Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Good luck with that, I found it an uphill job.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    ....and to the rabbit that anonymously docked points for alleged "bible thumping", give yourself an uppercut, it might improve your comprehension skills, I did not endorse any religion, anywhere or any way on this thread. A little tip for you.....decrying atheism is not a vote for the bible, there is a position called agnosticism you may not have thought of..........:rolleyes:
     
  3. John Perry
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    John Perry Senior Member

    In a previous discussion under the thread title "two hulls which has least drag". I sketched two hulls, both with the same displacement, wetted surface, waterline length and both with semi-elliptical cross sections and circular arc waterlines as viewed from above. Some time after this discussion petered out I got around to running the Michelet drag prediction program for the two hulls, but with a rather less extreme difference in B/T than I originally considered. The results are below.

    Comparing these Michelet results with those from Moland as referenced by Ad Hoc I note that the effect of B/T on total drag as predicted by Michelet is considerably more than for Moland's results. Also, whereas Moland indicates a wider shallower hull to have slightly less drag than a deeper narrower hull Michelet predicts the reverse.

    At, say, 5m/s, a fairly typical speed for a 10m LWL multihull in good conditions, Michelet predicts 7.7% greater total drag for the wide shallow hull than for the deep narrow one. I would say that such a drag difference is not insignificant in the context of competitive yacht racing!

    I do not know how Michelet works so I have no idea how suitable it is for this kind of drag comparison, I am just stating the results so that Leo or others can decide whether they are relevant.

    One point that I would make is that increasing B/T with constant displacement and constant LWL increases the hydrostatic pitch stability of the hull, for small pitch angles, in direct proportion to B. Both the Moland results and the Michelet predictions are for calm water but if one hull were to pitch more than the other in rough water this would also affect the comparison. I suspect that it would be a complex exercise to predict how pitch stability impacts on sea keeping and in turn on the extent to which sea keeping affects performance, both by direct effect on hull drag and by effect on the rig. However, catamarans with low pitch stability, e.g. narrow beam canoe stern hulls, do have a bad reputation for pitching.
     

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  4. Ad Hoc
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    Ad Hoc Naval Architect

    Hi John,

    Interesting, thanks for the contribution.

    It is not just about how the program works, but how accurate it is!

    If i look at the Series 64, using your values of length, displacement, speed and the variation in B/T, there are differences.

    If I take a few spot checks, the Series 64 shows a range of 2-4% difference in B/T from 2 to 4. Whereas over a consistent range the results you obtained from Michelt is 7-9%. I assume you kept the transom immersion the same with your 2 hulls?

    I would suggest that at higher Fn's there is less correlation. than at lower Fn.s Im sure Leo would like to comment?

    It still figures in well with the results by Warren et al. Where they conducted a very similar exercise. Keeping all parameters the same, but varying the hull shape. Not as easy as it seems.

    Various Shapes constant L-D ratio.jpg

    Each had their own "sweet" spot, but all very consistent.

    Indeed.

    As also previously noted design is greater than the sum of its individual parts. So whilst one may elect to use a more narrow hull, citing a gain of say 4%, the changes to the hull form requires mitigation in the seakeeping aspect. The mitigation effects may well outweigh the gains in resistance, such as an increase in weight owing to appendages to control pitch and drag of said appendages, for example.

    It is interesting when you look at varying the L/D ratio, i.e. more slender. A clear correlation that the more slender the lower the accelerations.

    length variations.jpg

    Which indicates how the improved resistance characteristics can be used, that of a multihull. Thus, the Pitch instabilities can be mitigated. The down side is the increase in roll stiffness, which has undesirable snap type accelerations, and of course the interference drag of the 2 hulls too. And of course having to be creative with the structure to keep the overall weight down.
     
    Last edited: May 16, 2014
  5. John Perry
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    John Perry Senior Member

    I chose very simple hull shapes with canoe sterns for this exercise. That way I did not need to consider whether or not Michelet would be including the immersed transom area in the hull wetted surface or whether Michelet was making good assumptions about flow separation at an immersed transom.

    The two hull shapes I used are shown in the pictures below. The yellow parts are above the waterline and so are irrelevant to the hydrodynamics. You can see that if you split the grey underwater part of the narrow hull along the centreline then rejoin the two halves you get the underwater part of the wider hull. Hence it is certain that these two hulls have the same wetted surface so, not surprisingly, Michelet did predict the same frictional resistance for the two hulls.

    Sea keeping is digressing somewhat from the original thread topic, but I do wonder how to interpret your graph for Accelerations vs Length. The graph shows data for two hulls which have slightly different values for Lpp/Disp. Are these two hulls the same displacement but different LPP, or are they two hulls having the same LPP but different displacements, or are they two hulls differing both in displacement and LPP? I think this does make a difference to how the graph should be interpreted.

