Draft effect on Resistance

Discussion in 'Hydrodynamics and Aerodynamics' started by jesdreamer, Dec 6, 2015.

  1. jesdreamer
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    jesdreamer Junior Member

    Could it be that a deep draft hull might require more power than a hull of similar displacement but more wetted area and thus less draft -- I realize friction at lower speed ranges should be greater but I am talking of wavemaking resistance. It seems to me that pushing the water out of the way from a lower draft hull would take less power than from a deeper draft hull of similar displacement. I realize the total work (as defined by force and distance) might be theoretically the same but I can't get over the idea of less distance to move each element of water perhaps being more important and that the lower draft hull might take less power for similar speed -- Could time be a critical factor here?? The work to move "x" elements of water a longer distance might be similar to the work to move more elements a shorter distance but since power is work within a specific time, it would seem to me that the water displacement would have to move faster in the deep draft case to accomplish the same volume displacement within the same time and therefore require more power even though "work" in the 2 cases would be similar -- Any thoughts on the subject??
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Take two identical boats, same length same displacement.

    if one of the hulls has more WSA, owing to its shape (in what ever arrangement) than the other, it shall have more drag. So long as their is always residuary resistance present, this remains so. Unlike if the hull then becomes fully submerged...
     
  3. jesdreamer
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    jesdreamer Junior Member

    Not questioning wetted area effect on friction

    I realize that more wetted area yields more frictional drag, and stated this belief in original post -- but our question is per the effect of draft (not wetted area) on wavemaking resistance. With deeper draft, the hull has to move the water further away in the same time as a shallower draft hull moves the same volume of displaced water -- but moves it less distance. Now if both have to move this displaced water in the same time (in order to have the same speed) it seems to me that the deeper draft hull has to move it's displaced water faster -- thus consuming more power to do so -- Is this analysis reasonable??
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I am aware of the question.

    Additionally, the Fn v length-displacement ratio is a contributing factor.
     
  5. DCockey
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    DCockey Senior Member

    First consider a streamline object which moves beneath the surface. If it is just below the surface waves will be generated and there will be wave making resistance. If the object submerges deeper below the surface and moves at the same speed then surface waves will still be generated but the waves will be smaller and the wave making resistance will be less. Submerged even deeper the waves will be even smaller and the wave making resistance even less.

    Now consider two hulls of same length, same displacement and thus same length displacement ratio, but one is stretched vertically and compressed horizontally so that it is deeper and narrower beam. The deeper hull will be narrower and will have lower wave making resistance due to the deeper distribution and displacement and the narrower beam.
     
  6. DCockey
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    DCockey Senior Member

    Assuming the same displacement and similar shapes, the deeper, narrower hull would move water less, not more.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'll add...
    Wave resistance scales (roughly) like beam squared.
    Doubling the draft, while keeping length and displacement constant, means
    halving the beam, and therefore wave resistance will be reduced by a factor of 4.

    Of course, surface area (and hence skin-friction) will be greater the more you
    tend to move from (say) semi-circular cross-sections and towards
    semi-elliptical ones.

    jesdreamer: A simple program like Michlet can help you investigate the
    delicate interplay between components of resistance for some simple
    mathematical shapes. However, it will require some investment of time to learn.
    I'd start with the first example in the manual (a simple Wigley hull) and
    vary the draft to see the effect on resistance. You probably won't need to
    anything more than that to get some answers.
     
  8. jesdreamer
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    jesdreamer Junior Member

    Displaced water movement

    I just totally disagree with this -- if the one hull has more wetted area to yield less draft then it displaces a greater "area" of water a shorter distance (as hull moves through the water, not stationary) -- vs the deeper draft hull which displaces a smaller "area" of water a greater distance -- If so then it seems to me that to accomplish each case within the same incremental time period, requires the water displaced the greater distance to do so at greater speed and if "work" done in each case is equal, then to accomplish this work withing the same time period the deeper draft water has to be displaced at a greater speed and thus requires more power -- please explain how this analysis is wrong??
     
