach, Va
    I run a 1983 43 post and after talking with other charterboats in the area i have noticed that they all are running 4 blade props. I run 6-71 TI and usally burn around 31 an hour at cruise of 2150 RPM but am starting to wonder if I replaced the props with 4 blades if I will increase my cruise speed and fuel numbers.

    Any thoughts??
  2. Henning

    Henning Senior Member

    Joined:
    Sep 22, 2009
    Messages:
    940
    Location:
    Ft Lauderdale FL
    The less blades you have on a prop, the more efficient it is. It's much a matter of HP and diameter. The key limitation is prop diameter and how much power you can transfer per blade. The more blades, the more power you can transfer, but also the more drag you encounter which wastes power. A longer blade also has more leverage and less slippage transferring a greater percentage of power per blade. Smaller diameter more bladed props spool up to speed quicker and will get you on the step faster, but won't be as efficient once you're there.

    Call Frank and Jimmy's prop shop in Ft Lauderdale and talk to them to get you set up with the best prop for your application.
  3. blueboat

    blueboat New Member

    Joined:
    Jan 23, 2005
    Messages:
    16
    Location:
    My main boating areas are Lewes, DE, and Hatteras,
    I have a 1982 42' Post. My Capital gears had to be replaced. I went with Twin Disc MG5085SC gears but the ratio was 1:1.42 vs 1:1.54 for the Capitals.

    At first I could only get one gear, so I put it in on the port side. The props originally were 3 bladed Columbias 26 x 28. I put a new 26 x 27 on the Twin Disc side. The result was bad noise and vibration. I thought it was the install but nobody could find anything wrong.

    It was a big decision to put the second Twin Disc in because I thought I might be throwing good money after bad. In the end, I put it in along with a set of 24 x 27 4 blade custom wheels that were actually cheaper than off-the-shelf Columbias.

    The boat is now smoother than it was originally and speed is what it was before I changed gears. It actual seems a little faster at cruise.

    Since I changed the gear and the wheels at the same time, I'm not sure exactly what made the improvement. My theory is that the 3 bladed prop didn't have enough blade area to carry the load at the higher shaft speed.

    Engines: 6V92 J&T 530 HP
    2300: 29.7 knots
    1900: 23 - 24 knots
  4. kapoc

    kapoc New Member

    Joined:
    Nov 14, 2009
    Messages:
    14
    Location:
    Virginia beach, Va
    That is the info I was looking for thanks.....did the 4 blades do anything to your fuel numbers???
  5. Beau

    Beau Senior Member

    Joined:
    Jan 15, 2010
    Messages:
    2,265
    Location:
    Beaufort, NC
    Kapoc,

    One thing for sure, you will notice a tremendous difference in the "bite" of those four blade props at low maneuvering speeds
  6. kapoc

    kapoc New Member

    Joined:
    Nov 14, 2009
    Messages:
    14
    Location:
    Virginia beach, Va
    Thanks for all the advice i'm going to be looking into getting the prop change now anybody have a gross est on what price range i should be expecting??
  7. Beau

    Beau Senior Member

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    Jan 15, 2010
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    2,265
    Location:
    Beaufort, NC
    Kapoc

    I'm no help on cost estimate, but do suggest you stay with nibral for durablity.
  8. blueboat

    blueboat New Member

    Joined:
    Jan 23, 2005
    Messages:
    16
    Location:
    My main boating areas are Lewes, DE, and Hatteras,
    I would suggest calling Ralph at Digital Prop Shop at Herrington Harbor North Marina, 389 Deale Road, Tracys Landing, MD 20779. Office phone is 800-804-7757, cell is 410-867-8878.

    He got me some kind of custom props from Asia. I don't remember the name, but he'll know. They were cheaper than Michigans and the blades looked very sophisticated.

    He recommended 26 x 28, but because I was a little overwheeled before, I went with 26 x 27 which are near perfect. This is with 1.42 ratios on a 42 with 530 HP 6V92s. The numbers on a 46 with 6-71s would probably be a lot different.

    Disclaimer: I make no money from this. I hope it helps.