axe bow design

Discussion in 'Boat Design' started by john L, Nov 20, 2011.

  1. john L
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    john L Junior Member

    Anyone ready to comment on the performance of the bows on the AC catamarans in San Diego. Stuffing the bows seems much less fatal
     
  2. Doug Lord
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    Doug Lord Flight Ready

    True, but there is sure a lot of spray. No doubt that shape is faster than a traditional shape on those boats.Great video in good wind today!
    See the thread in "Multihulls".....
     
  3. john L
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    john L Junior Member

    I was impressed by the speed but much more by the recovery. Looked for the capsizes and most were diagonal with bows recovered. I guess it slows enough to bring the apparent wind back to abeam, but other than one example with Korea in a race else where I couldn't find an example of the classic pitch pole.
     
  4. john L
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    john L Junior Member

    Does this change your thinking about Ama shapes?
     
  5. Doug Lord
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    Doug Lord Flight Ready

    ------------------
    No, not at all. I think the ama is designed very well. What's not good in my opinion is the daggerboard: a curved lifting foil would be far better and it would counter the pitchpole tendency these boats have to excess. It's proven technology and used on a lot of beachcats now including A Class cats, C Class cats, the Nacra 20 and I think even on the Marstrom 32 as well as on almost every racing trimaran.
    I'd also steal a little design feature pioneered by Martin Fisher: a little ridge on the Wildcat hull that tends to break the water off except I'd probably extend it forward a bit more.
    ----
    PS-I don't know for sure but didn't the reverse bow on small cats come about way before the "axe-bow" on ships?
     
  6. john L
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    john L Junior Member

    bows for amas

    Hope this is not a double post.
    My ama question was really aimed at the work you did on a self righting tri, and whether you would change the ama forms given this successful X bow business.
    My interest is directed towards autonomous tri's and given that they have lousy helms-machines, stability and recovery are prerequisites.

    The Xbow idea in commercial practice seem to date back as least as far as 2005.
    http://www.ulsteingroup.com/kunder/ulstein/cms66.nsf/doc/3A6662B6DEF379B3C12576C70031E17D

    or just the ulstein site for lots of large scale application.
    I haven't had much luck with dating it in multihull application.
    I would have thought that people like shuttleworth might have used it
    if it was in the air in the GBFerries days.
     
  7. tom28571
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    tom28571 Senior Member

    It seems to me that the X bow gains some of the characteristics of the bulbous bow without being limited to a small speed range. Not the bow wave cancelling part but the wave slicing, re-emerging and buoyancy elements. Are there any other benefits?

    Having watched the bow of my Korean War destroyer shudder while struggling to emerge from big waves, I wonder if the ax bow might have helped with that issue. Under less severe conditions the high flared bow was clearly much better for normal operations.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    ========================
    John, the bows on my tri model and probably on the full size boat are entirely different than the AC 45 hulls. They are designed to contact the water intermittently, not to be sailed "on" on a regular basis. Since they may make contact at high speed the hull is a planing configuration. Since they could be
    submerged momentarily the hull is ,essentially, double-ended.
    However, the bows are reverse bows with, maybe , a little more "reverse" than the AC 45 hulls.
    Here is a link to the thread: http://www.boatdesign.net/forums/mu...-trimaran-test-model-36058-12.html#post502724

    ama pictures prior to carbonation:
    -click on image-
    Left, hull sitting upright, bow down; Right-transom with planing surface shown.
     

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  9. garydierking
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    garydierking Senior Member

    The reverse bow shape was very popular on warships in the early 20th century.

    [​IMG]
     
  10. cthippo
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    cthippo Senior Member

    I wonder if some of that wasn't because ramming was still considered a viable tactic.
     
  11. JRD
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    JRD Senior Member

    Yeah, that and smacking headlong into icebergs!
     
  12. john L
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    john L Junior Member

    My interest in the "Axe" bow was provoked by the performance of the AC45 catamarans in San Diego and Plymouth. If you look at the movie coverage available on U tube and have sailed fast catamarans and pitch poled then I'm sure that you will see that the recoveries from bow submergence are remarkable. Note that there are not chins below or the surface.
    I haven't been able to see what the cross sections look like over the first six feet or so (2 m.) Perhaps someone has seen them ?
     
  13. Doug Lord
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    Doug Lord Flight Ready

    ================
    John, in my opinion, the AC 45's have a great tendency to pitchpole-compared to the wider Extreme 40's. That tendencey is slightly mitigated by the vertical lift generated by the boats daggerboard(slight angle allows some lift).
    I think the reverse bow has very little to do with either causing or recovering from a pitch pole but does reduce drag in combination with the forward section shape in waves. However, I think a ridge, similar to the one on Fischers Wildcat could help to shed the spray sooner.
    I'm convinced that a combination of a small rudder t-foil and curved lifting daggerboard would prevent most of the instances of pitchpole or attempted pitch pole we saw in the windy conditions in Plymouth. And eliminate it 100% in lighter conditions.
     
  14. john L
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    john L Junior Member

    I certainly agree that curved boards may help. My personal experience with rudderT foils on a twin rudder setup was somewhat unpredictable motion on heeling as the weather foil emerged. Any suggestions?
     

  15. Doug Lord
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    Doug Lord Flight Ready

    ==================
    John, check out the Mayfly thread in "Multihulls" here and look at the pictures of the "L" rudders on Fischers A Class cat. Note that the foils are angled down a bit. May help or not though I don't think he would intentionally allow the boat to heel so far the windward rudder would come out. But maybe in racing he'd pull it up? The leeward rudder foil would be horizontal with "X" amount of heel but you'd have to guess at "X".
    There are also pictures of Landy's rudder foils on his A class. I think small rudder foils combined with curved lifting foils is a marriage made in heaven as long as the whole thing is very carefully designed.
     
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