Torque ratings

Discussion in 'Outboards' started by AMZ, Jul 22, 2012.

  1. AMZ
    Joined: Mar 2007
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    Location: Bellingham, Wash.

    AMZ Junior Member

    I'm wondering if there's any published information available on the torque output for outboard motors, specifically in the 15-30 horsepower range. I'd like to try and narrow down what engine would be most suitable for a boat I'm interested in.
    I've googled until I'm blue in the face and pretty much come up empty-handed
    In my ideal world there'd be a more empirical method available than the ballpark approach of "well, based on your size and weight, you should be okay with such and such".
    Am I being unrealistic here?
    Any constructive feedback would be appreciated.
     
  2. sottorf
    Joined: Sep 2007
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    sottorf member

    it is very hard to find any useful data on torque ratings or even proper power curves for OBMs as nobody publishes.

    I recommend you take a look at www.boattest.com and try and find the closest match of boat and use the associated test data to try and reverse engineer the engine infromation you require.
     
  3. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    the torque peak is very close to the HP peak and of about the same number
    Not a lot of use using the torque number as you cant really change your gearbox ratio and propellor diameter as you could for an inboard
     
  4. Frosty

    Frosty Previous Member

    The torque of a 2 stroke is meaningless and difficult to put a number on. needles to say its pretty low at certain RPM
     
  5. AMZ
    Joined: Mar 2007
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    AMZ Junior Member

    Thanks for the reply. I was mostly interested in what size outboard/propeller would get my boat to the speed I wanted. I can do the math from the torque rating at a given speed, but the horsepower doesn't get me there.
    AZ
     
  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Um, yes, you are going about it the hard way. Engine mfgs will build several hp ratings for a given block. They may also have slightly different prop diameter and gear ratios. If you have a screaming fast boat, go with the highest hp rating for that displacement. It will have a small diameter and a lot of pitch and a high gear ratio. If you have an average sort of boat, go with the middle hp rating. If you have a tub, go with the lowest hp rating for that block, and low gear ratio if available. That is how you work the torque issue. Have a little faith in the folks that design these things (and thank them for doing all these calculations for you) :)

    More important is to choose a brand with a good reputation, resale value, and parts support in your area.
     
  7. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    If you know power and rotational speed you can calculate torque.

    Power = Torque * Rotational Speed

    Torque = Power / Rotational Speed

    Corresponding units and/or conversion factors are needed.
     
  8. powerabout
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    powerabout Senior Member

    I can think of some 100,000hp 2 strokes that make that power at 100rpm
    thats make quite a big torque number
    http://www.boattest.com/resources/view_news.aspx?newsid=3055
     
  9. AMZ
    Joined: Mar 2007
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    AMZ Junior Member

    That's not the half of it. Large, direct-reversible slow-speeds have an effective speed range of about 40 RPM. There's also a crtical speed range that has to be passed through quickly when accelerating or decelerating to avoid setting up a harmonic that would damage or destroy the engine.

    Cool engines, though. Years ago I worked on container ships with Sulzer RND's and RTA's and B&W/MAN L-series engines.
     
  10. AMZ
    Joined: Mar 2007
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    AMZ Junior Member

    From Wikipedia (for the RTA96);
    torque 7,603,850 newton metres (5,608,310 lbf·ft) @ 102 rpm
     
  11. Frosty

    Frosty Previous Member


    Thats cheating. That engine is blown so a good charge of air is possible. Its is a wet sump 2 stroke,--not quite the same as a 15 to 35 outboard.
     
  12. AMZ
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    AMZ Junior Member

    Dry sump on that one...
    Same for the B&W's.
     
  13. Frosty

    Frosty Previous Member

    Now we are getting into terminology. Dry sump can mean scavanged its still oil lubricated bearings.

    The big 100,000HP in question has white metal bearings.
     

  14. powerabout
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    powerabout Senior Member

    I'm with ya Frosty
    my bad
     
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