  9. jesdreamer
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    jesdreamer Junior Member

    Beam effect on wave resistance

    I agree with Leo's comment as to beam effect on wave resistance and understand it to have a major effect as indicated by the formulas -- But I am trying to keep this analysis focused on the effect of draft -- perhaps we could think of a light boat vs a heavy one of same hull configuration, and as originally posed, neglect friction which of course increases with wetted area?? (I don't remember weight entering into any of the resistance formulas) -- but volume displaced with light vs heavy would change, thus the way I posed original question to involve equal displaced volumes. It should take about the same "work" to displace a narrow, but deep volume as it might to displace a wide shallow volume of equal cc -- but the incremental elements of the narrow deep volume have to move further and to accomplish the displacement within the same time should therefore have to move faster & thus require more power to do so -- This seems to make sense to me
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    You are arguing with some heavy hitters above, jesdreamer, they are not making it up, simply reporting what empirical observation shows, and if you are sceptical why not make some simple models and do some tests yourself. You appear to have developed a notion that water is being displaced downwards further in the deep draft type, which you assume requires more work, if one followed the "logic" of that, why isn't that energy being returned in the after part of the boat when the bottom is rising toward the surface again ? The whole idea is a misapprehension, but even the internal logic is flawed, it sounds like you think it is "heads I lose, tails they win".
     
  11. John Perry
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    John Perry Senior Member

    I dont understand Leo. In a previous post here (which I cannot now find) I presented the graph below which I generated using your very own Michelet computer program. This compares the drag of two hulls which both have the same length, the same displacement, the same wetted surface area, the same general form (i.e. both are canoe stern hulls with eliptical underwater cross sections) but they have different beam and draft. One hull has 1m wl beam, the other has 0.6m wl beam. Needless to say, the wide hull has less draft than the narrow one.

    If it were true that wave resistance scales (roughly) like beam squared (at constant displacement), then the wider of these two hulls would have (roughly) 2.77 times more wave resistance than the narrower one. The graph below shows that the wider hull does have a bit more wave resistance than the narrow one but the difference is nothing like a factor of 2.77, and indeed to me it would be very counter-intuitive that changing the beam while keeping the same displacement could make such a huge change in wave resistance.

    I wonder what's wrong here?
     

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  12. jesdreamer
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    jesdreamer Junior Member

    Simply reporting what empirical observations shows????

    1) -- I am not trying to argue with anyone, just trying to understand what might be taking place -- rather than blindly following some empirical info.
    2) -- I don't think much water is being displaced downward with forward motion of the hull -- but I do visualize a lot of water being pushed out of the way sideways (& do feel that the shallower draft hull would exhibit more hydrodynamic buoyancy under forward motion but I am not questioning this which I am willing to accept based on common sense).
    3) -- I have tried to propose 2 situations which involve the same "work" as defined by mass and distance moved -- and I specified that both situations involve the same work -- but questioned the relative power involved (which is work completed within in a specific time) and since I believe the deep draft involves moving some displacement water further, if within the same time, by F=MA we would have more acceleration requiring higher force and therefore more power --
    4) -- Energy into the water during the motion discussed is returned as wake. So if we limit discussion to draft (only), then it would seem that the shallower draft hull would generate less violent wake
    5) -- I don't see how the question can be a misapprehension -- are you suggesting that a deeper draft hull does not move water farther out of the way??
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    It's no good belting your head against a brick wall and wondering why it won't yield, you can only go on what empirical observation shows, not insist it should conform with some pre-formed idea you have. You'd be best to study hydrodynamics from the ground up, than to try and have practice conform to your theory. You have stated that energy is returned as wake, which I offer no comment on as to it's veracity, but if this excess energy expended by deep draft, as you imagine, is being returned proportionately, what is the fuss about ?
     
  14. jesdreamer
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    jesdreamer Junior Member

    Original question was about Power

    The fuss is about HP -- It seems to me that power involved is the main criteria whether hull is for oar powered skull or for diesel powered commercial use
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Because I foolishly over-stated the case in the early hours.
    I should have said if displacement is not kept constant.
    Then if the beam is halved, weight is halved and Rw is 1/4 of the original value.
     
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