Light Power Cat

Discussion in 'Props' started by fritzdfk, Apr 26, 2015.

  1. fritzdfk
    Joined: Jul 2004
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    Location: Alaska

    fritzdfk Junior Member

    I have a light custom power catamaran, 18'. It has a single Yamaha T9.9 high thrust outboard with a stock Yamaha three blade 11 3/4 X 9 1/4 high thrust prop. The engine WOT rpm rating is 4000-5000. I am getting 5100 rpm and 9 knots WOT. I am wondering if a little more pitch will give me a higher top end. Yamaha does not make an aluminum 11 3/4 prop with more pitch. They do make a plastic 11 3/4 X 11 prop that I am thinking about trying. Can the aluminum 11/3/4 X 9 1/4 prop be given more pitch? There are a variety of other props available all less diameter but with more pitch. Am I better off with the large diameter props?
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Don't get too greedy, the poor thing can't be expected to run the same diameter and increase the pitch, whilst maintaining the current WOT rpm reading ! :D It would be a matter for experiment, I think, to see which propellor suits best, but you don't run the engine at WOT other than for testing purposes, do you ? The product line of outboard makers typically has propellors for each motor decreasing diameter, as pitch increases. But, the odds are you would be better to stick with the larger diameter unit at that modest boat speed. Is the recommended rpm range really going as low as 4000, or is it 4500 ? By what you say, you are under-propped, albeit slightly, so you have room to increase pitch, but I think I'd prefer the finer pitch if headwinds e.g., washes off much speed.
     
  3. SaltOntheBrain
    Joined: Feb 2007
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    Location: crosbyton, TX

    SaltOntheBrain Senior Member

    That close to your target RPM, you might just have a prop shop add a tiny bit of cup.
    Or make some templates so you can do it yourself and get it back to original if it doesn't work.
    If the 9.9 isn't remote steered, you may try to see if you can borrow another that isn't high-thrust (lower-geared). See how it performs and base your next step on the results.

    LF
     
  4. Steve W
    Joined: Jul 2004
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    Location: Duluth, Minnesota

    Steve W Senior Member

    Probably your best choice would be to sell the motor and buy one more suited to your boat. The 9.9 Yamaha high thrust has about the lowest gearing of any motor on the market in order to swing that big prop, specifically to push heavy boats at displacement speeds, I cant think of a worse choice for a small, light cat. I have one on my 30ft, 7000lb Gemini sailing cat and it is perfect for that.

    Steve.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Pretty hard to judge the suitability of the motor to the boat, without knowing more about the design of it and the weight, "light" is a flexible term !
     
  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    You should be able to get an aftermarket propeller with more pitch. They have the generic ones with the interchangeable hub insert.
     
  7. fritzdfk
    Joined: Jul 2004
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    Location: Alaska

    fritzdfk Junior Member

    The boat is a semi displacement design so is not meant to be very fast. It is an adapted, by the designer, sail boat hull. About 900 pounds or so.
    http://www.ikarus342000.com/ECOmotorboat.htm
    I had a 20hp outboard on it and had trouble with ventilation. The T9.9 with its extra long shaft and large diameter slower turning prop so far doesn't have the ventilation issues. I also redesigned a fairing forward of the leg and perhaps that made a difference.
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The recommended WOT rpm range for that engine is actually 4500-5500, so I'd say you don't have much scope to go up in pitch, at all. I think you will find you just be tinkering without much benefit, if there was more speed potential with a 9.9 than what you are getting, it would be revving higher than 5100 with the "fan" now fitted.
     
  9. fritzdfk
    Joined: Jul 2004
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    Location: Alaska

    fritzdfk Junior Member

    The engine is a 2003 model and in the owners manual and the service manual the rpm range is given as 4000-5000.
     

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

    My apologies, you are correct. I have never heard of a modern outboard with the maximum rpm range less than 4500. It was the standard 9.9, 4500-5500, apparently, not yours. Pretty simple here, try a higher pitched prop just to see what happens, but if you are going to push it along at maximum revs to get a little speed, then your fuel burn will be greater than what it would be with a larger engine, working in a more efficient rpm range. Once you go past 80% of the maximum revs the engine will pull, fuel flow goes ballistic.
     
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