Catamaran design loads

Discussion in 'Boat Design' started by mdatrpz, Feb 10, 2008.

  1. mdatrpz
    Joined: Feb 2008
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    mdatrpz Junior Member

    Maybe the more experienced can offer some insights here:

    I am trying to build a cumulative list of loads that apply to a free-twin masted sailing/cruising catamaran.

    Hydrostatic forces on hulls at rest
    Hydrodynamic forces due to wave action (varying frequency and intensity)
    Wing sail force on each mast inducing a bending moment on mast
    Masts weight inducing normal force on hull
    Bending moments on each mast along the center of effort of the deck/hull
    Keel loading on hulls - resist lateral sail force
    Hulls exert bending moments on bridgedeck due to varying wave action/pitching
    Bridgedeck has weightloads on hull
    Frontal impacts on either hull (not recommended but possible)
    Other point impacts (not recommended but possible)
    General weight of contents on hulls and bridgedeck
    Shear force of focused loads on hulls (marine diesel weight/water tanks/etc)
    Propeller shafts forward thrust on hulls

    Anybody have comments on others?


  2. Guest625101138

    Guest625101138 Previous Member

    A few more:
    1. Rudder loads under normal hard-pressed sailing and if beached.

    2. Torsional load through bridge deck - bow of one hull submerged and stern of other hull submerged creates substantial torsional load in a quartering sea.

    3. Crew weight on decks and bridge (cabin) roof.

    Rick W.
  3. Guest625101138

    Guest625101138 Previous Member

    I just read the initiating thread on this set of criteria.

    Other things that may be of use:

    The overturning moment is limited by the overall beam. Basically the mast forces can be reduced for a set displacement by bringing the hulls closer together.

    Centre of buoyancy moves forward as the sails load up.

    A non-ventilating rudder is about three times more effective for its area than a stern hung rudder. Likewise with lee boards/keels.

    If there is no length constraint then a monohull will have about 40% less drag than a catamaran of the same displacement.

    An optimised long slender hull, such as those used on a cat, will be increasingly fuller in the ends as the design speed increases relative to its hull speed.

    There is an optimum spacing for cat hulls depending on the speed but it is not particularly significant.

    High aspect foils are more aerodynamically efficient but with sails, the overturning moment comes into play when determining the optimum sail aspect.

    In the limit, a catamaran is a monhull so performance to windward can be assessed as if it is a monohull without wave interference from the lifted hul.

    It is possible to design hull shape and/or place keels such that a cat will sail ballanced through a range of windspeed.

    Rick W.
  4. masalai
    Joined: Oct 2007
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    masalai masalai

    My "being serious hat is firmly on"

    Design criteria to reduce risk of "pitch-polling" as this issue seems to be regularly mentioned - more so by non cat sailors...
    Merits of various sail configurations "expressed in numbers" so as to form some basis of consistency in comparison eg...

    velocity generated per sq metre sail area with true wind beam on, highest pointing angle, VMG...

    then ability to de-power or reef or pull down/stow (roller-reefing or other method) at any point or wind velocity or effective usable limit ranges...

    survivability of the crew in inverted hulls (a place to be dry, sheltered & ability to access food etc)

    Analysis of frequency of "urban myths" and other incidents at sea being designed to inhibit/minimise as being considered by you (how often is frontal impact occurring in relation to sea miles as a global value) and does that warrant a change in current thinking. In other words, justify the additional changes in a cost benefit analysis to current practice.
  5. mdatrpz
    Joined: Feb 2008
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    mdatrpz Junior Member

    Thanks a bunch.

    Keep the LOADS going. Maybe we can follow up with CRITERIA soon.

    Focus on those FORCES....

    (please, no Skywalker jokes here)

  6. mdatrpz
    Joined: Feb 2008
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    mdatrpz Junior Member

    Some more FORCES:

    "I had completed the final weight calculation to reduce wave making drag"

    "These loads can increase by up to 3 tonnes if shock loading is taken into account due to the boat decelerating by falling sideways off a wave or burying the bow."
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