what would be the design cost?

Discussion in 'Multihulls' started by UpOnStands, Sep 26, 2017.

  1. UpOnStands
    Joined: Nov 2015
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    UpOnStands Senior Member

    Jeff, not real deal breaker, just a small issue that came to mind. Pity there is no CAD plug-in that can tell you what is the best trade-off :p
    But it seems that if the submerged portion of the surface piercing foil must be 50%?? 100%?? chord longer than the bottom exit foil then this penalty is acceptable.
    this would be offset of course by the 1m or so reduction in feed length.
    The downhaul pulley would be wet a lot of the time so hopefully 4mm Dyneema running over a solid PTFE saddle would be sufficiently durable.
     
  2. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    My take would be that the trade off is greater than just altering the cord length, I'd be adding some design complexity over the simple through case with the topsides being also "notched" to accept the arrangement so no longer a simple in plane part of the longtitudinal structure, add the berthing arrangement to floating pontoons to interact with the boards exit point & on top of that the cantilevered part of the board becomes longer with the bearing points of same getting closer together with resulting increase in load/moments. Maybe a half way solution exists where the case is installed deck to bottom leaning out & at full draft projection of the boards a bench top or other additional structure coincides with the upper load/bearing point this separates the bearing points giving an advantage in moment at the same overall draft with similar result ?- this also allows the option of a longer board installation to the vessel.
    Jeff.
     
  3. UpOnStands
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    UpOnStands Senior Member

    umm, not sure I like the idea of having the top of the board at mid-height inside a case inside the hull. If I understand you correctly.
    Wouldn't you need a popsicle stick to ensure board retrieval? Suppose the stick could be hinged so when the board is retracted the stick lies on the deck.
     
  4. waikikin
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    waikikin Senior Member

    Nice one, you'd need good refrigeration & slick hoisting system to hold your popsicle while bringing the board back up;)
    You apparently plan to have a downhaul line to deploy the board/s- very viceversa to rely on an uphaul & if underway assisting the outboard mounted daggers is fraught with mob danger if required/jammed. The thing to do for reliability is adopt the simplest & strongest option.. that's why daggers usually go top to bottom & project above the deck
    For the mid/bench height option an internal access hatch could be provided but comes at a complication this could aid in rigging of the downhaul purchase.
    Not sure why your boom is going to interact with the boards as I haven't seen your layout beyond this potential issue- typically/often the dagger cases are mounted behind the mast bulkhead so clear of headsails and forward of chainplates/shrouds so clear of main boom also. There must be some geometry to the rig that is not typical/familiar?
    Jeff.
     
  5. UpOnStands
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    UpOnStands Senior Member

    Very unfamiliar. Semi-open deck catamaran. Two unstayed masts, one per hull, set at the 75% aft position. Two jibs, each 30 m2, on furlers. The jibs are outhauled by Hoyt type booms that pivot on the same axis as the forestay. The forestays run over/thru the mast top and are tensioned aft.
    The Hoyts are close to the hull tops so the conventional vertical dagger in hull does not work.
    Their inclined axis of rotation means that there is vertical space on the centerline of the bridgedeck aft of the fwd crossbeam.
    The boards would need uphaul and downhaul if mounted on the sides.
     
    Last edited: Oct 14, 2017
  6. Steve W
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    Steve W Senior Member

    Most of Malcolm Tennants cats use cantilevered boards, ie, the top of the board is down inside the trunk rather than at deck level when deployed saving quite a bit of weight. The smaller ones at least, such as the GBE and Turissimo 9m, etc use a simple spade handle attatched to the top of the board to raise it which then does fold over horizontal when in the up position.

    Steve.
     

  7. UpOnStands
    Joined: Nov 2015
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    UpOnStands Senior Member

    Thanks Steve for the info. Will definitely look at this as indenting the hull is not efficient in terms of stress transfer.
    just checked on the Tennant web site
    Turissimo 10 disp = 2495 kg
    Turissimo 9 displ = 1300 kg
    What is going on here?
     
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