Resistance factors, planing hull at low speed

Discussion in 'Hydrodynamics and Aerodynamics' started by Mr Efficiency, Dec 6, 2010.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    We all know a planing hull isn't ideal for low speed operation, due to higher resistance from the hard chine interfering with smooth flow of water and the "suck" of the submerged transom. Would some know-it-all, er....expert tell me in ballpark figures which of those two is the more retarding, and by what margin ? Shapes vary of course, but think of a boat where the chine is immersed from at least midway back. I am particularly interested in a power cat in this regard, where chines will be immersed from well forward.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    To clarify low speed, I am thinking displacement "hull speed" for a boat that length.
     
  3. Alik
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    Alik Senior Member

    Pls refer to Savitsky/Mercier paper 'Resistance of transom-sten craft in the pre-planing regime', 1973.

    For planing power catamrans immersion of chines is not the major problem. Usually on planing cats hulls are too close together. So there will be very unfavorable interaction of hulls around FnL=0.5 where resistance is likely to increase by 40% compared with hull specifically designed for this speed.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The interaction of the waves from each sponson in the tunnel ? That could do my head in......what is the shorthand version of the forces at work there ?
     
  5. Perm Stress
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    Perm Stress Senior Member

    For monohull, planing hull below hull speed has resistance ~2-3 times more as hull of same length and comparable proportions, but designed for displacement sailing.
     
  6. Easy Rider
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    Easy Rider Senior Member

    Perm,
    That's what I thought on a thread awhile back but I got extreme resistance ... even from TAD. I thought a 40' Willard would take about 1/4 to 1/5 as much power to go 7 knots as a Grand Banks 36 or 42. Lots of guys here thought there wouldn't be much difference. I'll probably always wonder about that.

    Easy Rider
     
  7. Perm Stress
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    Perm Stress Senior Member

    A while ago, when studying all sorts of resistance calculation methods, I got this kind of results. At first it was a surprise.
    After some thinking about, it should not be a surprise:
    Planing hull is anything, but streamlined shape, and anything but smooth volume distribution from stem to stern...
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, that is a lot, so would the transom be the greatest contributor to the increased resistance ? Or the abrupt change at the chines ? Or it really impossible to generalize ? I know from experience that a small planing hull travelling slowly and having weight shifted forward so the transom is much higher than normal, will run noticeably more freely, so my guess is the transom drag is the main retardant......but I have no further proof. Of course there are plenty of dedicated displacement hulls with hard chines, though not with a straight run aft. What is never seen is a deep submerged transom for true displacement craft.
     
  9. Alik
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    Alik Senior Member


    2-3 times? Can't be that much.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Not sure you'll get many replies if those you seek an authorative reply from by instantly ridiculing them.

    To answer your question, it is neither. Whilst they both contribute, the length displacement ratio is the biggest factor that effects resistance.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, I shouldn't phrase enquiries as if I am talking in a diplomatic cable. :p

    What I mean is, to what is the extra resistance, over and above what a similar sized round bilge displacement vessel would have, to be attributed ? I "guess" the "suction" at the transom above all else, but maybe the chines are significant contributors.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    No, just ask a simple question. If you ask a question with smart arse phrasing, you’ll get similar smart arsed replies, and probably incorrect too. But that is your prerogative how you view “experts” and thus the manner in which you expect to understand things you currently do not.

    Define “extra resistance”?..... Each boat has a resistance based upon many factors. What may be true for one boat/design is not or may not be applicable to another, for many varied reasons. Thus you are not comparing apples with apples.

    Having a large skeg that adds frictional resistance may be the death of one design, yet the saviour of another to prevent directional instability, as an example.

    Naval architecture cannot be boiled down to one liners and absolutes. The design of a boat is greater than the sum of its individual parts, always has been always will be. And in this case, hydrodynamics, whilst being more of an “exact science” than the holistic Naval Architecture, does not always yield absolutes, owing to the aforementioned example.

    A round bilge has less resistance than a hard chine at low Froude numbers, but so what? So long as the whole design, satisfies the SOR, who cares which is slightly better or not? The objective is to meet the SOR, not to state which is better in ‘one minor out of context’ hypothetical situation.

    To push this point further in any form to obtain some type of absolute underlies a lack of understanding of what naval architecture is about. A naval architecture requires trends of what happens when XXX is done and then when YYY is done under various different conditions, not absolutes.
     
  13. Boston

    Boston Previous Member

    from what I've been able to learn these last few years its not the chines at all that really add to resistance, its the run aft, the displacement for length and the transom drag. I think it was Apex who once told me you could just about run a brick at displacement speeds and it wouldn't make that much of a difference. Bellow hull speed of course. Above its a whole nother ball game.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Alright, that seems like a vote for transom drag as the principal villain, and of course we are talking at or slightly less than hull speed.
     

  15. Boston

    Boston Previous Member

    ya I'd wait for some more experienced hands to come into the fray before settling on any definite conclusions
     
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