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Thread: High Thrust vs. Standard Thrust Outboard Choice

  1. #1
    Join Date
    Apr 2006
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    Puget Sound, WA
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    Default High Thrust vs. Standard Thrust Outboard Choice

    I'm trying to decide between these two choices of 9.9 HP motors to power a Poulsbo boat. Its 18 ft by 5'-6" and will likely weight around 1000 pounds fully loaded for camp cruising. These boats are displacement hulls and I have no intention of trying to plane it. I would guess that the high thrust would be great for situations of trying to power against wind and/or tide, but would like run higher RPM for typical conditions. Seems I would be actually wasting fuel and making more noise for no gain except in situations mentioned earlier. Am I thinking about this correctly?

  2. #2
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    Aug 2016
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    Hobart, Tasmania, Australia
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    Hi rayven. I went with the standard Yamaha 9.9 4 stroke for my displacement boat weighing around 1200kgs. I purchased my engine with a shallow pitched propeller and have been happy with the engine. My propeller grips the water poorly in reverse gear and can make maneuvering more difficult. Perhaps the big prop on the hi thrust model is more effective in reverse? Good luck

  3. #3
    Join Date
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    Not all "high thrust" units are alike. They may have any combination of:

    Engine itself tuned to put the peak torque at a lower RPM;

    Lower gear unit with lower gear to allow lower prop RPM; and/or

    Larger and/or differently pitched prop.

    With that latter, some are pitched a bit flatter on the theory that it's like low gear on a car, while some larger props are pitched more steeply but with engine and lower unit providing much slower RPM.

    Then there are "dual" units that I don't understand at all.

    Normal outboard props put plenty of power into the water at speed but being small tend to osterize the water when pulling at very low speed and have poor "bollard pull". In general, "high thrust" is designed to put more useful thrust into the water at lower speeds, hence their popularity for trolling or sailboat motors.

    You might take an assumed cruising speed of six knots (or whatever you figure your hull should top out at laden, something a little less than "hull speed") and figure what RPM will get you there (use the pitch but toss in maybe 30% slip. If that's a good RPM, you're in the right place.

    At these speeds, the larger the prop and greater the pitch and lower the prop RPM the more efficiently she'll meet the load. For every prop, there's an ideal RPM for the load and given that load other speeds are a compromise. You're trying for the most likely compromise.

  4. #4
    Join Date
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    Quote Originally Posted by Ian McColgin View Post
    .....
    Normal outboard props put plenty of power into the water at speed but being small tend to osterize the water when pulling at very low speed and have poor "bollard pull"..
    The vocabulary gain in this place is amazing.

    Osterize - who knew?
    Someday, I'm going to settle down and be a grumpy old man.

  5. #5
    Join Date
    Aug 2016
    Location
    Brooklyn, NY
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    i thought maybe he was thinking more along these lines:


  6. #6
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    Quote Originally Posted by coelcanth View Post
    i thought maybe he was thinking more along these lines:

    Yes... he was being all 'poeticalish'-like...
    David G
    Harbor Woodworks
    http://www.harborwoodworking.com/boat.html

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  7. #7
    Join Date
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    Quote Originally Posted by P.I. Stazzer-Newt View Post
    The vocabulary gain in this place is amazing.

    Osterize - who knew?

    If the offered definition is as Ian intended, then I am especially curious about "bollard pull."

    Jeff

  8. #8
    Join Date
    Sep 2010
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    468

    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    We have a Yamaha FT 9.9 GEL (Long schaft - thrust) on a 27 ft 2.2 ton keelyacht and are very happy with it`s performance: economical, powerfull and has really good stopping capabilities in reverse. If I knew what " Osterize" was maybe I could tell you wether it does it or not ;-)

    Greetings from Germany,

    Alan

  9. #9
    Join Date
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    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    I had in mind the classic Oster food processer, the daddy and mommy of them all.



    I used to enjoy demonstrating with a high power high speed outboard. Say 100 HP. If I had the tow line tight and was well braced, the boat could power up but I held it firmly as the prop turned the water to froth - osterizing the water.

    If I gave the boat some slack, a chance to get a little speed on, it would pluck me right off.

    Bollard pull is the first test. No way on. No jerk pull. Just static pull. The science of prop design is interesting but in general, blade area has a lot to do with how much horsepower you can put into the water. The other variables are pitch and prop rpm. A flatish pitch with plenty of rmp can put power into the water, rather like driving in low gear, but too much rpm and you're just spinning water out sideways and osterizing the water with lots of froth. Of course, too steep a pitch and the prop is just stalled. So, it becomes a judgement call.

    G'luck

  10. #10
    Join Date
    Sep 2010
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    468

    Default Re: High Thrust vs. Standard Thrust Outboard Choice

    Thanx for clearing that up Ian! I can now report that our Yamaha 9.9 Thruster does not "Osterize" the water neither does it commit "Bollard Pulling"....at least not in my presence, thank goodness!

    Greetings from Germany,

    Alan.

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