    Suppose for now that both hulls have the same displacement but differ in length. The graph shows that the longer hull has lower vertical accelerations across a wide range of wave height. I dont find it at all surprising that a longer hull has lower vertical accelerations, after all if we consider a hull long enough to span many wave lengths pitching will become insignificant. The magnitude of the difference in vertical accelerations for only a fairly small difference in Lpp/disp is perhaps more surprising, but this data does not include any information about the wave encounter frequency or the natural pitching frequency or pitch damping of the two hulls.
     

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  6. Ad Hoc
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    Ad Hoc Naval Architect

    But it still does not address the inconsistencies with published data at the higher Fns.

    The nomenclature used by the author is that of the normal L/D ratio. I'm sorry I assumed you would understand this, since data pertaining to hulls is, in general, referred to by the length-displacement (L/D) ratio....unless otherwise noted.

    Indeed.

    And that's the thing about hull design. As your hulls, for example, show no location/indication of where the propulsors would be, they are just 3D shapes for amusement. But when taking such into account for real design its effect on the overall PC, can have a significant effect on the amount of actual installed power required, far more than 2-4% "absolute" gains noted by software programs, especially at one end of an often "out of context" range. Does it still met the SOR..it is that simple.

    The design and by 'design' I mean everything that is to be considered in having a working operating and safe vessel on the water, is greater than the sum of its individual parts.

    Those that wish to keep looking at absolutes of one single isolated parameter, enjoy your programs for amusement :p
     
  7. oldsailor7
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    oldsailor7 Senior Member

    Quote. " But when taking such into account for real design its effect on the overall PC, can have a significant effect on the amount of actual installed power required." Quote.

    And if that power is sailpower, and the power is being applied to the hulls at the CE of the total sail plan, then there will be a rotary moment compared to the drag centre of the hulls, causing a bow down force.
    In the Buccaneer 24, for instance, the well rounded buttocks of the Vaka will have a beneficial downward force helping to counter this effect.
    I wonder if this may contribute to the fact that no B24 has ever been reported to have pitch poled. :?:
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    aaahh..
    If sail is the power...a whole load more variables to consider beyond that of "shape" :eek:
     
  9. John Perry
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    John Perry Senior Member


    But is that not one of the main techniques by which scientific knowledge is advanced? It is always tempting to short cut research programs by varying multiple parameters in one set of measurements but more often than not you end with a complete muddle that way, not knowing what is influencing what. And it often makes sense to experiment with simplified systems first, then see how the knowledge gained translates to more complicated systems.
     
  10. John Perry
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    John Perry Senior Member

    The same was said of the design of the 108ft LOA Club Med catamaran (and its sister ships) after it won the round the world race at the turn of the millennium, skippered by Grant Dalton. Michelet would not give any insight into this since it does not allow for squat and changing pitch trim with varying Fn, however Leo's new program Flotilla might perhaps be applicable. I havent yet tried Flotilla (too much sailing to do) but if it predicts changes in pitch trim then it is a simple matter to calculate the moment associated with that pitch trim based on the hydrostatic pitch stability and compare that with the moment generated by the rig. To mean anything I think you would need to try some different stern shapes and see whether any are signifcantly different to others.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    You're confusing scientific research which is then independently peer reviewed with a computer program that is being used beyond its limits of applicability and validation.
     
  12. John Perry
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    John Perry Senior Member

    The program is being used within its stated limits of applicability and I believe that there has been some validation of this program, see Leo's website. The program is based on methods that are described in papers at least some of which I think are peer reviewed, for what that's worth these days.

    Note that in my first post on this topic I wrote "I do not know how Michelet works so I have no idea how suitable it is for this kind of drag comparison, I am just stating the results so that Leo or others can decide whether they are relevant."

    I would be interested to hear what others think. I think I have used the program correctly but if someone thinks I may have made a mistake they can run it themselves and show me where I have gone wrong.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    "some validation", implies uncertainty.

    These two statements are at variance with each other. It just proves my point, as you have yet to answer this simple question:

    One does not need to ask questions of the author of a software program, the user manual should provide the answers.

    If the program is far too complex to understand or the science, logic and verification behind the program is not understood, the results will be somewhat meaningless. No matter how pretty the colour plots are ;)
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    FF's sake Ad Hoc <removed>
    There would be absolutely zero chance anyone would ever go sailing with you (by the way, do you sail? - no, I wonder why?) - because I'm sure any poor crew stuck with you would knife stick and roll you off the transom in the middle of the night.
     
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  15. Rastapop
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    Rastapop Naval Architect

    @John and Adhoc
    I'm not entirely sure which point is now the current focus of your argument, and if I've got the wrong end of the stick I apologise and I'll butt out ;)
    ...but I'd expect the main difference in drag between these two hull shapes to be the increased wavemaking resistance of the wider one.

    [​IMG]
     
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