Center nacelles and wave pounding in cats.

Discussion in 'Multihulls' started by xarax, Sep 1, 2007.

  1. kerosene
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    kerosene Senior Member

  2. doug kay
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    doug kay Junior Member

    Well it would certainly increase the fore and aft strength of the bridge deck but I can't see it doing anything to stop pounding since it would reduce it at the peaks but increase it in the troughs. Do you have special software to produce those beautiful pictures.
     
  3. brian eiland
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    brian eiland Senior Member

    Nacelles and CB mounting possibilities

    I touch on some of this topic in these discussions:

    http://www.boatdesign.net/forums/showpost.php?p=36348&postcount=36

    http://www.boatdesign.net/forums/showthread.php?p=36348#post36348

    ..under ASYMMETRIC CENTERBOARDS
    http://www.boatdesign.net/forums/showthread.php?t=13389&page=2

    DynaRig MotorSailer
    http://www.runningtideyachts.com/dynarig/

    Basically you need adequate clearance and surfaces that don't present a 90 degree impact surface to the water being thrown at it.
     
  4. Pericles
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    Pericles Senior Member

  5. brian eiland
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    brian eiland Senior Member

    ReOpen this Subject

    I am looking to reopen this subject, and examine the largest number of existing designs out there that have made an attempt to provide a modified surface area in the wingdeck area to combat the wave slamming that can occur on many catamarans.

    I searched the forum for "bridgedeck slamming" and came up with primarily 3 subject threads with that wording in their titles, but this one's title seems to embrace concept most closely.

    Here is another subject thread with some good postings on the subject:
    http://www.boatdesign.net/forums/multihulls/slamming-waves-open-bridgedeck-crossbeams-22833.html
    ....and a more recent one here:
    http://www.boatdesign.net/forums/multihulls/bridgedeck-clearance-37666.html


    I recall bring this subject up back in 2003, not long after joining the forum:
    http://www.boatdesign.net/forums/boat-design/big-cat-alt-cbs-sail-rigs-2225.html
    First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam).
    The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves.


    My 'nacelle plate' actually extended out to the front of the bridgedeck quite a bit to "attack" the water formation that might slap at the wing.

    Hopefully we will get a number of participants supplying photos of good and poor examples, that can be analyzed as to why or why-not they worked.
     
  6. Wavewacker
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    Wavewacker Senior Member

    So, as I'm trying to follow this, yes, I'm a beginner, The Micromega 5, sailed by the french twins across the Atlantic had a center pod or control cockpit that sat (I believe) slightly higher than the hulls and was as pointed as a Formula One car. Is that type of arrangement a solution?

    Another small cat, the ECO6, is rather strange looking but seems to be well received as a shall coastal cruiser that has a flat cabin that dives down to the leading edge of the boat incorporating both hulls. I wondered how waves might slam on the cabin, or would these boats be lighter and go over waves?

    And, how badly would a dory style hull slam, if it were say 42" high at the bow, at about 20' and say 800 pounds (hope that makes since) being the same asymetrical design?
     
  7. brian eiland
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    brian eiland Senior Member

    Could you provide links to those vessels...can't seem to find them. posting photos also really helps. If you need help with this just ask...by email preferable
     
  8. brian eiland
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    brian eiland Senior Member

    Hey Pericles, could you provide an update on the link?...doesn't seem to work now
     
  9. brian eiland
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    brian eiland Senior Member

    Could you provide an update on that link? ...it doesn't seem to work now.
     
  10. brian eiland
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    brian eiland Senior Member

    Lagoon 43 'sneezing'

    I believe this 'self inficted' phenomenon is what is occuring on the Lagoon 43 powercat and its reported 'sneezing' problem. The bows of that vessel are fairly full in shape, and fairly close together. The forward motion of the vessel is forcing a lot of water into a more restrictive channel down between the hulls. Add to this a bit of choppy sea, and/or a downward pitching bow, and that extra water gets 'sneezed' back out front of the vessel to be blown back over the bow as a spray.

    And that very minium the V-shape nacelle they provided does little or nothing to really help....mostly cosmetic if you ask me.

    I wrote about this sneezing problem somewhere else on some forums, but can't seem to recall where :confused:

    In all fairness I do not recall reports about slamming on these vessels,...only sneezing. Would be great to hear first hand from some owners. I tried several times to get a ride on one of these vessel in some nasty conditions, but never got the chance.
     

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  11. Frosty

    Frosty Previous Member

    The owner of the Catana 45 power cat behind me right now as I type can confirm this.

    A sneeze out of the front can do nothing in my opinion for forward momentum.

    It handy for stowing the anchor chain.
     
  12. Dryfeet
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    Dryfeet Junior Member

    So did/does the Gougeon 32 have a sneezing problem or noticeable slamming issue? It is afterall, both narrow AND low bridgedeck clearance. Or does the fineness of the hulls minimize this issue (versus something fat like the Lagoon43)? While width can help reduce wave train interaction from the hulls, it seems to me that the need to 'straddle' more waves might result in increased slamming. Bridgedeck clearances for offshore cats often are given in terms of vertical clearance but surely slamming would be greater for a 3' Brdk clearance on a 20' beam versus a 26' beam with the sam clearance?
     
  13. brian eiland
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    brian eiland Senior Member

    Sneezing vs Slamming

    I agree Frosty, it would do nothing for momentum, but I'll bet you could live with this much better than slamming.

    Do you think you could elicit some personal stories from that other owner??
     
  14. brian eiland
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    brian eiland Senior Member

    Fineness of the hulls can be both good and bad. Fine can reduce the squeezing between effect. But fine can pitch down more by the bow before gaining sufficient bouyancy

    In most cases these vessels are not staddling multiple waves.
     

  15. Alik
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    Alik Senior Member

    Minimal vertical clearance t for powercats I use for our designs:

    For displacement cats:
    - t=4.5...6% of length
    - t=0.5*h1/3, where h1/3 is anticipated significant wave height

    For planing cats:
    - t=2...3% of length

    These and other recommendations are published in some my papers on catamaran design.
     
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