ISO highest power/weight ratio. Need some outboard suggestions!

Discussion in 'Outboards' started by salukikev, Nov 18, 2011.

  1. salukikev
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    salukikev Junior Member

    Hi Everybody!
    So I'm glad to have found this forum while researching outboards. I'm working on building a proof of concept prototype for a home built watercraft, and the quickest way to get to the finish line will most likely be to use an outboard boat motor, although some inboard motors might have a shot too.
    Essentially I'm looking for the highest available power to weight ratio for a motor in the 30-100hp range.
    My research so far led me to this page, in which the Yamaha c70 seems to be about ideal (as an example). So, I'm looking for something comparable in terms of power and weight. Less than 200lbs and more than 40hp.

    Of course things like reliability figure in as well, but other factors like cosmetics don't factor in at all (no cowl is required).
    I would love to hear some suggestions as to which model/specification motor would be a good bet with the outline generally specified above.

    Thanks in advance for any help!
    -k
     
  2. DCockey
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    DCockey Senior Member

    Visit the outboard manufactuers web sites which list the power ratings and weights of their offerings. Then divide the power rating by the weight.

    At the bottom of the page you referenced: Last Updated on 4/9/02 It looks to be out of date.
     
  3. salukikev
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    salukikev Junior Member

    I know it was out of date, it was just the best example of a list that would be helpful to me. I was looking for suggestions for particular models/manufacturers so that I wouldn't HAVE to go to every manufacturers website and collect specs on each individual motor (assuming they have them all posted).
     
  4. DCockey
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    DCockey Senior Member

    If you are in the US the outboard brands in the over 30 HP range are:
    Evenrude
    Mercury
    Honda
    Yamaha
    Suzuki
    Tohatsu
    Go to the websites, make your own list based on your needs.

    Is this part of a college project or similar?
     
  5. DCockey
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    DCockey Senior Member

    Also Nissan though they may be the same as Mercury.
     
  6. cyclops2
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    cyclops2 Senior Member

    Just read where

    All Mercury, UNDER 30 hp are, relabeled Tohatsu motors.

    If you want me to believe that.
    I will believe, Bond Bread is really relabeled ........
     
  7. salukikev
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    salukikev Junior Member

    I did visit some of the manufacturer's websites already, but since all options are on the table currently, (new, used, modified, rebuilt, etc.) the manufacturers websites aren't always so helpful, and certainly speak from the perspective of selling me their motor rather than experience across the board.
    This isn't a college project, but an internal development project for myself and my (small) product design company, so I'd be glad to save some money by buying a used/rebuilt model since many benefits of a new motor (cosmetics, cowling, warranty, longevity, etc.) won't be useful to my goal. In fact, I expect my project would immediately void any warranty from the get-go.


     
  8. salukikev
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    salukikev Junior Member

    I've been in discussion about purchase of this motor. Any opinions on whether this is a good deal or not? 55hp for 202lbs isn't too bad.
     
  9. tom28571
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    tom28571 Senior Member

    You did notice that this is a jet and not a propeller lower unit, didn't you?
     
  10. salukikev
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    salukikev Junior Member

    I did notice that, and I believe the jet is a plus for my purpose. I will readily admit though that I'm not terribly familiar. I would suppose that the concern is regarding power loss compared to a prop?
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    And thats a big concern as is the power loss too, IMO (and some experience too) around 30% or more..
     
  12. meren
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    meren Junior Member

    I have found following equations to fit in estimating maximum rated kilowatts for boats in case planing hull (under 8 m) is well designed:

    Pmax = (0,137 * minimum operative weight^0,6)^2*0,44*deadrise anlgle ^0,3) [kW],[kg],[deg]

    Other equation for maximum speed most likely to pass manouverable test for CE (boats with LH<8m):

    Vmax=45*Minimum operative displacement^0,33-(1,5+Bc^1,5) [t],[kn],[m]
     
    Last edited: Nov 26, 2011
  13. DCockey
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    DCockey Senior Member

    And if the deadrise angle is 0, ie no deadrise?

    Over what range of parameters do you expect this formula to be valid? How precise is it?
     
  14. meren
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    meren Junior Member

    These "cook book" equations are derived from sample of outboard powered boats which passed RCD manouverable test, some of boats "barely" and else with some margin left. Minimum operative weight (300...3000 kg) deadrise 15...28 degrees Lenght of hull below 8,0 m. Can't say these are more precise than -+15%. Maximum 90 degrees turning radius [m], [kn] with full throttle in tests 6* Lh for boats under 30 knots and 6 * Lh + 2*(Vmax-30) for faster ones. (Lh = lenght of hull)
     

  15. DCockey
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    DCockey Senior Member

    Thanks for the explaination. Presumably some of the boats have maximum power and resulting speed limited due to the test?
     
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