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boating mathematics

Discussion in 'The Lounge' started by leckig, Aug 7, 2007.

  1. I am a little lost in the way you guys talk about boat motor power.

    Gas motors, you give their power in HPs, thats easy, easy to convert to Kw and of course to PS (metric horsepowers).

    Now for electric, troling motors you use, if I understand right, lb of force, pounds of force. What is that and how does that convert to metrics? I have been looking for the answer for a while and cannot find it.

    It seems to me more logical to have electric motors be measured in Kw, it would make it easier to judge battery life out of that.

    Also, I see some ald outboards on john boats that are 10 HP and they are very little. Than I see some new (but 2 stroke) 10 HP outboards and they are just huge, not much smaller than 225 HP standing right by it. How strange?
     
  2. misfit

    misfit MOD SQUAD

    greg,there is a formula for converting thrust and also a formula for estimayting running time for electric motors.if i can find the info again,i'll post it.
    as for size of outboards,the newer ones are bigger,and the four stroke motors are much bigger than 2 strokes,simply by the nature of their design.there are many more parts to a 4 stroke,and other differences,make them necessarily bigger and heavier.
     

  3. LakeRaider

    LakeRaider EEEEEK!

    One horsepower = 746 watts. Twelve lbs of thrust = one horsepower. Iv,e seen all kinds of trust equals hp ratings. Maybe somebody here can make things clearer? :eek: Raider
     
  4. misfit

    misfit MOD SQUAD

    multiply volts times maximum amp draw of motor to get watts.divide watts by 746 for app. hp.

    example----40 pound thrust,12 volt motor = 40x12=480 watts/746= app. .64 hp.

    typically,your trolling motor will draw i amp per hour per pound of thrust,depending on conditions.in essence,if you have a 40 lb. thrust motor,it will draw 40 amps per hour from the battery at full power.this is not set in stone,but is probably pretty close for most motors.
     
  5. Excellent! That sounds very realistic. Thanks misfit!
     
  6. misfit

    misfit MOD SQUAD

    you're welcome,greg.
     
  7. Watogo big boy. I knew you would find it.
     
  8. There is no conversion from pounds force to horsepower. One is a measure of thrust, the other a measure of work over time...you can't make a direct comparison. The thrust accounts for the shape, size and efficiency of the prop, whereas horsepower ratings do not.

    http://www.minnkotamotors.com/support/faq.asp?pg=general&lp=gb&pt=11

    If you want a comparison in the metric system, 1 pound of force is equal to 4.45 Newtons. So a 55 lb trolling motor gives you 245 Newtons of thrust.
    For those who don't think in Newtons, think of it as roughly 25 Kg.
     
  9. misfit

    misfit MOD SQUAD

    i have read that info also.and it is basically true.there really isn't a correlation between horsepower and thrust,and the numbers given above are more of a coparison of power produced by the motor,and not at the prop,which is totally different and dependant on factors you mentioned,as well as others.
    the more important numbers to me are the run time at different settings,which i think is probably more important to greg than actual power produced.
    and since i'm not polish,i don't have a clue as to most metric measurements:D ;)
     
  10. H2O Mellon

    H2O Mellon Hangin' With My Gnomies

    Man Misfit, get back out on Hoover. This is a post for the boring winter months!!!!!:) :p It's too much for my simple mind to handle! :eek:
     
  11. Hook N Book

    Hook N Book The Original Hot Rod Staff Member

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    Actually there is a conversion but not neccessarily the same type of thrust...but nonetheless, thrust.
    2.5 lbs. thrust=1 H.P. That's what we use in the areospace industry as a conversion factor. Granted this does not take in to account the additional resistance of water versus ambient air. :D
     
  12. misfit

    misfit MOD SQUAD

    and i was gonna say it's just a trolling motor,it ain't rocket science:p :D

    i never realized the wife was talking about the air when she commented on the "ambiance" in that restaurant the other night:confused:
     
  13. Hook N Book

    Hook N Book The Original Hot Rod Staff Member

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    My point exactly...! It was getting too complicated. :D

    They mean the same thing...Should have said ambient temperture...! :eek:
     
  14. misfit

    misfit MOD SQUAD

    now that you mention it,it was rather chilly.that must be why she was complaing about the ambiance:D
     
  15. Hook N Book

    Hook N Book The Original Hot Rod Staff Member

    6,110
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  16. misfit

    misfit MOD SQUAD

    yippee!!!i never won anything before:D
    thought i won a woman's heart once,but found out she just wanted my body for her own personal lustful reasons:(:eek:
     
  17. You mean "DREAMT" - right????
     
  18. there was supposed to be that laughing smiley after my comment - instead my dumbarse put it at the top of my post..........

    haha...........
     
  19. creekcrawler

    creekcrawler Konfused Kayaker

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    Don't do any math, just add a lot of chrome.
    Nobody'll ask any questions.
     

    Attached Files:

  20. heck, I should have thought about that - Newton is related to the weight being moved, and pound of force, well, a pound is easily calculated to metrics.