Outboard prop modifications

Discussion in 'Props' started by Slowboating, Oct 22, 2021.

  1. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    I have a Yamaha 50hp highthrust outboard. It pushes my Bayliner 2670 at a comfortable cruising speed of 6 knots at 3200 rpm and 9.5 knots at 5100 rpm.
    Roughly in there anyway

    Using a prop slip calculator, I found that it’s efficiency is only around 48%

    the prop is 14” diameter and 11” pitch.

    I have a spare propellor the same, so I thought I’d modify it. I didn’t want to increase the pitch, as I think it’s just fine for the application and due to space, I can’t increase the diameter. It’s as big as the leg will handle already. So, why not play with the surface area!

    I work in an aluminum boat building shop, so I thought I’d have a crack at it. Here’s the first blade I modified. It will increase the surface area of the prop by 22” squared
     

    Attached Files:

  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I don't think you will gain anything. Is that the prop that came with the engine ?
     
  3. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    It’s the spare, but yes, and I agree, it probably won’t gain anything except in wind or waves, it will probably hold the boat a little more. It’s an experiment for now, but will be shrouded with a rice nozzle (kort nozzle) later. That’s where I think it will have the most benefits.
     
  4. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Hmm where to start.

    The best way to up prop efficiency has always been slow the wheel down and increase the diameter. Obviously not possible on an outboard with a fixed diameter and gear ratio.

    Fishing boats have a limited draft and are constantly chasing the most thrust possible from a given horsepower. In the early 2000s the popularity of high dar props rose with the prevailing belief that more blade was better. Recently there has been a resurgence in the fish master style props which have a very low surface area. Suffice to say I'm guessing if the same boats who went one way 10 years ago are going back, it was less a monumental improvement than they had hoped.

    As to the kort nozzle. They were wildly adopted initially on the steel boats, but weren't perfect. A boat that swung an open 66 could maybe swing a nozzles 58 or if lucky a 60. For the guys with the room that swapped equal size for equal size saw fairly significant gains but with the diameter loss situations it was less spectacular. I'd say anecdotally about half kept the nozzle past 10 years. In an inboard on a retrofit a nozzle is insufferable to maneuver.

    Granted a nozzle on an outboard would not hinder steering as the unit itself would move. Keep in mind a nozzle is much more than a ring around a prop. Kaplan props are specific to nozzles, and the ring wing dimensions matter. One upside is done well a true 14 inch nozzle should be more oomph than a 14 open.

    With all that said. Consider the operating rpm your prop is operating at. Then ask yourself if your new high skew high dar custom prop is gonna equate to long lower unit bearing life.
     
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  5. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    Well, I have some answers for you!
    The prop spins around 1500rpm. At 3500 rpm. (I went to those numbers because they’re simple)

    I anticipate that the prop will more be efficient at cruising rpm and less efficient at WOT, prop speed. I’m not trying to gain top end in this case.
     
  6. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Do you think that prop will be balanced well enough to hold up long term to 1500 rpm?
     
  7. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    I’ll have it balanced properly, though I’ll give it the initial balance myself. I think it will be just fine.
     
  8. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Prop research with your own prop is fine. Have fun. But don't expect to learn anything that corporations, universities, and military labs with annual budgets that exceed your entire lifetime earnings didn't know before we were born.

    @comfisherman 's description of movement in the profession field has nothing to do with new information. It's an insider's view of how salesmen, who are by definition professional liars, exploit and encourage shifts in fashion to earn sales commissions from hardworking folks who trust them.

    Read Dave Gerr's Propeller Handbook for a solid introduction. There's a few things in the book I object to, but only because I'm pedantic, and I apply reductio ad absurdum to pretty much everything. For anyone reasonable it's solid.
     
    Last edited: Oct 24, 2021
    baeckmo likes this.
  9. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    My information within the industry pertains to boats that are heavy relative to power, with fixed propeller diameter. The owners/ captains are on the water daily for months and know their vessels rather well. For the guys rocking open wheels, several have recently gone to fishmaster style wheels. The trend for the last 20 years had been higher and higher blade surface area props like a teiginbridge. One fairly prominent boat was hauled a few weeks ago and had swapped its original prop to a fishmaster, the owner had said it seemed to hook up better at slower speeds when heavy.

    Now, when push comes to shove this fall my new boats getting a veem, but some rather astute seafaring guys are trending another direction.
     
  10. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    And after all the research I’ve done, leaning on the backs of those who spend the millions in scientific R&D work, surface area was the best solution for my scenario. Can’t go bigger, don’t need more pitch, can’t buy a blade with the surface area increase that I wanted, we must modify. I’m not going into this blindly, even though it’s definitely an experiment. The nature of this post was intended as a show and tell.
     
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  11. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

    Most excellent @Slowboating . I'm doing a very expensive project myself, reinventing the wheel that Albert Hickman invented in 1914. Things can go as senselessly out of fashion as they can go into fashion.
     
  12. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    @DogCalvery

    I WANT TO SEE
     
  13. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry I aim to misbehave.

  14. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    More blade area if required, is best accomplished with more blades, the penalty with any increase in blade area is added frictional drag
     

  15. DogCavalry
    Joined: Sep 2019
    Posts: 1,781
    Likes: 917, Points: 113
    Location: Vancouver bc

    DogCavalry I aim to misbehave.

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