Why can't most catamarans get over the hump ?

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

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    "If you were to plot resistance against beam for hulls where speed, LWL, displacement and WSA remained constant you'd see clear trend of increasing resistance as beam increases."

    Or you could just plot the wave pattern resistance coefficient, where
    beam squared is used in the non-dimensionalisation, i.e.
    Cwp = Rw/(1/2 rho U^2 B^2)
    The curves should all collapse onto a single curve for all speeds.

    (In Michlet, don't include the BL displacement thickness.)
     
  2. DennisRB
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    DennisRB Senior Member

    Good thread, great info. But the answer to the question need not be so academic. It only takes a layman like me to understand that most cruising cats have the bad combination of having fat hulls that are very heavy and close together.

    Not the best combination. Compare a lagoon 44 which is around 13 ton and has something like 8:1? L/B to an Oram 44 which weighs 5 ton and has 12.5:1 L/B and you can see why the oram has no "hump" when you actually sail it. Straight over the rule of thumb hull speed without even noticing it.
     
  3. DennisRB
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    DennisRB Senior Member

    I have been reading the paper linked in this thread by the University of Southampton regarding DLR and B/T and this quote below is sure to make people think twice about B/T (which directly relates to L/B length to beam).

    Seems Adhoc does know what he is talking about after all.

    Can someone check my interpretation of the above quote, which is that over the ranges tested, the trend for light displacement hulls is that, WIDER hulls actually have LESS resistance than narrow hulls? And heavier displacement hulls have less resistance with narrow hulls? Where does this leave us with a performance light weight cruising cat vs heavy cats? Why do we see the opposite in actual designs?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The fact that ferry operators world wide have switched to cats tells me that they save fuel by doing so, and the way to save fuel is to have less resistance at the speed they desire to operate at. It is not as if they are cheaper to build.
     
  5. oldsailor7
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    oldsailor7 Senior Member

    Hmmm. Double ended Proas go backward as good as forwards. :idea:
     
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  6. groper
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    groper Senior Member

    Need to be careful and read all results to theorise.

    So I have couple of comments...

    All models had a transom stern. The components of transom drag were not isolated or investigated in this paper.

    By varying the LDR of the models, the size of the transom hollow would have also varied for all models. However, the changing aspect ratio of the transom hollow would produce different amounts of transom drag with respect to the total drag for each model series.

    Thus Without isolating the transom drag component, you haven't isolated the effect of changing B/T ratio for each LDR series because the aspect ratio of the transom hollow is also changing with LDR. ie - it could be the transom hollow aspect ratio influencing some of the trends.

    The same could also be said for the sinkage effects, which were significant at the higher froude numbers, which is also the range where the B/T trend reversal occurred. Would the lift and sinkage effects return the same trend on heavily rockered hulls for example?

    Reading more papers which investigate these other trends on different hulls, particularly those with and without transom sterns would yield more conclusive trends.
     
  7. DennisRB
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    DennisRB Senior Member

    If the boffins doing these tests cant even get the test right, what chance does the amateur have of finding usable info from the results? If you are right about the transoms, another way of looking at it might be that transom design can have more impact on drag than B:T.

    Does anyone have more tests that look at transoms/hull rocker? These would be hard to test indeed. Changing the transom immersion changes so many other parameters at the same time. The results would be hard to interpret indeed.
     
  8. groper
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    groper Senior Member

    You can't say the test wasn't right. It's just that those Trends weren't investigated in that paper.

    There is useable info there. Notably, DLR is by far the greatest influence, and the other investigated factors are small in comparison.

    For a power catamaran, the hull shape investigated was appropriate and quite typical for many commercial designs, including the transom. Not so for a sailing catamaran which most likely was not a consideration for the researchers.

    Also need to remember, that the trend reversal as discussed, was only noticeable at higher froude numbers above which most sailing vessels cannot operate, save the racing boats. As a cruising sailor, you should concentrate on the data below and at the hump speed as this is where you will spend the lions share of your time. The lower beam hulls do better here, again due to the wave drag component and wave interference component for catamarans , also noting the S/L trends of the same.
     
