charts/rules of thumb for efficiency of different hulls (mono/cat/disp vs plane)

Discussion in 'Hydrodynamics and Aerodynamics' started by big_dreamin, Oct 20, 2016.

  1. big_dreamin
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    big_dreamin Junior Member

    Hi

    I've already stumbled across this searching for this topic:

    [​IMG]

    And i'm wondering if there are other charts like it OR rules of thumb that might apply for changing variables around. (ie less or more weight, longer vs shorter, even wider speed range, etc)

    I'm trying to mentally grok certain principles of hull design in a visual form outside of just text (or maybe there are even online simulations or calculators of some sort??) - alternately if someone can refer me to books or engineering texts I might get from the library that has charts or explanations like that would be just as good. :)

    I have other questions i'd like to ask inspired by that chart alone but unless I know if that's highly representative of how it always tends to work it's premature to pick brains.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Let me guess, that chart is from a source promoting the virtues of a displacement cat ? The performance at 20 knots seems optimistic.
     
  3. mydauphin
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    mydauphin Senior Member

    Depends... on a lot... But I dont think that a dispalcement Cat can beat a planning hull at cruising speed if all else is the same including speed in fuel economy. But of course we are not looking at hull design either.
     
  4. jehardiman
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    jehardiman Senior Member

    By the love of Mike!, you know that one change leads to many others that cannot be forseen. FWIW, hydrodymanicists are born, not made. If you cannot see the flow, you never will.

    Pass the salt.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    There is maybe a ghost of a chance that we of the great unwashed may glean a snippet or two ! :( And be able to spell "hydrodynamicist" :D
     
  6. big_dreamin
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    big_dreamin Junior Member

    Yes, that being said i'm aware the normal limit of 'hull speed' is the big bulbous blob of water in front of the boat which aren't really generated by really long skinny hulls. For speed I seem to remember a figure something like 1.34 x LWL used for monohulls, except that that 1.34 number becomes different numbers as the width to length factor gets skinnier and skinnier.

    I'm positive someone has done all the calculations and they're just sitting in a book somewhere, I just need to know the title and where to find that book. :)
     
  7. gonzo
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    gonzo Senior Member

    That chart shows a planing monohull as more fuel efficient than a displacement monohull of the same power and displacement. That is absolutely wrong.
     
  8. jehardiman
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    jehardiman Senior Member

    OK, Dyslexia strikes again. Visualizing flow?...not so much. I've never seen anyone who could not see it right off visualize it after "study". That's what makes CFD visualization programs so scary...people believe what a computer represents to them, not what they should grok on their own.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Well spotted. Yeah, that is crazy, at say, 8 knots, the displacement hull would be getting much better economy than the same size planing boat.
     
  10. Joakim
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    Joakim Senior Member

    The same chart is here http://www.catamarans.com/news/2006/04/catcomparison.asp
    It shows very high consumptions at low speeds. At 6 knots it would be very easy to get well below 1 liter/Nm. Even this can do 0.7 liter/Nm at 5.7 knots and 1 liter/Nm at 6.7 knots http://www.boattest.com/review/beneteau/2487_swift-trawler-44

    According to the chart all the boats are tested with 2*250 HP, so they should have equal consumption at WOT. The planing cat takes 3 l/Nm at 27 knots, which equals to about 80 l/h (way too low for 2*250 HP, even diesel should be 100-110 l/h). So the planing monohul and displacement cat at 25 knots should consume the same 80 l/h and about 3.2 l/Nm. And the displacement monohul at 17 knots should be at 4.7 l/Nm.

    The engines can't be the same or at least their WOT rpm is very different.

    Also the planing monohull curve doesn't look at all like a typical planing hull. More like a semidisplacement hull optimized for low speeds. A planing hull should have a maximum around 10-15 knots and then quite flat or even decreasing 20->30 knots.

    What's even the point testing a displacement hull with 2*250 HP, when 1*100 hp is more than enough?

    Not much real information in that chart!
     
  11. TANSL
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    TANSL Senior Member

    Do not confuse the efficiency of a boat, to be assessed at cruising speed for example, with fuel consumption at, say, 8 knots. They are totally different things.
    Perhaps the first thing you should do is define how the efficiency is evaluated: speed vs consumption, speed vs. power, ...
    Nor should we forget that the efficiency of a propulsive system, any how efficiency is defined, can't be the same at any speed.
     
  12. mydauphin
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    mydauphin Senior Member

    All boats are very efficient at 8 knots... a small engine can push a big boat at 8 knots.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I am referring to the chart posted by the OP that shows a displacement monohull getting less NMPG at all speeds, than a planing monohull of the same length, weight, and installed power. That would be very hard to construe. The chart really isn't that helpful, inasmuch as installed power being the same for each type is not realistic, anyone knows a boat with more speed potential will need greater power, to realise that potential, at equal boat weights. The chart is intended to promote displacement cats, but seems to have got a few things wrong.
     
  14. TANSL
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    TANSL Senior Member

    Mr Efficiency, I totally agree with you regarding the chart in post # 1. It is totally inconsistent.
     

  15. Richard Woods
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    Richard Woods Woods Designs

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