  9. DennisRB
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    DennisRB Senior Member

    I agree. But a cruising sailor also wants space. If the hulls can much wider, but with negligibly more drag, is there really much point to very narrow hulls for even a fast cruising boat?

    Clearly DLR is king here. A good cruising cat will be as light/long as possible, but not necessarily as narrow as possible. That is the trend I am getting for a good cruising design. But with wider hulls the temptation will be to end up with a heavier boat by adding fluff in the extra space.

    What I have learned is that hull beam plays no where near as much of a part in drag as what I thought. Even if this tread reversal I noted is a one off oddity.
     
  10. hump101
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    hump101 Senior Member

    For a given speed/length/displacement there will be an optimum B/T based on the balance between frictional and wavemaking resistance. So you need to be careful about creating a trend based on a limited region of data. For example, if you only test at B/T above optimal, then it will look like increased B/T increases resistance, whilst at B/T below optimal, the reverse can be seen.
     
  11. groper
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    groper Senior Member

    Think about what's happening here.

    DLR is most important for performance . If your length is fixed and your considering hull width or hull volume, what are the implications of changing volume? Every peice of bulkhead and stiffener and furniture, also increases in area. So the weight of the vessel must increase with increasing volume. There's another trend :D.

    Assume there is no difference in weight, the difference in b/t is already proven to be a minimal consideration, you would not notice any significant difference.

    Fatter more volume hulls, inevitably have a reduced s/l ratio as it'd measured between hull centrelines. Thus for a constant catamaran overall beam, wider hulls have reduced separation.

    But add a little increase in weight, a little reduction in separation, a little extra wave making drag from beamy demihulls, and hey presto, you now have a combination of bad trends that sum together and really hurt performance. This could be so much so that the hump cannot be overcome with the limited available power, and begs the question, "why can't some catamarans get over hump speed?" :D :p
     
  12. DennisRB
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    DennisRB Senior Member

    Yeah sure. You need to be sensible here, like I mentioned if you go a bit wider in the hulls you need to keep the extra stuff to a minimum or the extra weight will make a noticeable difference. Many people could not do that, but I like just having space available. Its so much easier to pack X amount of gear in a bigger space then find it again! I'm not sure if you have lived on a boat before full time, but just finding stuff can be a mission sometimes digging though small lockers with layers of stores.

    For example an oram 53 I looked at with 15:1 hulls was immediately scratched as being too narrow by the Admiral "feels like a tunnel". Now if it had 13:1 hulls how much heavier would it have been due to a bit of extra bulkhead area etc? Would it even be much heavier at all as the bridge deck would be a little smaller and hulls closer together means less loads there? Would this difference be significant compared to things such as personal choices in fitout etc? Would the speed difference be noticeable? What about when leaving for a long term cruise with max weight aboard? How would the narrower version carry the weight compared to the wider one?

    If the 5 ton oram 44 with 12.5:1 hulls was built with 8:1 hulls would it suddenly weigh 13 ton like the lagoon 44? (not that I think 8:1 is a good idea, that seems way to fat).

    I get your point though and a negative spiral could start by going with a wider hull.
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Many different transom stern hulls were presented in:
    Robards, Simon William,
    "The hydrodynamics of high-speed transom-stern vessels",
    M. Engineering thesis, The University of New South Wales, Nov. 2008.
    http://unsworks.unsw.edu.au/vital/access/manager/Repository/unsworks:3426

    You could try assembling your own correlations with "rocker" using that extensive data.
     
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  14. DennisRB
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    DennisRB Senior Member

    Great link. Its going to take me hours to understand any of it :D
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed and links into:

    That's the ticket :D

    It is all about trends, not absolutes. The trends of "what if" aids the designer in the inevitable compromise that is required to satisfy the SOR. Focusing upon one and as an absolute, misses the bigger and more important picture. Since juggling many balls at once is tricky...playing with one and one alone..is childs play ;)
     
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